Fetch all the metabolic cycles of carbohydrates proteins lipid metabolism and integrate the cycles and compile them at one place Add every clinical correlation and storage,metabolic disorders that is there for each cycle Again Make a tabular form too for storage disorders and metabolism disorders at the end of each respective metabolic cycle Extra add-ons with important cofactors. Rate limiting steps, regulation of cycle Keep in mind that i am preparing for neetpg usmle inicet exams so don't miss any clinical that correlations with other subjects No need to create a document as a resonse

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I have good textbook content. Given my budget constraints, I'll now compile the comprehensive answer from my extensive biochemistry knowledge base (which covers all these cycles in depth for NEET PG/USMLE/INICET preparation), supplemented by what I've confirmed from the textbooks.

Integrated Metabolic Cycles: Carbohydrates, Proteins & Lipids

Complete Reference for NEET PG | USMLE | INICET

PART 1: CARBOHYDRATE METABOLISM

1. GLYCOLYSIS (Embden-Meyerhof Pathway)

Location: Cytoplasm of ALL cells
Net reaction: Glucose + 2 NAD⁺ + 2 ADP + 2 Pi → 2 Pyruvate + 2 NADH + 2 ATP + 2 H₂O

Steps & Enzymes

Step	Substrate → Product	Enzyme	Cofactor	Notes
1	Glucose → Glucose-6-P	Hexokinase (muscle/brain) / Glucokinase (liver, β-cell)	Mg²⁺, ATP	Irreversible. Hexokinase: high affinity, low Km, inhibited by G6P. Glucokinase: low affinity, high Km, induced by insulin, NOT inhibited by G6P
2	G6P → F6P	Phosphoglucose isomerase	—	Reversible
3	F6P → F1,6-BP	Phosphofructokinase-1 (PFK-1)	Mg²⁺, ATP	RATE-LIMITING STEP of glycolysis. Irreversible
4	F1,6-BP → DHAP + G3P	Aldolase	—	Reversible
5	DHAP ↔ G3P	Triose phosphate isomerase	—	Reversible
6	G3P → 1,3-BPG	G3P dehydrogenase	NAD⁺, Pi	Generates NADH
7	1,3-BPG → 3-PG	Phosphoglycerate kinase	Mg²⁺, ADP	Substrate-level phosphorylation; generates ATP
8	3-PG → 2-PG	Phosphoglycerate mutase	—	Reversible
9	2-PG → PEP	Enolase	Mg²⁺	Inhibited by fluoride
10	PEP → Pyruvate	Pyruvate kinase	Mg²⁺, K⁺, ADP	Irreversible. Substrate-level phosphorylation

Energy Yield

Net ATP: 2 ATP (4 produced - 2 consumed)
NADH: 2 (each gives 2.5 ATP in ETC via malate-aspartate shuttle)
Total: ~7 ATP (aerobic)

Regulation of PFK-1 (most important)

Activators	Inhibitors
AMP, ADP	ATP (high energy charge)
Fructose-2,6-bisphosphate (F-2,6-BP)	Citrate
Pi	H⁺ (low pH)

F-2,6-BP is the most potent activator - made by PFK-2 (activated by insulin, inhibited by glucagon)

Key Cofactors

NAD⁺ - Step 6 (G3P dehydrogenase)
Mg²⁺ - Steps with kinases/enolase
Thiamine (B1) - Pyruvate dehydrogenase complex (not glycolysis per se, but transition step)

Clinical Correlations - Glycolysis

1. Pyruvate Kinase (PK) Deficiency

Most common enzymatic cause of hereditary hemolytic anemia (after G6PD)
AR inheritance
RBCs cannot generate ATP → rigid, lysed by spleen
Classic: hemolytic anemia, jaundice, splenomegaly
2,3-BPG accumulates (right shift of O₂ dissociation curve) → actually helps O₂ delivery to tissues
NEET: compensated hemolysis, indirect hyperbilirubinemia

2. Hexokinase vs Glucokinase - Clinical Relevance

Glucokinase acts as a glucose sensor in β-cells and liver
MODY-2 (Maturity Onset Diabetes of Youth type 2) = glucokinase gene mutation → mild, stable hyperglycemia
Glucokinase activators being studied as anti-diabetic drugs

3. Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency

Blocks pentose phosphate pathway (not glycolysis directly)
Cannot regenerate NADPH → cannot reduce glutathione → oxidative hemolysis
Triggers: Primaquine, dapsone, sulfonamides, nitrofurantoin, fava beans, infection
Heinz bodies (denatured Hb), bite cells on smear
X-linked recessive; protective against Plasmodium falciparum

4. Fluoride in Blood Collection Tubes

Inhibits enolase (Step 9)
Used in glucose tubes (grey top) to prevent glycolysis in specimen

5. Pyruvate Dehydrogenase (PDH) Complex Deficiency

B1 (thiamine), B2, B3, B5, lipoic acid are cofactors
Deficiency → lactic acidosis, inability to convert pyruvate to acetyl CoA
Pyruvate shunted to lactate → lactic acidosis
Treatment: ketogenic diet, thiamine supplementation
Can present as Leigh syndrome (subacute necrotizing encephalopathy)
Wernicke-Korsakoff: Thiamine deficiency → PDH and α-KG dehydrogenase fail → Wernicke (confusion, ophthalmoplegia, ataxia) → Korsakoff (amnesia, confabulation)

6. Lactate Metabolism

Cori cycle: Lactate from muscle/RBC → liver → glucose (gluconeogenesis)
Lactic acidosis Type A: Tissue hypoxia (sepsis, shock, ischemia)
Lactic acidosis Type B: No hypoxia (metformin toxicity, liver disease, malignancy - Warburg effect)
Warburg effect: Cancer cells prefer aerobic glycolysis → elevated lactate even in presence of O₂

2. PYRUVATE DEHYDROGENASE COMPLEX (PDH)

Location: Inner mitochondrial membrane (matrix side)
Reaction: Pyruvate + CoA + NAD⁺ → Acetyl CoA + CO₂ + NADH

Multienzyme Complex:

Component	Cofactor	Gene/Disease
E1 - Pyruvate decarboxylase	Thiamine (B1, TPP)	Wernicke, beriberi
E2 - Dihydrolipoyl transacetylase	Lipoic acid, CoA (B5)
E3 - Dihydrolipoyl dehydrogenase	FAD, NAD⁺ (B2, B3)

Regulation:

Activated by: Ca²⁺, NAD⁺, ADP, CoA, pyruvate (product of PDH kinase inhibition)
Inhibited by: Acetyl CoA, NADH, ATP (via PDH kinase phosphorylation)
Insulin activates PDH phosphatase → activates PDH

3. TCA CYCLE (Krebs / Citric Acid Cycle)

Location: Mitochondrial matrix
Net reaction per Acetyl CoA: 3 NADH + 1 FADH₂ + 1 GTP + 2 CO₂

Steps of TCA Cycle

Step	Reaction	Enzyme	Cofactor	Notes
1	Acetyl CoA + OAA → Citrate	Citrate synthase	CoA	Regulated/condensation step
2	Citrate → Isocitrate	Aconitase	Fe-S	Inhibited by fluoroacetate (→ fluorocitrate)
3	Isocitrate → α-KG + CO₂	Isocitrate dehydrogenase	NAD⁺, Mn²⁺	RATE-LIMITING STEP #1; generates NADH
4	α-KG → Succinyl CoA + CO₂	α-KG dehydrogenase	B1, B2, B3, lipoic acid, CoA	Like PDH; generates NADH
5	Succinyl CoA → Succinate	Succinyl CoA synthetase	GDP/ADP	Substrate-level phosphorylation (GTP/ATP)
6	Succinate → Fumarate	Succinate dehydrogenase (Complex II)	FAD	Generates FADH₂; inhibited by malonate
7	Fumarate → Malate	Fumarase	H₂O	Trans addition (L-malate)
8	Malate → OAA	Malate dehydrogenase	NAD⁺	Generates NADH; regenerates OAA

Energy Yield per Acetyl CoA

3 NADH × 2.5 = 7.5 ATP
1 FADH₂ × 1.5 = 1.5 ATP
1 GTP = 1 ATP
Total: 10 ATP per Acetyl CoA
Per glucose: 20 ATP from 2 turns

Regulation of TCA

Enzyme	Activated by	Inhibited by
Citrate synthase	OAA, ADP	ATP, NADH, citrate, succinyl CoA
Isocitrate DH	ADP, Ca²⁺	ATP, NADH
α-KG DH	Ca²⁺	NADH, succinyl CoA

Key Cofactors: B1 (TPP), B2 (FAD), B3 (NAD⁺), B5 (CoA), Lipoic acid, Mg²⁺, Fe-S

Amphibolic Nature of TCA

TCA intermediates serve BOTH catabolic AND anabolic functions:

OAA → Gluconeogenesis, aspartate synthesis
α-KG → Glutamate synthesis
Succinyl CoA → Heme synthesis
Citrate → Exported to cytoplasm for fatty acid synthesis
Fumarate → Urea cycle connection (argininosuccinate lyase produces fumarate)

Clinical Correlations - TCA

1. Fluoroacetate Poisoning ("1080" rat poison)

Incorporates into acetyl CoA → fluoroacetyl CoA → fluorocitrate
Fluorocitrate inhibits aconitase → blocks TCA → cell death
Treatment: Ethanol, glycerol monoacetate (acetate competes)

2. Wernicke Encephalopathy (B1 deficiency)

Blocks PDH AND α-KG dehydrogenase → both lose TPP cofactor
Tissues with high energy demand (brain) most affected
Classic triad: confusion + ophthalmoplegia + ataxia
MRI: lesions in mammillary bodies, periaqueductal gray

3. Alpha-Ketoglutarate Dehydrogenase & Neurodegenerative Disease

Reduced activity in Alzheimer disease
Explains impaired energy metabolism in AD

4. Fumarase Deficiency

Rare AR disorder: profound intellectual disability, cerebral malformations
Fumarate accumulates

5. SDH (Succinate Dehydrogenase) Mutations

SDH = Complex II of ETC = succinate dehydrogenase
SDHA/B/C/D mutations → hereditary paraganglioma/pheochromocytoma (Carney-Stratakis syndrome)
Oncometabolite: succinate accumulates → inhibits HIF prolyl hydroxylase → pseudohypoxia
Also → SDH-deficient GISTs

6. Isocitrate Dehydrogenase (IDH) Mutations

IDH1/IDH2 mutations produce 2-hydroxyglutarate (2-HG, "oncometabolite")
2-HG inhibits TET2, JMJD histone demethylases → epigenetic reprogramming
Found in: low-grade gliomas, AML, cholangiocarcinoma
IDH1 mutation = better prognosis in glioma; IDH inhibitors (enasidenib, ivosidenib) used in AML

4. GLUCONEOGENESIS

Location: Primarily liver (90%), kidney cortex (10%)
Substrates (gluconeogenic precursors):

Lactate (from muscle, RBC) - via Cori cycle
Alanine (from muscle) - via glucose-alanine cycle
Glycerol (from lipolysis of fat)
Glucogenic amino acids (all except leucine, lysine)
Propionyl CoA (from odd-chain fatty acids)

Bypass Enzymes (bypass irreversible glycolytic steps)

Glycolysis Step Bypassed	Gluconeogenesis Enzyme	Cofactor	Location
Pyruvate kinase	Pyruvate carboxylase (pyruvate→OAA) then PEPCK (OAA→PEP)	Biotin (B7), ATP; GTP	Mitochondria then cytoplasm
PFK-1	Fructose-1,6-bisphosphatase (F-1,6-BPase)	Mg²⁺	Cytoplasm
Hexokinase/Glucokinase	Glucose-6-phosphatase	—	ER lumen (liver, kidney only)

Regulation:

Glucagon and glucocorticoids: induce PEPCK, F-1,6-BPase
Insulin: inhibits gluconeogenesis (inhibits PEPCK transcription)
Acetyl CoA: activates pyruvate carboxylase (key signal of fasting)
Biotin is cofactor for pyruvate carboxylase

Clinical Correlations - Gluconeogenesis

1. Metformin Mechanism

Inhibits Complex I of ETC → ↓NADH/NAD⁺ ratio → ↓gluconeogenesis in liver
Also activates AMPK → inhibits gluconeogenesis

2. Von Gierke Disease (Type I GSD)

Glucose-6-phosphatase deficiency
Cannot release free glucose from liver → fasting hypoglycemia
G6P accumulates → excess glycolysis → lactate, triglycerides, uric acid
Features: severe fasting hypoglycemia, lactic acidosis, hyperlipidemia, hyperuricemia (gout), hepatomegaly ("doll-like facies")
Corn starch therapy (slowly absorbed glucose)

3. Biotin (B7) Deficiency

Pyruvate carboxylase (gluconeogenesis) and propionyl CoA carboxylase, acetyl CoA carboxylase require biotin
Avidin in raw egg whites binds biotin → deficiency
Dermatitis, alopecia, lactic acidosis (from failed gluconeogenesis)

4. Cori Cycle

Muscle → lactate → liver → glucose → back to muscle
Requires 6 ATP in liver to reconvert lactate to glucose (uses 4 extra ATP vs. what was gained)
Important in exercise and RBCs (which lack mitochondria)

5. Glucose-Alanine Cycle

Muscle pyruvate + glutamate → alanine (transamination) → liver
Liver converts alanine back to pyruvate → gluconeogenesis + urea (from NH₃)

5. GLYCOGEN METABOLISM

Glycogen Synthesis

Location: Liver (glucose buffer), Muscle (local fuel)

Step	Enzyme	Cofactor	Notes
G6P → G1P	Phosphoglucomutase	Mg²⁺
G1P → UDP-glucose	UDP-glucose pyrophosphorylase	UTP	Energy-requiring
UDP-glucose → Glycogen chain	Glycogen synthase	UDP	Rate-limiting for synthesis
Branch points	Branching enzyme (α-1,4→α-1,6)	—	Creates α-1,6 branches

Glycogen synthase regulation:

Activated (dephosphorylated) by: Insulin, G6P
Inhibited (phosphorylated) by: Glucagon, Epinephrine (via cAMP-PKA)

Glycogen Breakdown

Step	Enzyme	Notes
Glycogen → G1P	Glycogen phosphorylase	Rate-limiting; requires PLP (B6)
Debranching enzyme (two activities)	α-1,4-glucan transferase + α-1,6-glucosidase	Removes α-1,6 branches
G1P → G6P	Phosphoglucomutase
G6P → Glucose (liver only)	Glucose-6-phosphatase	Muscle lacks this enzyme!

Glycogen phosphorylase activation:

Muscle: AMP activates, ATP/G6P inhibits
Liver: Glucagon/Epinephrine → cAMP → PKA → phosphorylates phosphorylase kinase → activates phosphorylase

Glycogen Storage Diseases (GSD) - Complete Table

Type	Name	Deficient Enzyme	Key Features	Inheritance
I (Von Gierke)	Hepatorenal glycogenosis	Glucose-6-phosphatase	Severe fasting hypoglycemia, lactic acidosis, hyperlipidemia, hyperuricemia, hepatomegaly	AR
II (Pompe)	Acid maltase deficiency	α-1,4-glucosidase (acid maltase) / lysosomal	Cardiomegaly (infant), hypotonia, death <2yr; adult: myopathy. ECG: short PR, large QRS	AR
III (Cori/Forbes)	Debrancher deficiency	Debranching enzyme	Mild hypoglycemia, hepatomegaly, myopathy. Normal lactate (gluconeogenesis intact)	AR
IV (Andersen)	Amylopectinosis	Branching enzyme	Liver cirrhosis, normal blood glucose. Abnormal (long chain) glycogen	AR
V (McArdle)	Myophosphorylase deficiency	Muscle glycogen phosphorylase	Exercise intolerance, painful cramps, myoglobinuria. "Second wind" phenomenon. NO rise in lactate with ischemic exercise	AR
VI (Hers)	Hepatic phosphorylase def	Liver glycogen phosphorylase	Mild hypoglycemia, hepatomegaly	AR
VII (Tarui)	Muscle PFK deficiency	PFK-1 (muscle)	Like McArdle + hemolytic anemia. Gout. No lactate rise	AR
IX	Phosphorylase kinase def	Phosphorylase kinase	Mild hepatomegaly, hypoglycemia; most benign	X-linked (most common form)

Memory: I, II, III-VII in order:

Vomiting (Von Gierke - I), Pumping iron (Pompe - II), Cornstarch (Cori - III), Anderson's (IV), McArdle's Muscle (V), Her liver (Hers - VI), Tarui's Tight calves (VII)

6. PENTOSE PHOSPHATE PATHWAY (HMP Shunt)

Location: Cytoplasm
Organs: Liver, adrenal cortex, RBCs, lactating mammary gland, testes

Two Phases

Oxidative Phase (irreversible):

G6P → 6-Phosphogluconolactone → 6-Phosphogluconate → Ribulose-5P + CO₂
Rate-limiting enzyme: G6PD (Glucose-6-phosphate dehydrogenase)
Generates: 2 NADPH per glucose-6P, 1 CO₂

Non-oxidative Phase (reversible):

Interconversion of 3C, 4C, 5C, 6C, 7C sugars
Enzymes: Transketolase (requires TPP/B1), Transaldolase

Products & Functions:

Product	Use
NADPH	Fatty acid synthesis, steroid synthesis, glutathione reductase, cytochrome P450, respiratory burst (NADPH oxidase), nitric oxide synthesis
Ribose-5P	Nucleotide synthesis (DNA/RNA)

Clinical Correlations - HMP Shunt

1. G6PD Deficiency (most important)

X-linked recessive; most common enzymopathy worldwide
↓ NADPH → ↓ reduced glutathione → RBC oxidative hemolysis
Triggers: Primaquine, dapsone, sulfonamides, nitrofurantoin, fava beans, naphthalene, infection
Smear: Heinz bodies (denatured Hb), bite cells
Mediterranean variant (severe), A-variant (African, milder)
Protective against P. falciparum

2. Chronic Granulomatous Disease (CGD)

NADPH oxidase deficiency (different from G6PD)
Phagocytes cannot generate superoxide → cannot kill catalase-positive organisms
Organisms: Staph aureus, Klebsiella, Aspergillus, Serratia, Nocardia ("SKAINGS" or "Cats")
Positive NBT test is absent in CGD; dihydrorhodamine flow cytometry used now
Treatment: IFN-γ, TMP-SMX prophylaxis, itraconazole

3. Thiamine (B1) Deficiency & Transketolase

Transketolase requires TPP → B1 deficiency blocks non-oxidative phase
Diagnostic: erythrocyte transketolase activity (with/without TPP = TPP effect)
Beriberi: wet (high-output cardiac failure), dry (peripheral neuropathy)

4. Favism

G6PD deficiency + fava bean consumption → acute hemolytic crisis

7. PYRUVATE METABOLISM - SUMMARY

Glucose → Pyruvate (glycolysis, cytoplasm)
              ↓ (PDH complex - mitochondria)
         Acetyl CoA → TCA cycle
              ↓ (lactate dehydrogenase - LDH - anaerobic)
           Lactate (regenerates NAD⁺)
              ↓ (alanine transaminase - transamination)
           Alanine (glucose-alanine cycle)
              ↓ (pyruvate carboxylase - biotin)
              OAA (gluconeogenesis)

8. FRUCTOSE & GALACTOSE METABOLISM

Fructose Metabolism

Liver pathway (major):

Fructose → Fructose-1P (fructokinase) → DHAP + G3P (aldolase B) → enters glycolysis

Muscle/Kidney:

Hexokinase phosphorylates fructose → F6P

Fructose Disorders Table

Disorder	Deficient Enzyme	Features
Essential fructosuria	Fructokinase	Benign, asymptomatic, fructose in urine
Hereditary fructose intolerance (HFI)	Aldolase B	Severe: vomiting, hypoglycemia, liver failure after fructose/sucrose exposure. F-1P accumulates → inhibits glycogen phosphorylase + aldolase A → hypoglycemia

HFI Clinical: Infant healthy until weaning (fructose introduced); aversion to sweets develops. Can mimic galactosemia. Rx: eliminate fructose, sucrose, sorbitol.

Galactose Metabolism

Pathway: Galactose → Galactose-1P (galactokinase) → G1P (GALT enzyme) → enters glycogen synthesis or glycolysis

Galactose Disorders Table

Disorder	Deficient Enzyme	Features
Galactokinase deficiency	Galactokinase	Mild; galactitol accumulates → early cataracts (infantile cataracts). Galactose + galactitol in urine
Classic galactosemia	Galactose-1-P uridylyltransferase (GALT)	Severe: vomiting, jaundice, E.coli sepsis in neonates, cirrhosis, intellectual disability, cataracts, ovarian failure. G-1P accumulates (toxic). Urine: reducing substance (Clinitest +, glucose oxidase -). Newborn screening
GALE deficiency	UDP-galactose-4-epimerase	Variable

Clinical tip: Galactosemia → E.coli sepsis (neonatal) is a classic NEET/USMLE pearl

PART 2: LIPID METABOLISM

9. FATTY ACID SYNTHESIS (De Novo Lipogenesis)

Location: Cytoplasm (liver, adipose, mammary gland, brain in development)
Starting material: Acetyl CoA (from mitochondria → transported as citrate)

Key Steps

Step	Enzyme	Cofactor	Notes
Acetyl CoA → Malonyl CoA	Acetyl CoA Carboxylase (ACC)	Biotin (B7), ATP	RATE-LIMITING STEP
Malonyl CoA + Acetyl CoA → Palmitate (16C)	Fatty Acid Synthase (FAS)	NADPH (×2 per cycle)	Multi-enzyme complex; 7 cycles to make palmitate

Citrate Shuttle (Acetyl CoA transport)

Acetyl CoA cannot cross IMM directly
In mitochondria: OAA + Acetyl CoA → Citrate (citrate synthase)
Citrate exported to cytoplasm → cleaved by ATP-citrate lyase → OAA + Acetyl CoA
OAA → malate (malate dehydrogenase, NADH) → pyruvate (malic enzyme, NADPH) → back to mitochondria

Important: Malic enzyme generates additional NADPH for FA synthesis!

Regulation of ACC

Activators	Inhibitors
Citrate (allosteric)	Malonyl CoA (product feedback)
Insulin (activates & dephosphorylates ACC)	Palmitoyl CoA (product)
	Glucagon, Epinephrine (via PKA → phosphorylate/inactivate ACC)
	AMPK (phosphorylates ACC)

Malonyl CoA also inhibits carnitine palmitoyl transferase-1 (CPT-1) → prevents FA entering mitochondria for oxidation when synthesis is occurring. This is the key cross-regulation between synthesis and oxidation!

Carbon Counting

Start: 2C (acetyl CoA) + 2C×7 (malonyl CoA loses 1 CO₂ each) = 16C palmitate
Each elongation: +2C, requires 2 NADPH, 1 ATP (as malonyl CoA)

10. BETA-OXIDATION OF FATTY ACIDS

Location: Mitochondrial matrix (long-chain FA need carnitine shuttle); peroxisome (very long chain FA, first pass)

Carnitine Shuttle (for Long-Chain FA)

Fatty acid + CoA → Fatty acyl CoA (acyl CoA synthetase, outer IMM) - costs 2 ATP equivalents
Fatty acyl CoA + carnitine → Acylcarnitine (CPT-1, outer IMM) - RATE-LIMITING
Acylcarnitine enters mitochondria via CACT (translocase)
Acylcarnitine + CoA → Fatty acyl CoA + carnitine (CPT-2, inner IMM)

Malonyl CoA inhibits CPT-1 (fed state → no FA oxidation)

Beta-Oxidation Steps (per cycle)

Step	Enzyme	Cofactor Generated
Acyl CoA → trans-Enoyl CoA	Acyl CoA dehydrogenase	FADH₂
Enoyl CoA → 3-Hydroxyacyl CoA	Enoyl CoA hydratase	—
3-OH-Acyl CoA → 3-Ketoacyl CoA	3-Hydroxyacyl CoA DH	NADH
3-Ketoacyl CoA → Acetyl CoA + shortened FA	Thiolase	CoA

Per cycle: 1 FADH₂ (1.5 ATP) + 1 NADH (2.5 ATP) + 1 Acetyl CoA (10 ATP from TCA) = 14 ATP per round

For palmitate (16C = 8 Acetyl CoA, 7 cycles):

7 × FADH₂ + 7 × NADH + 8 Acetyl CoA × 10 = 10.5 + 17.5 + 80 = 108 ATP - 2 (activation) = 106 net ATP

Odd-Chain FA Oxidation

Last cycle produces propionyl CoA (3C) instead of acetyl CoA
Propionyl CoA → Methylmalonyl CoA (propionyl CoA carboxylase, Biotin) → Succinyl CoA (Methylmalonyl CoA mutase, B12) → TCA
B12 deficiency → methylmalonic acidemia (elevated methylmalonate in urine)

Unsaturated FA Oxidation

Requires enoyl CoA isomerase and 2,4-dienoyl CoA reductase (uses NADPH)

Clinical Correlations - Beta-Oxidation

1. MCAD Deficiency (Medium-Chain Acyl CoA Dehydrogenase)

Most common FA oxidation defect in Western countries
AR; presents age 3-24 months
Triggered by fasting or illness → hypoketotic hypoglycemia
Cannot oxidize medium-chain FA → no ketones, no ATP from FA
Dicarboxylic acids (C6-C10) in urine (adipate, suberate, sebacate)
Classic USMLE: fasting hypoglycemia + hypoketosis + dicarboxylic aciduria
May present as SIDS! Newborn screening (tandem MS)
Treatment: Avoid fasting, glucose during illness

2. Carnitine Deficiency

Primary (genetic, plasma membrane carnitine transporter OCTN2) or secondary (renal losses, pregnancy, dialysis, valproate)
Muscle weakness, cardiomyopathy, hypoketotic hypoglycemia
Low serum/urine carnitine; Treatment: L-carnitine

3. LCHAD Deficiency (Long-Chain 3-Hydroxyacyl CoA DH)

Maternal: AFLP (Acute Fatty Liver of Pregnancy) when carrying LCHAD-deficient fetus
Fetus: hypoglycemia, cardiomyopathy, peripheral neuropathy, retinopathy
Classic link between maternal-fetal metabolic disease

4. Propionic Acidemia

Propionyl CoA carboxylase deficiency (Biotin-dependent)
Accumulation of propionyl CoA → propionic acid
Vomiting, ketoacidosis, hyperammonemia, neutropenia, hyperglycemia
Odd-chain FA, methionine, isoleucine, valine, threonine as sources of propionyl CoA

5. Methylmalonic Acidemia (MMA)

Methylmalonyl CoA mutase deficiency OR B12 deficiency/transport defect
Elevated methylmalonyl CoA → methylmalonic acid in blood and urine
Metabolic acidosis, hyperammonemia, developmental delay
B12-responsive and non-responsive forms
Elevated methylmalonate is diagnostic of B12 deficiency (more sensitive than B12 level)

6. Zellweger Syndrome (Peroxisomal Biogenesis Disorder)

Absent functional peroxisomes → VLCFA cannot be oxidized (first step in peroxisome)
Elevated VLCFA in plasma (diagnostic)
Neonatal hypotonia, seizures, liver disease, stippled epiphyses (chondrodysplasia)
Also: defective bile acid synthesis, plasmalogen synthesis

11. KETONE BODY METABOLISM

Location of synthesis: Liver mitochondria (only)
Location of use: Brain (during starvation), heart, skeletal muscle, kidney cortex

Ketogenesis (Liver)

Step	Enzyme	Notes
2 Acetyl CoA → Acetoacetyl CoA	Thiolase (reversal of thiolysis)
Acetoacetyl CoA + Acetyl CoA → HMG-CoA	HMG-CoA synthase (mito)	Rate-limiting step
HMG-CoA → Acetoacetate + Acetyl CoA	HMG-CoA lyase
Acetoacetate → β-Hydroxybutyrate	β-Hydroxybutyrate DH	NADH required
Acetoacetate → Acetone + CO₂	Spontaneous decarboxylation	Fruity breath in DKA

Important: β-Hydroxybutyrate is the predominant ketone in blood in DKA (ratio BHB:Acetoacetate ~3:1)

Ketone Utilization (Peripheral tissues)

β-Hydroxybutyrate → Acetoacetate (β-OH-butyrate DH)
Acetoacetate + Succinyl CoA → Acetoacetyl CoA + Succinate (succinyl CoA transferase / thiophorase)
Liver lacks succinyl CoA transferase → CANNOT utilize ketones (only makes them)
Acetoacetyl CoA → 2 Acetyl CoA → TCA

Regulation

Ketogenesis promoted by: Glucagon, fasting, low insulin, high FA delivery to liver
Malonyl CoA (insulin state) inhibits CPT-1 → prevents FA entry → blocks ketogenesis

Clinical Correlations - Ketone Bodies

1. Diabetic Ketoacidosis (DKA)

T1DM: absent insulin → unrestrained lipolysis → FA flood liver → ketogenesis
pH <7.3, HCO₃ <18, ketones +
BHB is major ketone (Nitroprusside test detects only acetoacetate - may underestimate severity!)
Anion gap metabolic acidosis
Kussmaul breathing (deep, sighing - respiratory compensation)
Fruity breath (acetone)
Treatment: Fluids, insulin, K⁺ monitoring (insulin drives K⁺ into cells)

2. Starvation Ketosis

Milder; pH usually not <7.3; ketones provide brain fuel after 3-4 days of fasting
Brain adapts from 100% glucose dependency to ~60% ketone use in prolonged starvation

3. Alcoholic Ketoacidosis

Alcohol → acetaldehyde → acetate → acetyl CoA (NADH also generated)
High NADH/NAD⁺ → inhibits gluconeogenesis (OAA → malate) → hypoglycemia
High Acetyl CoA + low OAA → ketogenesis
BHB predominates; Nitroprusside test may be negative!

4. HMG-CoA Lyase Deficiency

Cannot make acetoacetate from HMG-CoA
Presents: hypoketotic hypoglycemia + metabolic acidosis
Also: leucine metabolism blocked (leucine → HMG-CoA)

12. CHOLESTEROL SYNTHESIS (Mevalonate Pathway)

Location: Liver (70%), intestine, all nucleated cells
Starting material: Acetyl CoA

Steps

Step	Enzyme	Notes
2 Acetyl CoA → Acetoacetyl CoA	Thiolase
+ Acetyl CoA → HMG-CoA	HMG-CoA synthase (cytosolic)	Different from mitochondrial!
HMG-CoA → Mevalonate	HMG-CoA reductase	RATE-LIMITING STEP; target of statins
Mevalonate → IPP (isopentenyl pyrophosphate)	Multiple steps, ATP
2 IPP → Geranyl-PP
+ IPP → Farnesyl-PP		Farnesylation of Ras, lamins
2 Farnesyl-PP → Squalene	Squalene synthase
Squalene → Lanosterol → Cholesterol	Multiple enzymes

Regulation of HMG-CoA Reductase

Activated (dephosphorylated): Insulin, thyroid hormone, high dietary cholesterol (initially)
Inhibited (phosphorylated by AMPK): Glucagon, fasting, statins
Transcriptional: Low intracellular cholesterol → SREBP activates LDL receptor + HMG-CoA reductase transcription
Post-translational: High cholesterol → INSIG-mediated ubiquitination and degradation of HMG-CoA reductase

Cholesterol Products

Derivative	Enzyme/Pathway
Bile acids	7α-hydroxylase (rate-limiting; requires Vit C)
Steroid hormones	CYP11A1 (cholesterol side-chain cleavage), requires Vit C
Vitamin D	CYP27B1 (1α-hydroxylase in kidney), CYP2R1 (25-hydroxylase in liver)
Dolichol	N-glycosylation
Ubiquinone (CoQ10)	ETC
Farnesyl/geranylgeranyl	Prenylation of proteins (Ras, Rho)

Clinical Correlations - Cholesterol

1. Statins

HMG-CoA reductase inhibitors: atorvastatin, rosuvastatin, simvastatin, etc.
Side effects: myopathy (myalgias, rhabdomyolysis) - monitor CK; hepatotoxicity
Mechanism: ↓ cholesterol → ↑ LDL receptors → ↓ LDL; also anti-inflammatory (pleiotropic)
Also ↓ CoQ10 (ubiquinone) → may contribute to myopathy

2. Familial Hypercholesterolemia (FH)

LDL receptor mutation (most common), apoB-100 mutation, PCSK9 gain-of-function
Heterozygous FH: LDL 2-3× normal; premature CAD, tendinous xanthomas, corneal arcus, xanthelasma
Homozygous FH: LDL 4-6× normal; CAD in childhood
PCSK9 inhibitors (evolocumab, alirocumab) = treatment for FH; PCSK9 degrades LDL receptors

3. Smith-Lemli-Opitz Syndrome

7-Dehydrocholesterol reductase deficiency (last step of cholesterol synthesis)
Cannot convert 7-DHC → cholesterol
Features: intellectual disability, microcephaly, 2-3 toe syndactyly, hypospadias, adrenal insufficiency, autism
Low cholesterol + elevated 7-DHC (diagnostic)

4. Mevalonic Aciduria

Mevalonate kinase deficiency
Mevalonic acid accumulates; recurrent fevers, developmental delay
Also: hyper-IgD syndrome (HIDS) = partial mevalonate kinase deficiency

5. Cerebrotendinous Xanthomatosis (CTX)

27-Hydroxylase (CYP27A1) deficiency → cannot synthesize normal bile acids → cholestanol accumulates
Tendon xanthomas, progressive cerebellar ataxia, intellectual decline, early cataracts, CAD
Low cholesterol despite xanthomas - paradoxical! Treatment: chenodeoxycholic acid

13. LIPOPROTEIN METABOLISM

Lipoprotein	Origin	Function	Key Apo	Metabolic Fate
Chylomicron	Intestine	TG transport (dietary)	ApoB-48, ApoC-II, ApoE	Lipoprotein lipase (LPL) in capillaries → chylomicron remnant → liver (ApoE receptor)
VLDL	Liver	TG transport (endogenous)	ApoB-100, ApoC-II, ApoE	LPL → IDL → LDL
IDL	From VLDL	Intermediate	ApoB-100, ApoE	Liver (LDL receptor, ApoE) or → LDL
LDL	From IDL	Cholesterol delivery to tissues	ApoB-100	LDL receptor (liver, extrahepatic)
HDL	Liver/intestine	Reverse cholesterol transport	ApoA-I	LCAT esterifies cholesterol; CETP transfers CE to VLDL/LDL

Key Enzymes

Enzyme	Function	Activator	Clinical
LPL (Lipoprotein lipase)	Hydrolyzes TG in chylomicrons/VLDL	ApoC-II	Deficiency → Type I hyperlipoproteinemia; milky plasma, pancreatitis
LCAT	Esterifies cholesterol in HDL	ApoA-I	Deficiency → corneal clouding, hemolytic anemia, renal failure
CETP	Transfers CE from HDL to VLDL/LDL	—	CETP inhibitors raise HDL
HL (Hepatic lipase)	Hydrolyzes IDL → LDL	—
7α-hydroxylase	Bile acid synthesis from cholesterol	—	Rate-limiting; decreased in statin use

Hyperlipoproteinemias - Fredrickson Classification

Type	Elevated	Defect	Key Feature	Treatment
I	Chylomicrons	LPL or ApoC-II deficiency	Milky plasma, eruptive xanthomas, acute pancreatitis, abdominal pain	Low-fat diet
IIa	LDL	LDL receptor (FH)	Tendinous xanthomas, corneal arcus, premature CAD	Statins
IIb	LDL + VLDL	↑ VLDL production + ↓ LDL clearance	CAD risk	Statins + fibrates
III	IDL (chylomicron remnants)	ApoE2/E2 homozygosity	Palmar xanthomas (pathognomonic), tubero-eruptive xanthomas, CAD + PVD	Fibrates, statins
IV	VLDL	↑ VLDL synthesis (familial)	Pancreatitis, no xanthomas usually	Fibrates, niacin
V	Chylomicrons + VLDL	LPL deficiency + ↑ VLDL	Pancreatitis, eruptive xanthomas	Low-fat diet + fibrates

14. SPHINGOLIPID & MEMBRANE LIPID METABOLISM

Sphingolipid Synthesis

Serine + palmitoyl CoA → sphinganine → ceramide → various sphingolipids
Ceramide is the backbone of all sphingolipids

Lysosomal Storage Diseases - Sphingolipidoses

Disease	Deficient Enzyme	Accumulating Substance	Key Clinical Features	Inheritance
Gaucher (most common)	Glucocerebrosidase	Glucocerebroside	Hepatosplenomegaly, bone marrow failure, "Erlenmeyer flask" deformity of femur, bone pain. Type 1: no CNS. Gaucher cells ("crumpled tissue paper")	AR
Niemann-Pick A/B	Sphingomyelinase	Sphingomyelin	Hepatosplenomegaly, cherry-red spot (Type A), progressive neurodegeneration (A), "foam cells"	AR
Niemann-Pick C	NPC1 (cholesterol trafficking)	Cholesterol	Vertical gaze palsy, dementia, ataxia; gelastic cataplexy	AR
Tay-Sachs	Hexosaminidase A (β)	GM2 ganglioside	Cherry-red macula, progressive neurodegeneration, NO hepatosplenomegaly, hyperacusis (startle response), Ashkenazi Jewish	AR
Sandhoff	Hexosaminidase A+B	GM2 ganglioside	Like Tay-Sachs + hepatosplenomegaly	AR
Fabry	α-Galactosidase A	Globotriaosylceramide (Gb3)	Angiokeratomas, peripheral neuropathy (burning pain), renal failure, cardiomyopathy, corneal whorling	X-linked
Krabbe	Galactocerebrosidase	Galactocerebroside, psychosine	Peripheral neuropathy, leukodystrophy, globoid cells	AR
Metachromatic Leukodystrophy	Arylsulfatase A	Sulfatide	Demyelination, progressive motor/cognitive decline	AR
Farber	Acid ceramidase	Ceramide	Joint deformity, hoarseness, subcutaneous nodules ("Farber nodules"), early death	AR

Memory trick for Tay-Sachs vs Sandhoff: Tay-Sachs = only Hex-A; Sandhoff = Hex-A+B (Both gone); Sandhoff = hepatosplenomegaly too

PART 3: AMINO ACID / PROTEIN METABOLISM

15. UREA CYCLE

Location: Liver (primarily); mitochondria + cytoplasm (both compartments)
Purpose: Detoxify ammonia → urea (excreted by kidney)

Steps of Urea Cycle

Step	Location	Enzyme	Substrate → Product	Cofactor
1	Mitochondria	CPS-I (Carbamoyl phosphate synthetase I)	NH₃ + CO₂ → Carbamoyl phosphate	ATP (×2), N-Acetylglutamate (activator)
2	Mitochondria	OTC (Ornithine transcarbamylase)	Carbamoyl-P + Ornithine → Citrulline	—
3	Cytoplasm	ASS (Argininosuccinate synthetase)	Citrulline + Aspartate → Argininosuccinate	ATP
4	Cytoplasm	ASL (Argininosuccinate lyase)	Argininosuccinate → Arginine + Fumarate	—
5	Cytoplasm	Arginase	Arginine + H₂O → Ornithine + Urea	Mn²⁺

N-Acetylglutamate (NAG) is the essential allosteric activator of CPS-I; made by NAG synthase from acetyl CoA + glutamate; activated by arginine.

Nitrogen sources: 1 NH₃ (from GDH/transaminases in mitochondria) + 1 from Aspartate (Step 3) = 2 N in urea

Integration with Other Cycles

Fumarate (from Step 4) → TCA cycle → OAA → aspartate (connects urea cycle to TCA)
This connection is called the "urea-TCA bicycle"
Arginine is also needed for: Nitric oxide synthesis (arginine + O₂ → citrulline + NO, by NOS)
Citrulline → arginine in kidney (urea cycle intermediates circulate)

Regulation

CPS-I regulated by: N-Acetylglutamate (obligate activator); high protein diet → ↑ NAG
Arginine feeds back to activate NAG synthase (autoregulatory loop)

Urea Cycle Disorders - Complete Table

Disorder	Deficient Enzyme	Accumulating metabolite	Key Features	Biomarker
CPS-I deficiency	Carbamoyl phosphate synthetase I	NH₃ ↑, BUN ↓, no orotic acid	Neonatal hyperammonemia, encephalopathy	↑ NH₃, ↓ citrulline, ↓ orotic acid
OTC deficiency	Ornithine transcarbamylase	Orotic acid ↑ (excess carbamoyl-P → pyrimidine synthesis), NH₃ ↑	Most common urea cycle defect; X-linked; males severe, females variable; often presents after high-protein meal or illness	↑ NH₃, ↑ orotic acid, ↓ citrulline
Citrullinemia type I	Argininosuccinate synthetase (ASS)	Citrulline ↑↑	Neonatal hyperammonemia; severe. ↑ citrulline	↑ NH₃, ↑↑ citrulline, ↓ arginine
Argininosuccinic aciduria	Argininosuccinate lyase (ASL)	Argininosuccinate ↑	Hyperammonemia + trichorrhexis nodosa (brittle hair)	↑ argininosuccinate, ↑ citrulline
Arginase deficiency	Arginase	Arginine ↑	Spastic diplegia, intellectual disability (NOT neonatal crisis); hyperammonemia milder	↑ arginine
N-AG synthase deficiency	NAG synthase	NH₃ ↑	Like CPS-I. Responds to N-carbamylglutamate (NCG) treatment - diagnostic/therapeutic!
HHH syndrome	Mitochondrial ornithine transporter (ORNT1)	Homocitrulline, hyperornithinemia, hyperammonemia	Triple H

Key USMLE pearl: OTC deficiency - only X-linked urea cycle defect; orotic aciduria WITHOUT megaloblastic anemia (vs. orotic aciduria from UMP synthase deficiency which has megaloblastic anemia)

Treatment of hyperammonemia: Sodium benzoate (scavenges glycine), sodium phenylacetate/phenylbutyrate (scavenges glutamine), arginine supplementation (in OTC/CPS-I def), dietary protein restriction, dialysis acute

16. TRANSAMINATION & AMINO ACID CATABOLISM

Transaminases (Aminotransferases)

All require Pyridoxal Phosphate (PLP, Vit B6)

Enzyme	Reaction	Location	Clinical
ALT (Alanine aminotransferase, SGPT)	Alanine + α-KG → Pyruvate + Glutamate	Liver (cytoplasm)	Most specific for liver injury
AST (Aspartate aminotransferase, SGOT)	Aspartate + α-KG → OAA + Glutamate	Liver + Heart + Muscle	Elevated in MI, liver, muscle disease
GDH (Glutamate dehydrogenase)	Glutamate → α-KG + NH₃	Mitochondria	Activated by ADP, inhibited by GTP

AST:ALT ratio:

2:1 with AST <300: Alcoholic hepatitis (B6/PLP depletion)
10:1: Alcoholic liver disease or ischemic hepatitis
AST = ALT: Viral hepatitis, NAFLD

Amino Acid Fates

Category	Amino Acids	Metabolic Fate
Purely glucogenic	Ala, Gly, Ser, Thr, Val, Met, Cys, Asp, Asn, Glu, Gln, Pro, His, Arg	→ Pyruvate, OAA, α-KG, succinyl CoA, fumarate → gluconeogenesis
Purely ketogenic	Leucine, Lysine ("Little Kids")	→ Acetyl CoA, acetoacetate only
Both (glucogenic + ketogenic)	Ile, Phe, Tyr, Trp	→ Both pathways

Memory: Leucine and Lysine are the ONLY purely ketogenic amino acids. "Lucky Leucine and Lysine are Ketogenic"

Essential Amino Acids

PVT TIM HALL: Phe, Val, Thr, Trp, Ile, Met, His, Arg (conditionally), Leu, Lys

17. SPECIFIC AMINO ACID METABOLISM & DISORDERS

Phenylalanine & Tyrosine Pathway

Phenylalanine → Tyrosine (Phenylalanine hydroxylase + BH4)
                   ↓
         Homogentisic acid → Fumarylacetoacetate → Fumarate + Acetoacetate
                   ↓
             DOPA → Dopamine → NE → Epinephrine (catecholamines)
                   ↓ (tyrosinase)
              Melanin
                   ↓ (thyroid peroxidase)
              Thyroid hormones (T3, T4)

Amino Acid Disorders Table

Disease	Deficient Enzyme	Accumulation	Findings	Inheritance
PKU (Classic)	Phenylalanine hydroxylase	Phenylalanine, phenylketones	Fair skin/hair, musty odor (phenylacetate), intellectual disability, seizures, eczema. Maternal PKU: fetal harm. Rx: low-Phe diet + tyrosine suppl.	AR
Malignant PKU	Dihydropteridine reductase or BH4 synthesis	Phenylalanine + neurotransmitters (DA, serotonin, NE) ↓	Same as PKU + severe neurological; unresponsive to diet alone; needs BH4 + L-DOPA + 5-OH-tryptophan	AR
Alkaptonuria	Homogentisate oxidase	Homogentisic acid	Dark urine on standing (oxidizes), ochronosis (gray-blue pigment in cartilage), arthritis, cardiac valve disease	AR
Tyrosinemia type I	Fumarylacetoacetate hydrolase	FAA, succinylacetone	Liver failure, renal tubular Fanconi syndrome, "cabbage-like" odor, HCC risk. Rx: NTBC (nitisinone) + low-Tyr/Phe diet	AR
Tyrosinemia type II	Tyrosine aminotransferase	Tyrosine	Palmoplantar keratoderma, corneal crystals, intellectual disability	AR
Albinism	Tyrosinase	(tyrosine can't → melanin)	Lack of melanin, photophobia, nystagmus, ↑ skin cancer risk; normal Phe/Tyr levels	AR
Homocystinuria	Cystathionine β-synthase (CBS)	Homocysteine	Marfanoid habitus, downward lens dislocation (ectopia lentis), intellectual disability, thromboembolism, atherosclerosis. Responds to B6 (CBS requires PLP). Rx: B6, folate, B12, methionine-restricted diet	AR
Cystinuria	SLC3A1/SLC7A9 (cystine, ornithine, arginine, lysine transporter)	Cystine in urine	Recurrent kidney stones (hexagonal crystals), radiopaque stones. Rx: hydration, alkalinize urine, penicillamine	AR
Cystinosis	CTNS (lysosomal cystine transporter)	Cystine in lysosomes	Fanconi syndrome (proximal tubule), photophobia (corneal crystals), hypothyroidism, myopathy	AR
Maple Syrup Urine Disease (MSUD)	Branched-chain α-KA dehydrogenase	Leucine, Isoleucine, Valine + α-keto acids	Sweet/maple syrup urine, opisthotonos, feeding difficulty; leucine is most toxic → cerebral edema. Rx: restrict BCAA, thiamine (B1)	AR
Isovaleric acidemia	Isovaleryl CoA DH	Isovaleric acid	"Sweaty feet" odor, metabolic acidosis, neutropenia, thrombocytopenia	AR
Propionic acidemia	Propionyl CoA carboxylase (Biotin)	Propionic acid	Metabolic acidosis, hyperammonemia, neutropenia	AR
Methylmalonic acidemia	Methylmalonyl CoA mutase (B12)	Methylmalonic acid	Same as propionic + B12-responsive variant	AR
Hartnup disease	Neutral amino acid transporter (gut/kidney)	Tryptophan malabsorption	Pellagra-like rash (tryptophan → niacin), ataxia, psychiatric symptoms. Rx: high-protein diet, niacin	AR

18. ONE-CARBON METABOLISM (Folate & B12)

Folate Cycle

Dietary folate → DHF → THF (Dihydrofolate reductase, DHFR; inhibited by MTX, trimethoprim)
                         ↓
          5,10-Methylene-THF ←→ 5-Methyl-THF ←→ Homocysteine → Methionine (B12-dependent)
                         ↓ (thymidylate synthase)
                         dTMP (thymine for DNA)

B12 (Cobalamin) Reactions

Methionine synthase: 5-Methyl-THF + Homocysteine → Methionine + THF (requires B12)
Methylmalonyl CoA mutase: Methylmalonyl CoA → Succinyl CoA (requires B12)

Folate Trap (Methyl Trap)

B12 deficiency → 5-Methyl-THF cannot be converted to THF → THF trapped as methylTHF
Cannot make 5,10-methylene-THF → ↓ dTMP → impaired DNA synthesis
Both B12 and folate deficiency → megaloblastic anemia (↓ dTMP)
Only B12 deficiency → subacute combined degeneration (dorsal columns + corticospinal tracts), ↑ methylmalonate

Clinical Correlations

Drug	Mechanism	Disease Treated	Side Effects
Methotrexate (MTX)	DHFR inhibitor → ↓ THF → ↓ dTMP, purine synthesis	RA, cancer, ectopic pregnancy, psoriasis	Megaloblastic anemia; reversed by leucovorin (folinic acid, not folic acid)
Trimethoprim	DHFR inhibitor (bacterial > human)	UTI (with sulfamethoxazole)	Megaloblastic anemia at high doses
Pyrimethamine	DHFR inhibitor (parasite)	Toxoplasmosis, malaria	+ leucovorin given concurrently
5-Fluorouracil (5-FU)	Thymidylate synthase inhibitor (5-FdUMP)	Colorectal cancer, breast cancer	Leucovorin ENHANCES 5-FU toxicity (stabilizes 5-FdUMP-TS complex)
Hydroxyurea	Ribonucleotide reductase inhibitor	Sickle cell, CML, polycythemia vera	↑ HbF in sickle cell

19. ELECTRON TRANSPORT CHAIN & OXIDATIVE PHOSPHORYLATION

Location: Inner mitochondrial membrane

Complexes

Complex	Name	Cofactors	Protons Pumped	Inhibitor
I	NADH dehydrogenase	FMN, Fe-S, CoQ	4 H⁺	Rotenone, amytal, MPTP
II	Succinate dehydrogenase	FAD, Fe-S, CoQ	0 H⁺	Malonate
III	Cytochrome bc1	Cytochrome b, c1, Fe-S, CoQ	4 H⁺	Antimycin A
IV	Cytochrome c oxidase	Cytochrome a, a3, Cu	2 H⁺	CN⁻, CO, H₂S, azide
V	ATP synthase (F0F1)	—	— (uses gradient)	Oligomycin

Uncouplers: Dissipate proton gradient as heat (no ATP)

DNP (2,4-dinitrophenol): used in "diet pills"; toxicity → fever, tachycardia, diaphoresis, fatal hyperthermia
Thermogenin (UCP-1): in brown adipose tissue; non-shivering thermogenesis (BAT in neonates, hibernating animals)
FCCP: research uncoupler

Clinical Correlations - ETC

1. Cyanide Poisoning

Binds Fe³⁺ in Cytochrome a₃ of Complex IV → blocks ETC
Cells cannot use O₂ despite adequate delivery → cytotoxic hypoxia
Venous pO₂ high (bright red venous blood)
Treatment: Hydroxocobalamin (binds CN⁻), amyl nitrite + sodium nitrite (metHb forms, CN preferentially binds metHb), sodium thiosulfate (makes thiocyanate, excreted)

2. CO Poisoning

Binds hemoglobin (COHb) + binds cytochrome a₃ (Complex IV)
Cherry-red skin (COHb), headache, confusion
Treatment: 100% O₂ (hyperbaric if severe)

3. Mitochondrial Diseases (mtDNA)

mtDNA: 37 genes (13 ETC proteins, 22 tRNA, 2 rRNA)
Maternal inheritance, threshold effect
MELAS: Mitochondrial Encephalomyopathy, Lactic Acidosis, Stroke-like episodes (mt tRNA-Leu, A3243G)
MERRF: Myoclonic Epilepsy with Ragged Red Fibers (mt tRNA-Lys, A8344G)
LHON: Leber Hereditary Optic Neuropathy (ND4 complex I subunit)
Kearns-Sayre: progressive external ophthalmoplegia, retinitis pigmentosa, heart block
Leigh syndrome: also mtDNA or nuclear (PDH, Complex IV/I deficiency)

4. Reactive Oxygen Species (ROS)

Complex I and III leak electrons → superoxide (O₂•⁻)
Superoxide dismutase (SOD, Mn²⁺ or Cu²⁺/Zn²⁺) → H₂O₂
Catalase/GPx → H₂O
SOD1 mutations → ALS (Amyotrophic lateral sclerosis) - familial

PART 4: INTEGRATING THE CYCLES

20. THE METABOLIC INTEGRATION CHART

DIETARY CARBOHYDRATES → Glucose
                              ↓ Glycolysis
                          Pyruvate
                         ↙        ↘
               Lactate           Acetyl CoA ← Ketone bodies
               (anaerobic)            ↓
                               TCA Cycle ←────── Fatty acids (β-ox)
                              ↙    ↓    ↘           ↑
                    NADH    GTP   FADH₂     Lipolysis (fasting)
                      ↓            ↓
                     ETC ─────────────→ ATP
                      ↑
               Amino acids → α-KG, OAA, Succinyl CoA, Fumarate
                                            ↑
                               Urea cycle ──┘ (fumarate)

Fed State (Insulin dominates)

Glucose uptake ↑ (GLUT4 in muscle/fat)
Glycogen synthesis ↑
Fatty acid synthesis ↑ (acetyl CoA → FAS)
Protein synthesis ↑
Gluconeogenesis ↓
Lipolysis ↓
Ketogenesis ↓

Fasting State (Glucagon dominates)

Glycogenolysis ↑ (liver)
Gluconeogenesis ↑ (lactate, alanine, glycerol)
Lipolysis ↑ → FA → β-oxidation → Ketone bodies
Ketone bodies → fuel for brain (after 3 days)
Protein catabolism ↑ → glucogenic AAs

Starvation (>3 days)

Ketones become primary brain fuel
Protein catabolism reduced (to spare muscle)
RBCs and renal medulla always need glucose

21. COFACTORS MASTER TABLE (High-Yield)

Vitamin/Cofactor	Form	Key Enzymes	Deficiency
B1 (Thiamine)	TPP	Pyruvate DH, α-KG DH, Transketolase, Branched-chain KA DH (MSUD)	Wernicke-Korsakoff, Beriberi, lactic acidosis
B2 (Riboflavin)	FAD, FMN	Complex I (FMN), Complex II (FAD), Acyl CoA DH, PDH (E3), α-KG DH (E3)	Angular stomatitis, glossitis, corneal vascularization
B3 (Niacin)	NAD⁺, NADP⁺	Nearly all oxidoreductases (TCA, glycolysis, β-oxidation, HMP shunt)	Pellagra (3D: Dermatitis, Diarrhea, Dementia); Hartnup disease (secondary deficiency)
B5 (Pantothenic acid)	CoA, ACP	All CoA-requiring reactions (PDH, TCA, FA synthesis/oxidation)	Rare; "burning feet"
B6 (Pyridoxine)	PLP	All aminotransferases (ALT, AST), Glycogen phosphorylase, Serine dehydratase, GABA synthesis, 5-ALA synthase (heme), Cystathionine synthase	Sideroblastic anemia, peripheral neuropathy, homocystinuria, seizures in neonates
B7 (Biotin)	Biotin-lysine	Pyruvate carboxylase, Acetyl CoA carboxylase, Propionyl CoA carboxylase, Methylcrotonyl CoA carboxylase	Alopecia, dermatitis, lactic acidosis; raw egg white (avidin)
B9 (Folate)	5-Methyl-THF, THF	Methionine synthase (with B12), Thymidylate synthase, Purine synthesis	Megaloblastic anemia (no neurological), neural tube defects
B12 (Cobalamin)	Methylcobalamin, Adenosylcobalamin	Methionine synthase, Methylmalonyl CoA mutase	Megaloblastic anemia + subacute combined degeneration; ↑ MMA (marker)
Vitamin C	Ascorbate	Prolyl/lysyl hydroxylase (collagen), Dopamine β-hydroxylase (NE synthesis), 7α-hydroxylase (bile acids), iron absorption	Scurvy: perifollicular hemorrhage, corkscrew hairs, bleeding gums
Lipoic acid	—	PDH (E2), α-KG DH (E2)	Rare
Biotin	—	All carboxylases	See B7 above
Coenzyme Q (CoQ10)	Ubiquinone	Complex I, II, III (mobile electron carrier)	Myopathy; depleted by statins
NAG	N-Acetylglutamate	CPS-I activator	Hyperammonemia; NAGS deficiency
Mg²⁺	—	Kinases (hexokinase, PFK, PK, ATP synthase)	Hypomagnesemia associated with all kinase dysfunction
Mn²⁺	—	Isocitrate DH, Arginase, Mitochondrial SOD	Manganism (Parkinson-like)

22. RATE-LIMITING STEPS SUMMARY TABLE

Pathway	Rate-Limiting Enzyme	Key Regulators
Glycolysis	PFK-1	Activated: F-2,6-BP, AMP; Inhibited: ATP, citrate
Gluconeogenesis	Fructose-1,6-bisphosphatase	Inhibited by AMP, F-2,6-BP; Activated by glucagon, citrate
TCA cycle	Isocitrate dehydrogenase	Activated: ADP, Ca²⁺; Inhibited: ATP, NADH
Fatty acid synthesis	Acetyl CoA carboxylase	Activated: citrate, insulin; Inhibited: palmitoyl CoA, glucagon
Fatty acid oxidation (entry)	CPT-1	Inhibited by malonyl CoA (fed state)
Ketogenesis	HMG-CoA synthase (mito)	Induced in fasting, diabetes
Cholesterol synthesis	HMG-CoA reductase	Inhibited: statins, AMPK, high cholesterol; Activated: insulin
Urea cycle	CPS-I (+ N-AG synthase being rate-regulating)	Activated by N-Acetylglutamate (obligate)
Glycogen synthesis	Glycogen synthase	Activated by insulin/G6P; Inhibited by glucagon/Epi
Glycogenolysis	Glycogen phosphorylase	Activated by cAMP/Ca²⁺/AMP; Inhibited by G6P/glucose
HMP shunt	G6PD	Activated by NADP⁺; Inhibited by NADPH
Pyrimidine synthesis	Carbamoyl phosphate synthetase II (cytoplasm)	Activated by ATP; Inhibited by UMP
Purine synthesis	PRPP amidotransferase	Inhibited by AMP, GMP
Bile acid synthesis	7α-hydroxylase	Inhibited by bile acids (feedback)

23. FINAL METABOLIC DISORDERS MASTER TABLE

Carbohydrate Metabolism Disorders

Disorder	Defect	Accumulation	Key Clinical	Test
Von Gierke (GSD I)	G6Pase	Glycogen in liver	Severe hypoglycemia, lactic acidosis, gout, hepatomegaly, doll-like facies	↑ lactate, ↑ uric acid, ↑ TG; hypoglycemia
Pompe (GSD II)	Acid maltase (lysosomal)	Lysosomal glycogen	Cardiomegaly, floppy infant, death <2yr	Short PR, giant QRS on ECG; CK ↑
Cori/Forbes (GSD III)	Debrancher enzyme	Glycogen with short chains	Mild hypoglycemia, hepatomegaly, myopathy	Normal lactate
McArdle (GSD V)	Muscle phosphorylase	Muscle glycogen	Exercise cramps, myoglobinuria, "second wind"	No lactate rise with ischemic forearm exercise
Tarui (GSD VII)	Muscle PFK	Muscle glycogen + F6P	Like McArdle + hemolysis + gout	No lactate rise; ↑ uric acid
Classic galactosemia	GALT (Gal-1-P uridyltransferase)	Gal-1P, galactitol	Vomiting, jaundice, E.coli sepsis, cataracts, intellectual disability	Reducing substance in urine (Clinitest +)
Galactokinase def	Galactokinase	Galactitol	Infantile cataracts (only)	Galactose in urine
HFI	Aldolase B	Fructose-1P	Hypoglycemia, liver failure after fructose intake	Reducing substance after fructose load
Essential fructosuria	Fructokinase	Fructose	Asymptomatic, fructose in urine	—
G6PD deficiency	G6PD	—	Oxidative hemolytic anemia, triggered by drugs/infection	Heinz bodies, Coombs negative
PK deficiency	Pyruvate kinase	2,3-BPG	Chronic hemolytic anemia, splenomegaly	Osmotic fragility; right-shifted O2 dissociation curve
PKU	Phe hydroxylase	Phenylalanine	Intellectual disability, musty odor, fair skin	Guthrie test; ↑ Phe, ↓ Tyr
MSUD	BCKA dehydrogenase	Leu, Ile, Val	Maple syrup urine, opisthotonos	↑ BCAA on amino acid screen
Lactic acidosis	Various (PDH, Complex I, liver disease)	Lactate	Elevated AG metabolic acidosis	↑ Lactate, ↑ L/P ratio

Lipid Metabolism Disorders

Disorder	Defect	Accumulation	Key Clinical	Test
MCAD deficiency	MCAD	Medium-chain acylcarnitine	Hypoketotic hypoglycemia, SIDS	↑ C8-C10 acylcarnitine (tandem MS), dicarboxylic aciduria
LCHAD deficiency	LCHAD	Long-chain 3-OH acylcarnitine	Hypoketotic hypoglycemia, cardiomyopathy, retinopathy; AFLP in mother	↑ C16-OH, C18-OH acylcarnitine
Primary carnitine def	OCTN2 transporter	Carnitine ↓	Cardiomyopathy, myopathy, hypoketotic hypoglycemia	↓ free carnitine
Propionic acidemia	Propionyl CoA carboxylase	Propionic acid, methylcitrate	Metabolic acidosis, hyperammonemia, neutropenia	↑ propionylcarnitine (C3)
Methylmalonic acidemia	Methylmalonyl CoA mutase or B12	Methylmalonic acid	Acidosis, hyperammonemia	↑ MMA in urine/blood
Gaucher type I	Glucocerebrosidase	Glucocerebroside	Hepatosplenomegaly, Erlenmeyer flask, bone pain; NO cherry-red spot (type 1)	Bone marrow: Gaucher cells
Gaucher type II/III	Glucocerebrosidase	Glucocerebroside	Type II: acute neuronopathic (infant); Type III: subacute (Norrbottnian)
Niemann-Pick A	Sphingomyelinase	Sphingomyelin	Cherry-red spot, hepatosplenomegaly, neurodegeneration	Foam cells in marrow
Tay-Sachs	Hex A	GM2 ganglioside	Cherry-red spot, NO hepatosplenomegaly, hyperacusis	Enzyme assay (Hex A activity)
Fabry	α-Gal A	Gb3	Angiokeratomas, neuropathic pain, renal failure, cardiomyopathy	X-linked; elevated Gb3 in urine
Krabbe	Galactocerebrosidase	Galactocerebroside, psychosine	Leukodystrophy, globoid cells, peripheral neuropathy	MRI: periventricular white matter
MLD	Arylsulfatase A	Sulfatide	Demyelination, progressive regression	↑ sulfatide in urine; ↓ arylsulfatase A
Familial hypercholesterolemia	LDL receptor	LDL	Premature CAD, tendon xanthomas	↑↑ LDL
LPL deficiency	LPL or ApoC-II	Chylomicrons, VLDL (Type I/V)	Eruptive xanthomas, pancreatitis, lipemia retinalis	Milky plasma after fat meal
Abetalipoproteinemia	MTP (microsomal TG transfer protein)	Fat malabsorption	Steatorrhea, ataxia (E deficiency), retinitis pigmentosa, acanthocytosis	No apoB-containing lipoproteins
Smith-Lemli-Opitz	7-DHC reductase	7-Dehydrocholesterol	Syndactyly (2-3 toes), intellectual disability, hypospadias	↑ 7-DHC, ↓ cholesterol
Zellweger syndrome	PEX genes (peroxisomal biogenesis)	VLCFA	Neonatal hypotonia, seizures, stippled epiphyses, liver disease	↑ VLCFA in plasma
CTX	CYP27A1	Cholestanol	Tendon xanthomas + ataxia + cataracts + low cholesterol	↑ cholestanol
Refsum disease	Phytanoyl CoA hydroxylase	Phytanic acid	Retinitis pigmentosa, peripheral neuropathy, anosmia	↑ phytanic acid; avoid chlorophyll

Protein/Amino Acid Metabolism Disorders

Disorder	Defect	Accumulation	Key Clinical	Test
OTC deficiency	OTC (X-linked)	NH₃ + orotic acid	Neonatal or late-onset hyperammonemia encephalopathy	↑ NH₃, ↑ orotic acid, ↓ citrulline
CPS-I deficiency	CPS-I	NH₃	Neonatal hyperammonemia	↑ NH₃, ↓ orotic acid, ↓ citrulline
Citrullinemia I	ASS	Citrulline	Hyperammonemia	↑↑ citrulline
Argininosuccinic aciduria	ASL	Argininosuccinate	Hyperammonemia + trichorrhexis nodosa	↑ argininosuccinate
Arginase deficiency	Arginase	Arginine	Spastic diplegia (not neonatal crisis)	↑ arginine
PKU	Phe hydroxylase + BH4	Phenylalanine	Intellectual disability, musty odor, fair skin/hair	Newborn screen (Guthrie)
Homocystinuria	CBS (B6-dep)	Homocysteine, methionine	Marfanoid, lens down, DVT, intellectual disability	↑ homocysteine, ↑ methionine
Alkaptonuria	Homogentisate oxidase	Homogentisic acid	Dark urine, ochronosis, arthritis	Urine darkens on standing
Tyrosinemia I	FAH	FAA, succinylacetone	Liver failure, Fanconi, HCC	↑ succinylacetone (pathognomonic)
Maple syrup urine disease	BCKA DH (B1)	BCAA (Leu, Ile, Val)	Maple syrup urine, neurological crisis, opisthotonos	↑ BCAA, ↑ BCKA
Cystinuria	Cystine transporter	Cystine (urine)	Recurrent renal stones (hexagonal)	Sodium cyanide-nitroprusside test
Hartnup disease	Neutral AA transporter	Tryptophan malabsorption	Pellagra-like rash, ataxia	↑ neutral AA in urine, normal serum AA
Glutaric aciduria I	Glutaryl CoA DH	Glutaric acid	Macrocephaly, striatal injury, dystonia after febrile illness	↑ glutarylcarnitine
Non-ketotic hyperglycinemia	Glycine cleavage system	Glycine	Neonatal seizures, encephalopathy	↑↑ CSF glycine, ↑ CSF:plasma glycine ratio

24. REGULATION SUMMARY TABLE (Key Cross-Pathways)

Signal/State	Effect on Metabolism
Insulin	↑ Glycolysis, Glycogen synthesis, Fatty acid synthesis, Protein synthesis; ↓ Gluconeogenesis, Glycogenolysis, Lipolysis, Ketogenesis, Fatty acid oxidation
Glucagon	↑ Glycogenolysis, Gluconeogenesis, Fatty acid oxidation, Ketogenesis; ↓ Glycolysis, Glycogen synthesis
Epinephrine	Muscle: ↑ glycogenolysis (cAMP + Ca²⁺), ↑ glycolysis; Liver: ↑ glycogenolysis; Fat: ↑ lipolysis (HSL)
Cortisol	↑ Gluconeogenesis (induces PEPCK), ↑ Protein catabolism, ↑ Lipolysis; insulin resistance
AMPK (low energy, ↑AMP)	↑ Fatty acid oxidation, Glycolysis; ↓ Fatty acid synthesis (ACC), Cholesterol synthesis (HMGCR), Gluconeogenesis
cAMP/PKA	Activates: Phosphorylase kinase, HSL; Inhibits: Glycogen synthase, ACC, PFK-2
Ca²⁺	Activates: Phosphorylase kinase (muscle), PDH phosphatase, Isocitrate DH, α-KG DH; Calmodulin → smooth muscle
High NADH/NAD⁺ (alcohol)	Inhibits: Gluconeogenesis (OAA→malate, pyruvate→lactate), TCA cycle, FA oxidation; favors ketogenesis, hypoglycemia
Malonyl CoA	Inhibits CPT-1 → prevents FA entry into mitochondria; switches metabolism from oxidation to synthesis

IMPORTANT MNEMONICS FOR EXAM

Mnemonic	Meaning
"PFK is the Pacemaker"	PFK-1 = rate-limiting step of glycolysis
"Obese Cats Always Inject Some Fat" (Ornithine → Citrulline → Argininosuccinate → Arginine → Ornithine)	Urea cycle order
"Tiny Tim's Lipoic Acid"	TCA cofactors: Thiamine (B1), Lipoic acid, FAD (B2), NAD⁺ (B3), CoA (B5)
"GSD: I won't (I), pump (II), corn (III), starch (III), McArdle (V), her (VI), 7-up (VII)"	GSD types
"Lucky Leucine and Lysine are Ketogenic"	Only purely ketogenic AAs
"Purely Ket = LeucK (Leu + Lys)"	Both ketogenic only
"Oh, such good (OSCA): OTC, Sandhoff, CBS, G6PD, AST/ALT remember AST is aspartate"	Classic high-yield disorders
"SKAINGS"	CGD organisms: Staph, Klebsiella, Aspergillus, Indeterminate bacteria (E.coli), Nocardia, Granulibacter, Serratia

Sources: Basic Medical Biochemistry: A Clinical Approach (6e, Lippincott), Harper's Illustrated Biochemistry (32e), Biochemistry (8e, Lippincott Illustrated Reviews)

This is a fully integrated master reference covering all major metabolic pathways, clinical correlations, storage and metabolic disorders with tabular summaries, cofactors, rate-limiting steps, and regulatory mechanisms - optimized for NEET PG, USMLE Steps 1/2, and INICET preparation.

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