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Now compiling the comprehensive integrated response. This is a large but highly organized answer:

🧬 INTEGRATED METABOLIC CYCLES - Master Reference for NEET-PG / USMLE / INICET

PART 1: CARBOHYDRATE METABOLISM

1.1 GLYCOLYSIS

Location: Cytoplasm | Oxygen requirement: Anaerobic (can proceed without O₂)

Overall equation: Glucose + 2 NAD⁺ + 2 ADP + 2Pi → 2 Pyruvate + 2 NADH + 2H⁺ + 2 ATP

Glycolysis and Gluconeogenesis pathway diagram

Glycolysis (blue, left) vs. Gluconeogenesis (red, right) - note the 3 irreversible bypass points

Steps and Key Enzymes

Step	Substrate → Product	Enzyme	Cofactor	Notes
1	Glucose → G-6-P	Hexokinase (tissues) / Glucokinase (liver)	Mg²⁺, ATP	Hexokinase: low Km, inhibited by G-6-P; Glucokinase: high Km, induced by insulin
3	F-6-P → F-1,6-BP	Phosphofructokinase-1 (PFK-1)	Mg²⁺, ATP	Rate-limiting step of glycolysis
5	F-1,6-BP → DHAP + G-3-P	Aldolase	-	Splits 6C into two 3C
6	G-3-P ⇌ DHAP	Triose phosphate isomerase	-	-
7	G-3-P → 1,3-BPG	Glyceraldehyde-3-phosphate dehydrogenase	NAD⁺	Inhibited by arsenate
8	1,3-BPG → 3-PG	Phosphoglycerate kinase	ADP	Substrate-level phosphorylation
10	PEP → Pyruvate	Pyruvate Kinase	K⁺, Mg²⁺	Irreversible; 3rd rate-limiting step

Three Irreversible (Rate-Limiting) Steps - HIGH YIELD

Hexokinase/Glucokinase - Glucose → G-6-P
PFK-1 - F-6-P → F-1,6-BP (THE key regulatory step)
Pyruvate Kinase - PEP → Pyruvate

Regulation of PFK-1 (Master Regulator)

Activators	Inhibitors
AMP, ADP (low energy signal)	ATP (high energy)
Fructose 2,6-bisphosphate (insulin ↑)	Citrate (TCA running well)
Pi	Glucagon → ↓ F-2,6-BP → inhibits PFK-1

MNEMONIC - PFK-1 activators: "FAME" - F-2,6-BP, AMP, Mg²⁺, Energy low states

Net ATP Yield

Aerobic glycolysis: 2 ATP net (+ 2 NADH fed to ETC → 5 ATP more)
Anaerobic: Only 2 ATP net

Clinical Correlations

🔴 Pyruvate Kinase Deficiency (PKD)

Most common hereditary non-spherocytic hemolytic anemia (after G6PD deficiency)
RBCs cannot undergo aerobic metabolism → rely entirely on glycolysis
PK deficiency → ↓ ATP in RBCs → rigid membrane → hemolysis
Lab: ↑ 2,3-BPG (accumulates), Heinz bodies absent, Coombs negative
INICET/NEET-PG: "2,3-BPG elevated - right shift of O₂-Hb dissociation curve"

🔴 Arsenic Poisoning

Arsenate replaces phosphate in glyceraldehyde-3-phosphate dehydrogenase step
1,3-BPG → 1-arseno-3-PG (bypasses substrate-level phosphorylation at step 7)
Net ATP = 0 from glycolysis; patient presents with Mees' lines, GI symptoms, polyneuropathy

🔴 Fructose Intolerance vs Fructosuria

Fructokinase deficiency → Essential fructosuria (benign, fructose in urine)
Aldolase B deficiency → Hereditary Fructose Intolerance (HFI): fructose-1-P accumulates → hypoglycemia, liver failure, renal tubular acidosis
HFI patients develop aversion to sweet foods (protective)

🔴 Warburg Effect (Oncology)

Cancer cells preferentially use aerobic glycolysis even in presence of O₂
PET scan exploits ↑ glucose uptake by tumor cells (↑ ¹⁸F-FDG uptake)
Basis of FDG-PET in tumor staging

🔴 Lactate Acidosis

Type A: Tissue hypoxia (shock, cardiac arrest) - pyruvate → lactate, NAD⁺ regenerated
Type B: Drugs (metformin, linezolid), liver failure
Lactate level: proportional to tissue oxygen deficit - used to gauge shock severity

Metabolic Disorders Table - Glycolysis

Disorder	Deficient Enzyme	Accumulated Metabolite	Key Features
Pyruvate Kinase Deficiency	Pyruvate Kinase	Pyruvate, 2,3-BPG	Hemolytic anemia, RBC, AR inheritance
Essential Fructosuria	Fructokinase	Fructose	Benign, incidental finding
Hereditary Fructose Intolerance	Aldolase B	Fructose-1-P	Hypoglycemia, liver failure, vomiting after sweets
Galactosemia (Classic)	Galactose-1-P uridyltransferase	Gal-1-P, Galactitol	Cataracts, liver failure, E. coli sepsis in neonates
Galactokinase Deficiency	Galactokinase	Galactitol	Cataracts only, milder

1.2 GLUCONEOGENESIS

Location: Liver (primary, 90%) and renal cortex (10%) | Occurs when: Fasting, prolonged exercise, high protein diet

Substrates (glucogenic):

Lactate (Cori cycle) - from muscle
Alanine (Alanine cycle) - from muscle
Glycerol - from fat hydrolysis
Oxaloacetate - from TCA intermediates
Glucogenic amino acids (all except Leu, Lys which are purely ketogenic)
Propionyl-CoA (from odd-chain fatty acid oxidation)

Unique Bypass Enzymes of Gluconeogenesis

Bypassed Glycolytic Step	Gluconeogenic Enzyme	Location	Cofactor
Pyruvate kinase	Pyruvate Carboxylase (Pyr → OAA) then PEPCK (OAA → PEP)	Mito / Cyto	Biotin (PC), GTP (PEPCK)
PFK-1	Fructose-1,6-bisphosphatase (F-1,6-BP → F-6-P)	Cytoplasm	Mg²⁺
Hexokinase	Glucose-6-phosphatase (G-6-P → Glucose)	ER lumen	-

HIGH YIELD: Glucose-6-phosphatase is present only in liver, kidney, intestine - that is why only these organs can export free glucose. Muscle CANNOT release glucose into blood!

Regulation of Gluconeogenesis

Stimulated by	Inhibited by
Glucagon (↑ cAMP)	Insulin
Cortisol	AMP (inhibits F-1,6-BPase)
Fasting	Fructose-2,6-bisphosphate
High NADH:NAD⁺ ratio (alcoholism)	-

Clinical Correlations - Gluconeogenesis

🔴 Alcohol and Hypoglycemia

Ethanol → Acetaldehyde → Acetate; both steps use NAD⁺ → massive ↑ NADH
High NADH:NAD⁺ → OAA converted to malate (not available for gluconeogenesis)
Pyruvate → Lactate (not pyruvate for gluconeogenesis)
Result: Hypoglycemia + Lactic acidosis in alcoholics (especially after fasting)

🔴 Biotin Deficiency

Pyruvate Carboxylase (needs biotin) impaired → gluconeogenesis fails
Presents with: alopecia, dermatitis, neurological symptoms
Also: Pyruvate cannot enter TCA → lactic acidosis
Caused by: eating raw egg whites (avidin binds biotin) OR TPN without biotin

🔴 Von Gierke Disease (Type I GSD) - USMLE FAVORITE

Glucose-6-phosphatase deficiency
Glycogen and fat accumulate in liver and kidney
Classic: Fasting hypoglycemia, lactic acidosis, hyperuricemia (gout), hyperlipidemia, hepatomegaly
Doll-like face, protuberant abdomen
NO response to glucagon (can't release glucose from G-6-P)

🔴 Cori Cycle

Muscle: Glucose → Lactate (anaerobic glycolysis)
Lactate travels to liver via blood
Liver: Lactate → Pyruvate → Glucose (gluconeogenesis)
Glucose returns to muscle
Net: transfers energy deficit from muscle to liver

1.3 TCA CYCLE (Krebs / Citric Acid Cycle)

Location: Mitochondrial matrix | Occurs when: Aerobic conditions

Purpose: Oxidize Acetyl-CoA → CO₂ + NADH + FADH₂ + GTP

Glycogen cycle regulation showing cAMP pathway

The cAMP-mediated coordinated control of glycogen synthesis and breakdown - note reciprocal regulation

TCA Cycle Steps

Step	Reaction	Enzyme	Cofactor	Product
1	Acetyl-CoA + OAA → Citrate	Citrate Synthase	-	Citrate (6C)
2	Citrate → Isocitrate	Aconitase	Fe-S	Isocitrate
3	Isocitrate → α-KG	Isocitrate Dehydrogenase	NAD⁺, Mn²⁺	NADH + CO₂ (Rate Limiting Step)
4	α-KG → Succinyl-CoA	α-Ketoglutarate Dehydrogenase	TPP, Lipoate, CoA, FAD, NAD⁺	NADH + CO₂
5	Succinyl-CoA → Succinate	Succinyl-CoA Synthetase	GDP/ADP	GTP (substrate-level phosphorylation)
6	Succinate → Fumarate	Succinate Dehydrogenase	FAD	FADH₂ (Complex II of ETC)
7	Fumarate → Malate	Fumarase	H₂O	Malate
8	Malate → OAA	Malate Dehydrogenase	NAD⁺	NADH

Per Turn of TCA (from 1 Acetyl-CoA)

3 NADH, 1 FADH₂, 1 GTP, 2 CO₂
ATP equivalent: 3×2.5 + 1×1.5 + 1 = 10 ATP per acetyl-CoA

Rate-Limiting Step

Isocitrate Dehydrogenase - the committed step of the cycle

Regulation of TCA

Activators	Inhibitors
ADP, Ca²⁺, NAD⁺	ATP, NADH
Low energy state	High energy state
Substrate availability	Succinyl-CoA (product inhibition of α-KG DH)

Key Cofactors (ALL 5 for PDH and α-KG DH)

Thiamine (B1) - TPP
Riboflavin (B2) - FAD
Niacin (B3) - NAD⁺
Pantothenic acid (B5) - CoA
Lipoic acid - Lipoamide
Mg²⁺ - for Isocitrate DH

MNEMONIC: "The Lovely Nastily Flavored Compound" = Thiamine, Lipoate, NAD (niacin), FAD (riboflavin), CoA (pantothenate)

PYRUVATE DEHYDROGENASE COMPLEX (PDH) - Bridge from Glycolysis to TCA

Reaction: Pyruvate + CoA + NAD⁺ → Acetyl-CoA + CO₂ + NADH

Same 5 cofactors as α-KG DH (TPP, Lipoate, CoA, FAD, NAD⁺)

Regulation:

Activated by: Ca²⁺, ADP, NAD⁺, CoA, Pyruvate (substrate), Insulin (dephosphorylates PDH kinase)
Inhibited by: Acetyl-CoA, NADH, ATP, PDH kinase (phosphorylates and inactivates PDH)

Clinical Correlations - TCA/PDH

🔴 Thiamine (B1) Deficiency - USMLE/NEET HIGH YIELD

PDH and α-KG DH require TPP (thiamine pyrophosphate)
Deficiency → pyruvate and α-KG cannot be oxidized → ACCUMULATE
α-Ketoglutarate = same as α-ketoglutaric acid (used to measure B1 deficiency)
Dry Beriberi: peripheral neuropathy (sensorimotor)
Wet Beriberi: high-output cardiac failure, dilated cardiomyopathy
Wernicke's Encephalopathy: confusion, ophthalmoplegia, ataxia (classic triad)
Korsakoff Psychosis: confabulation, anterograde amnesia (mammillary body degeneration)
Wernicke-Korsakoff seen in alcoholics - give IV thiamine BEFORE glucose!

🔴 PDH Deficiency

Accumulation of pyruvate and lactate → lactic acidosis
Neurological damage, developmental delay
Blood: Elevated lactate, pyruvate, alanine
Treatment: High-fat, low-carb diet; thiamine supplements

🔴 Isocitrate Dehydrogenase (IDH) Mutations - Oncology

IDH1 (cytoplasm), IDH2 (mitochondria) mutations
Gain-of-function mutation → produces 2-hydroxyglutarate (oncometabolite)
Found in: Gliomas (grade II/III), AML, cholangiocarcinoma
IDH-mutant gliomas have better prognosis
Treatment: Ivosidenib (IDH1), Enasidenib (IDH2)

🔴 Aconitase Inhibition

Fluorocitrate (from fluoroacetate, rat poison) → inhibits aconitase → TCA halted
Citrate accumulates → inhibits PFK-1 → glycolysis also fails
"Citrate trap" - double inhibition of energy metabolism

🔴 Succinate Dehydrogenase (SDH) Mutations

SDH = Complex II of ETC (only TCA enzyme embedded in inner mito membrane)
SDHB/SDHD mutations → Hereditary Paraganglioma/Pheochromocytoma syndrome
SDH is a tumor suppressor; loss → Warburg-like shift

1.4 GLYCOGEN METABOLISM

Glycogenesis (Synthesis): Fed state, insulin-driven, liver + muscle Glycogenolysis (Breakdown): Fasting/stress, glucagon/epinephrine-driven

Glycogen synthesis and breakdown pathway

Central role of glucose-6-phosphate in glycogen metabolism

Glycogenolysis steps with phosphorylase, glucan transferase and debranching enzyme

Glycogenolysis: Phosphorylase removes 1→4 links, debranching enzyme handles 1→6 branch points

Glycogenesis Steps

Step	Enzyme	Notes
Glucose → G-6-P	Hexokinase/Glucokinase
G-6-P → G-1-P	Phosphoglucomutase
G-1-P + UTP → UDP-Glucose	UDP-glucose pyrophosphorylase
UDP-Glucose → Glycogen chain	Glycogen Synthase	Rate-limiting step; adds α(1→4) links; requires a primer
Branch formation	Branching Enzyme	Transfers ≥6 residues to form α(1→6)

Glycogenolysis Steps

Step	Enzyme	Notes
Glycogen → G-1-P	Glycogen Phosphorylase	Rate-limiting; cleaves α(1→4); cofactor = Pyridoxal Phosphate (B6)
Debranching	Debranching enzyme (2 activities)	Glucan transferase + α-1,6-glucosidase
G-1-P → G-6-P	Phosphoglucomutase
G-6-P → Glucose (liver only)	Glucose-6-phosphatase	NOT present in muscle; muscle uses G-6-P internally

Regulation - Reciprocal Control by cAMP

Coordinated glycogen cycle regulation by cAMP-dependent protein kinase

Signal	Effect on Glycogen Synthase	Effect on Phosphorylase	Net Result
Glucagon/Epinephrine (↑ cAMP)	Inactivated (phosphorylated)	Activated (phosphorylated)	Glycogenolysis
Insulin	Activated (dephosphorylated)	Inactivated	Glycogenesis
Ca²⁺ (muscle contraction)	-	Activated via Phosphorylase Kinase	Glycogenolysis
Glucose-6-phosphate	Activates synthase	Inhibits phosphorylase	Glycogenesis

Glycogen Storage Diseases - COMPLETE TABLE (HIGH YIELD)

Type	Eponym	Deficient Enzyme	Gene	Organ Affected	Key Clinical Features	Exam Tip
0	-	Glycogen Synthase	GYS2	Liver	Fasting hypoglycemia, hyperketonemia	Opposite: too LITTLE glycogen
Ia	Von Gierke	Glucose-6-phosphatase	G6PC	Liver, Kidney	Fasting hypoglycemia, lactic acidosis, hyperuricemia (gout), hyperlipidemia, hepatomegaly, doll-like facies	No response to glucagon; hepatocellular adenoma risk
Ib	Von Gierke variant	G-6-P translocase	SLC37A4	Liver, Kidney	Same as Ia + neutropenia + infections	Transportopathy
II	Pompe	Acid Maltase (α-1,4 glucosidase)	GAA	Lysosomes - all tissues	Infant: hypotonia, cardiomegaly, death by 2 yrs; Adult: myopathy	Only GSD with lysosomal enzyme deficiency; Treatment: Alglucosidase alfa (ERT)
III	Cori / Forbes	Debranching enzyme	AGL	Liver, Muscle	Milder Von Gierke; limit dextrin accumulates	Fasting hypoglycemia, hepatomegaly
IV	Andersen	Branching enzyme	GBE1	Liver	Progressive cirrhosis, liver failure, abnormal long-chain glycogen	Also called Amylopectinosis
V	McArdle	Muscle Phosphorylase	PYGM	Muscle only	Exercise intolerance, cramps, myoglobinuria, NO rise in lactate after exercise (ischemic forearm exercise test)	Blood glucose NORMAL; serum CK elevated
VI	Hers	Liver Phosphorylase	PYGL	Liver	Mild hypoglycemia, hepatomegaly; benign course
VII	Tarui	Phosphofructokinase (muscle)	PFKM	Muscle, RBC	Like McArdle + hemolytic anemia	PFK deficiency in RBCs too
IX	-	Phosphorylase kinase	PHKA	Liver, Muscle	Mild, X-linked

MNEMONIC for GSD types I-V: "Very Poor Carbohydrate Absorption Mechanism"

I = Von gierke (V = Very)

II = Pompe (P = Poor)

III = Cori (C = Carbohydrate)

IV = Andersen (A = Absorption)

V = McArdle (M = Mechanism)

1.5 PENTOSE PHOSPHATE PATHWAY (HMP Shunt)

Location: Cytoplasm | Active in: Liver, RBCs, lactating mammary gland, adrenal cortex, testis

Two phases:

Oxidative (irreversible): G-6-P → Ribulose-5-P + 2 NADPH + CO₂
Non-oxidative (reversible): Interconversion of sugar phosphates

Key Enzymes

Enzyme	Role	Clinical Relevance
Glucose-6-Phosphate Dehydrogenase (G6PD)	Rate-limiting; G-6-P → 6-Phosphogluconate	G6PD deficiency = most common enzyme deficiency worldwide
6-Phosphogluconate Dehydrogenase	→ Ribulose-5-P + CO₂ + NADPH	-
Transketolase	Non-oxidative phase; requires Thiamine (B1)	Activity measured in B1 deficiency (RBC transketolase activity)
Transaldolase	Non-oxidative phase	-

Functions of PPP

NADPH production - for: Glutathione reduction (GSH), Fatty acid synthesis, Cholesterol synthesis, Cytochrome P450, Respiratory burst (NADPH oxidase), Nitric oxide synthesis
Ribose-5-phosphate - for nucleotide synthesis (DNA/RNA)

Regulation

Controlled by NADPH levels (product inhibition of G6PD)
When NADPH is oxidized → pathway activated
Insulin ↑ G6PD expression

Clinical Correlations

🔴 G6PD Deficiency - USMLE/NEET CLASSIC

X-linked recessive (most common enzyme deficiency in humans, affects ~400 million people)
NADPH deficient → cannot regenerate GSH → oxidized hemoglobin → Heinz bodies
RBC membrane damage → intravascular hemolysis → hemoglobinuria (dark urine)
Triggers: Primaquine, dapsone, nitrofurantoin, sulfonamides, fava beans, infection (most common trigger!)
Peripheral smear: Heinz bodies, bite cells (macrophages nibble Heinz bodies)
Coombs test: NEGATIVE (not immune-mediated)
G6PD variants: G6PD A- (African), G6PD Mediterranean (severe), G6PD Canton (Asian)

🔴 Chronic Granulomatous Disease (CGD)

Defective NADPH oxidase in neutrophils (not G6PD, but uses same NADPH)
Cannot produce superoxide (O₂⁻) → cannot kill catalase-positive organisms
Recurrent infections with: S. aureus, Aspergillus, Nocardia, Pseudomonas, Serratia
DHR (dihydrorhodamine) flow cytometry: diagnostic
Treatment: Prophylactic TMP-SMX, itraconazole; IFN-γ; bone marrow transplant (cure)

🔴 Transketolase and Thiamine (B1)

Transketolase in PPP requires TPP; activity used to diagnose subclinical thiamine deficiency
RBC transketolase activity test: ↓ activity in thiamine deficiency

PART 2: PROTEIN/AMINO ACID METABOLISM

2.1 TRANSAMINATION

Location: Liver primarily (also kidney, heart) Purpose: Transfer amino group from amino acid to α-keto acid (reversible)

Key Reactions

Reaction: Amino acid + α-Ketoglutarate ⇌ α-Keto acid + Glutamate

Enzymes:

ALT (Alanine aminotransferase / SGPT): Alanine + α-KG ⇌ Pyruvate + Glutamate (liver-specific)
AST (Aspartate aminotransferase / SGOT): Aspartate + α-KG ⇌ OAA + Glutamate (liver + heart + muscle)
Cofactor: Pyridoxal phosphate (Vitamin B6) - for ALL aminotransferases

Alanine-Glucose Cycle (Cahill Cycle)

Muscle: Pyruvate + glutamate → Alanine (via ALT) → travels to liver
Liver: Alanine → Pyruvate (transamination) → Glucose (gluconeogenesis)
Nitrogen delivered to liver as alanine for urea synthesis
Glucose returned to muscle via blood
Net: Transfers N from muscle to liver without releasing free NH₃ into blood

Clinical Correlations

🔴 ALT vs AST in Liver Disease

Viral hepatitis: ALT > AST (AST:ALT ratio <1)
Alcoholic hepatitis: AST:ALT ratio >2:1 (AST elevated due to B6 deficiency and mitochondrial damage; classic USMLE)
Liver cirrhosis: AST > ALT (scar tissue)
MI: AST elevated (cardiac muscle), ALT normal

🔴 Pyridoxine (B6) Deficiency

Impairs ALL aminotransferases
Dermatitis, glossitis, cheilosis, peripheral neuropathy, sideroblastic anemia (B6 needed for heme synthesis too - ALA synthase)
Isoniazid (INH) causes B6 deficiency (INH is a B6 antagonist) → always supplement B6 with INH

2.2 UREA CYCLE (Ornithine Cycle / Krebs-Henseleit Cycle)

Location: Liver ONLY (hepatocytes) | Start in: Mitochondria, Finish in: Cytoplasm

Purpose: Convert toxic ammonia → non-toxic urea → excreted in urine

Urea Cycle diagram showing mitochondria and cytosol compartments

The urea cycle - note CPS-I and OTC in mitochondria; remainder in cytosol

Steps of the Urea Cycle

Step	Enzyme	Location	Cofactor	Notes
1	NH₃ + CO₂ + 2ATP → Carbamoyl phosphate	CPS-I	N-Acetylglutamate (NAG), Mg²⁺	Mitochondria; Rate-Limiting Step
2	Carbamoyl-P + Ornithine → Citrulline	OTC (Ornithine Transcarbamylase)	-	Mitochondria; most common defect
3	Citrulline + Aspartate → Argininosuccinate	Argininosuccinate Synthetase (ASS)	ATP	Cytoplasm; needs N from aspartate
4	Argininosuccinate → Arginine + Fumarate	Argininosuccinate Lyase (ASL)	-	Cytoplasm; fumarate → TCA
5	Arginine → Ornithine + Urea	Arginase	Mn²⁺	Cytoplasm; urea released

Key Points

2 N atoms in urea: 1 from NH₃ (CPS-I step), 1 from aspartate (step 3)
Net energy cost: 3 ATP per cycle (2 for CPS-I + 1 for ASS)
NAG activates CPS-I - formed from acetyl-CoA + glutamate by NAG synthase
Arginine is essential amino acid in urea cycle defects (body cannot make enough)
Fumarate links urea cycle to TCA cycle

Regulation of Urea Cycle

CPS-I activated by: N-Acetylglutamate (NAG) - obligate activator
NAG synthesis increased by: Arginine, high protein intake
Long-term: Gene induction by glucocorticoids, high protein diet

Ammonia Detoxification in Brain (NOT liver)

Brain: Glutamate + NH₃ → Glutamine (glutamine synthetase)
Glutamine is the non-toxic transport form of NH₃ in blood
High NH₃ depletes α-KG from TCA → ↓ energy production in neurons → encephalopathy

Urea Cycle Defects - COMPLETE TABLE

Defect	Enzyme	Inheritance	Key Lab Finding	Distinguishing Feature
OTC Deficiency	Ornithine transcarbamylase	X-linked (only XLR urea cycle defect)	↑ Orotic acid in urine, ↑ NH₃, ↓ citrulline	Most common; excess carbamoyl-P → pyrimidine synthesis → orotic acid
CPS-I Deficiency	Carbamoyl phosphate synthetase I	AR	↑ NH₃, ↓ citrulline, Normal orotic acid	No orotic acid (problem is before carbamoyl-P is made)
NAG Synthase Deficiency	N-Acetylglutamate synthase	AR	↑ NH₃, ↓ citrulline, Normal orotic acid	Treatable with N-carbamoyl-glutamate (NAG analogue)
Citrullinemia Type I	Argininosuccinate Synthetase	AR	↑↑ Citrulline, ↑ NH₃, ↑ orotic acid	Citrulline accumulates massively
Argininosuccinicaciduria	Argininosuccinate Lyase	AR	↑ Argininosuccinate, ↑ citrulline	Hair abnormality (trichorrhexis nodosa)
Argininemia	Arginase	AR	↑↑ Arginine, spastic diplegia	Spastic tetraplegia, intellectual disability; late onset
HHH Syndrome	Ornithine transporter (ORNT1)	AR	↑ Ornithine, ↑ NH₃, Homocitrullinuria	Ornithine cannot enter mitochondria

Key Differentiating Points (EXAM)

↑ Orotic acid: OTC deficiency (ONLY X-linked), Citrullinemia
Normal orotic acid: CPS-I, NAG synthase deficiency
OTC vs CPS-I: Both have ↑ NH₃ and ↓ citrulline, but OTC has ↑ orotic acid
All present with: lethargy, vomiting, encephalopathy shortly after birth; ↑ plasma glutamine

Clinical Correlations

🔴 Hyperammonemia - Management

Dietary: Low-protein diet, essential amino acid supplementation
Sodium benzoate / sodium phenylbutyrate: alternative nitrogen excretion pathways
Arginine supplementation (essential in cycle defects)
Dialysis for acute crisis
Rifaximin for hepatic encephalopathy (reduces gut NH₃ production)
Liver transplant (definitive for urea cycle defects)

🔴 Liver Failure and Hyperammonemia

Portosystemic shunting bypasses hepatocytes → NH₃ reaches brain
Hepatic encephalopathy: flapping tremor (asterixis), fetor hepaticus (musty odor), confusion → coma
Precipitants: GI bleed, infection, constipation, sedatives, high-protein meal

🔴 Reye's Syndrome

Mitochondrial dysfunction (often post-viral + aspirin use in children)
Impairs urea cycle enzymes + fatty acid oxidation
Presents: vomiting, encephalopathy, hepatic failure, hypoglycemia
NO jaundice (hepatocellular necrosis minimal)
Lab: ↑ NH₃, ↑ ALT/AST, ↑ PT, normal bilirubin

2.3 AMINO ACID CATABOLISM & SELECTED METABOLIC DISORDERS

Phenylalanine - Tyrosine Pathway

Phe → Tyr (Phenylalanine hydroxylase + BH4 cofactor) Tyr → DOPA → Dopamine → NE → Epinephrine (catecholamine synthesis) Tyr → Thyroxine (T3, T4) Tyr → Melanin (tyrosinase) Tyr → Homogentisate → Maleylacetoacetate → Fumarate + Acetoacetate (catabolism)

Disorder	Deficient Enzyme	Accumulated	Clinical Features	Inheritance
PKU (Phenylketonuria)	Phenylalanine Hydroxylase (or BH4)	Phenylalanine, Phenylpyruvate	Intellectual disability, seizures, fair skin/hair/eyes, musty odor, eczema	AR
Malignant PKU	BH4 (Tetrahydrobiopterin) synthesis or recycling	Phe	Same + neurotransmitter deficiency (DOPA, serotonin)	AR
Alcaptonuria	Homogentisate oxidase	Homogentisic acid	Black urine (oxidized homogentisic acid), ochronosis (dark pigment in cartilage), arthritis	AR
Albinism	Tyrosinase	Melanin not made	Lack of pigment in skin, hair, eyes; ↑ skin cancer risk	AR
Tyrosinemia Type I	Fumarylacetoacetate hydrolase (FAH)	Succinylacetone	Liver failure, renal tubular acidosis, cabbage odor, ↑ AFP	AR

Branch-Chain Amino Acids (BCAA) - Leucine, Isoleucine, Valine

Maple Syrup Urine Disease (MSUD)

Defective α-Keto acid dehydrogenase (branched-chain)
Same cofactors as PDH: TPP, lipoate, CoA, FAD, NAD⁺
Accumulation of leucine, isoleucine, valine and their keto-acids
Maple syrup smell in urine/cerumen
Neonatal seizures, encephalopathy, feeding difficulty
Thiamine is adjunct treatment (cofactor)
Dietary: Restriction of Leu, Ile, Val

Homocysteine Metabolism

Disorder	Deficient Enzyme	Accumulated	Key Features	Treatment
Homocystinuria (Classic)	Cystathionine β-synthase	Homocysteine	Marfanoid habitus, ectopia lentis (DOWNWARD), intellectual disability, thromboembolic events, early atherosclerosis	Pyridoxine (B6), folate, methionine restriction
Homocystinuria (MTHFR)	MTHFR (methylenetetrahydrofolate reductase)	Homocysteine	Mild-moderate; associated with neural tube defects (pregnancy), CVD risk	Folate, betaine
Elevated Hcy (B12/Folate deficiency)	Methionine synthase (needs B12, methylfolate)	Homocysteine, Methylmalonic acid (B12 only)	Megaloblastic anemia, subacute combined degeneration (B12)	B12, Folate

Ectopia Lentis:

Marfan syndrome: superotemporal displacement

Homocystinuria: inferonasal (downward/medial) displacement

Weill-Marchesani: inferonasal

Sulphite oxidase deficiency: variable

Organic Acidemias

Disorder	Defect	Accumulates	Features
Methylmalonic Acidemia	Methylmalonyl-CoA Mutase (needs B12-adenosylcobalamin)	Methylmalonic acid	Metabolic acidosis, hyperammonemia, neutropenia
Propionic Acidemia	Propionyl-CoA Carboxylase (needs Biotin)	Propionic acid	Severe metabolic acidosis, hyperammonemia
Isovaleric Acidemia	Isovaleryl-CoA Dehydrogenase	Isovaleric acid	"Sweaty feet" odor, metabolic acidosis

PART 3: LIPID METABOLISM

3.1 FATTY ACID SYNTHESIS (Lipogenesis)

Location: Cytoplasm | Occurs in: Liver, adipose tissue, lactating mammary gland Substrate: Acetyl-CoA (from glucose/amino acids via PDH or β-oxidation overflow) Shuttle: Acetyl-CoA cannot cross inner mitochondrial membrane → citrate shuttle (citrate exits mito → cleaved to acetyl-CoA + OAA in cytoplasm by ATP citrate lyase)

Steps

Step	Enzyme	Cofactor	Product	Notes
1	Acetyl-CoA + CO₂ + ATP → Malonyl-CoA	Acetyl-CoA Carboxylase (ACC)	Malonyl-CoA	Rate-limiting step; requires Biotin
2-7	Malonyl-CoA + NADPH → FA elongation	Fatty Acid Synthase (FAS)	Palmitate (16C)	7 cycles; uses 7 NADPH per cycle

Overall: Acetyl-CoA + 7 Malonyl-CoA + 14 NADPH → Palmitate + 7 CO₂ + 8 CoA

Regulation of Fatty Acid Synthesis

Activators of ACC	Inhibitors of ACC
Citrate (allosteric)	Palmitoyl-CoA (product inhibition)
Insulin (activates via dephosphorylation)	Glucagon/Epinephrine (↑ cAMP → phosphorylates ACC → inactive)
High carbohydrate diet	Malonyl-CoA inhibits CPT-I (prevents FA oxidation simultaneously)

KEY EXAM POINT: Malonyl-CoA INHIBITS Carnitine Palmitoyl Transferase-I (CPT-I), preventing entry of fatty acids into mitochondria for β-oxidation. Thus: when synthesis is ON, oxidation is OFF - mutual exclusivity

NADPH Sources for Fatty Acid Synthesis

Pentose Phosphate Pathway (major source in liver)
Malate enzyme (cytoplasmic malic enzyme)
Isocitrate dehydrogenase (cytoplasmic)

3.2 FATTY ACID OXIDATION (β-Oxidation)

Location: Mitochondrial matrix (long-chain FA) | Peroxisomes: Very long chain FA (VLCFA) Activation: Fatty acid + CoA + ATP → Acyl-CoA + AMP + PPi (by Acyl-CoA Synthetase, on outer mitochondrial membrane)

Carnitine Shuttle (Entry into Mitochondria)

CPT-I (outer membrane): Acyl-CoA + Carnitine → Acylcarnitine (rate-limiting for FA oxidation)
Translocase: Carries acylcarnitine across inner membrane
CPT-II (inner membrane): Regenerates Acyl-CoA inside matrix

Malonyl-CoA inhibits CPT-I = the switch between fat synthesis and oxidation

β-Oxidation Cycle Steps

Beta oxidation overview - palmitoyl-CoA → acetyl-CoA

Beta-oxidation: successive removal of 2-carbon acetyl-CoA units from palmitoyl-CoA

Step	Reaction	Enzyme	Cofactor	Product
1	Acyl-CoA → Enoyl-CoA	Acyl-CoA Dehydrogenase	FAD	FADH₂
2	Enoyl-CoA + H₂O → 3-Hydroxyacyl-CoA	Enoyl-CoA Hydratase	H₂O	-
3	3-Hydroxyacyl-CoA → 3-Ketoacyl-CoA	3-Hydroxyacyl-CoA Dehydrogenase	NAD⁺	NADH
4	3-Ketoacyl-CoA + CoA → Acetyl-CoA + Acyl-CoA (n-2)	Thiolase	CoA	Acetyl-CoA

Per cycle: 1 FADH₂ + 1 NADH + 1 Acetyl-CoA For Palmitate (C16): 7 cycles → 7 FADH₂ + 7 NADH + 8 Acetyl-CoA → NET 106 ATP

Metabolic Disorders - Beta-Oxidation

Disorder	Defect	Key Features	Lab	Notes
MCAD Deficiency (Most common FA oxidation disorder)	Medium-Chain Acyl-CoA Dehydrogenase	Hypoketotic hypoglycemia during fasting, vomiting, encephalopathy, sudden death	↑ Octanoylcarnitine (C8) on newborn screen, ↑ Medium-chain FA in urine	Most common; diagnosed on newborn screening; AR
LCHAD	Long-chain 3-hydroxyacyl-CoA DH	Hypoglycemia, cardiomyopathy, peripheral neuropathy, AFLP in mother (carrier)	↑ Long-chain acylcarnitines	Maternal AFLP associated
VLCAD	Very Long-chain Acyl-CoA DH	Cardiomyopathy, hypoglycemia	↑ C14:1 acylcarnitine
Primary Carnitine Deficiency	OCTN2 (plasma membrane carnitine transporter)	Hypoketotic hypoglycemia, cardiomyopathy, muscle weakness	↓ Serum carnitine, ↑ acylcarnitines	Treatment: Carnitine supplementation
CPT-I Deficiency	CPT-I (liver)	Hypoketotic hypoglycemia, liver dysfunction	↑ Free carnitine, ↓ acylcarnitines	Opposite pattern from most FA disorders
CPT-II Deficiency	CPT-II (muscle)	Myopathy, myoglobinuria with exercise	↑ Long-chain acylcarnitines
Jamaican Vomiting Sickness	- (external toxin)	Hypoglycin A (from unripe ackee fruit) → inhibits MCAD and SCHAD	Hypoketotic hypoglycemia, vomiting, encephalopathy	Not inherited

3.3 KETONE BODY METABOLISM

Location: Synthesis: Liver mitochondria | Utilization: Brain, heart, muscle, kidney (NOT liver - lacks ketolytic enzyme)

Conditions promoting ketosis: Fasting >24h, uncontrolled DM Type 1, prolonged exercise, ketogenic diet, alcoholism

Ketogenesis Steps (in liver mitochondria)

Step	Reaction	Enzyme	Notes
1	2 Acetyl-CoA → Acetoacetyl-CoA	Thiolase
2	Acetoacetyl-CoA + Acetyl-CoA → HMG-CoA	HMG-CoA Synthase	Rate-limiting step of ketogenesis; mitochondrial
3	HMG-CoA → Acetoacetate + Acetyl-CoA	HMG-CoA Lyase
4	Acetoacetate → β-Hydroxybutyrate	β-Hydroxybutyrate DH	Reversible; uses NADH
-	Acetoacetate → Acetone + CO₂	Non-enzymatic decarboxylation	Fruity breath in DKA

3 ketone bodies: Acetoacetate, β-Hydroxybutyrate (predominant in DKA), Acetone

Ketone Body Utilization (Peripheral tissues)

β-Hydroxybutyrate → Acetoacetate (β-HB DH)
Acetoacetate + Succinyl-CoA → Acetoacetyl-CoA + Succinate (Succinyl-CoA:3-ketoacid CoA transferase = Thiophorase - absent in liver!)
Acetoacetyl-CoA → 2 Acetyl-CoA → TCA

Clinical Correlations

🔴 Diabetic Ketoacidosis (DKA)

Type 1 DM (insulin deficiency) → ↑ glucagon → ↑ lipolysis → ↑ FA to liver → ↑ ketogenesis
Malonyl-CoA LOW → CPT-I active → FA enter mitochondria
pH <7.3, anion gap metabolic acidosis
Ketones: β-HB >> Acetoacetate in DKA (ratio 3:1 normally, up to 10:1 in DKA)
Nitroprusside test detects acetoacetate ONLY (NOT β-HB) → may UNDERESTIMATE DKA severity
Treatment: Insulin + fluids + potassium replacement

🔴 Starvation Ketosis

Glucose depletes after 24h → insulin ↓, glucagon ↑
Brain adapts to use ketone bodies after 3-4 days (reduces protein catabolism)
After prolonged starvation: brain gets 70% energy from ketones
Controlled; pH usually normal (buffering capacity adequate)

🔴 Alcoholic Ketoacidosis

Ethanol → ↑ NADH:NAD⁺ → favors β-HB over acetoacetate
Nitroprusside test may be NEGATIVE despite severe ketosis (measures only acetoacetate!)
Anion gap metabolic acidosis + history of alcohol + vomiting

3.4 CHOLESTEROL SYNTHESIS (Mevalonate Pathway)

Location: Liver (primary), intestine | Occurs in: Cytoplasm + ER

Substrate: Acetyl-CoA → Mevalonate → Cholesterol

Key Steps

Step	Enzyme	Notes
2 Acetyl-CoA → Acetoacetyl-CoA	Thiolase
Acetoacetyl-CoA + Acetyl-CoA → HMG-CoA	HMG-CoA Synthase	Cytoplasmic (≠ mitochondrial)
HMG-CoA → Mevalonate	HMG-CoA Reductase	Rate-limiting step - TARGET OF STATINS
Mevalonate → Isopentenyl pyrophosphate	Multiple steps	Requires ATP, NADPH
IPP → Squalene → Lanosterol → Cholesterol	Multiple steps

Regulation of HMG-CoA Reductase

Activated by	Inhibited by
Insulin	Glucagon
Thyroxine	Cholesterol (product inhibition, via SREBP)
-	Statins (competitive inhibitors)
-	PCSK9 (degrades LDL receptors - PCSK9 inhibitors increase LDL clearance)

Other products of Mevalonate Pathway

Ubiquinone (CoQ10) - electron carrier ETC
Dolichol - N-glycosylation of proteins
Farnesyl/Geranylgeranyl pyrophosphate - prenylation of Ras (oncoproteins)
Heme A (modified porphyrin)

STATIN SIDE EFFECT NOTE: Statins inhibit HMG-CoA Reductase → ↓ CoQ10 → myopathy (rhabdomyolysis risk). Also ↓ farnesyl-PP → cannot prenylate Ras (anti-tumor side benefit)

Lipoprotein Metabolism

Lipoprotein	Origin	Main Lipid	Apoprotein	Function	Receptor
Chylomicrons	Intestinal enterocytes	Dietary TG	ApoB-48, ApoC-II, ApoE	Transport dietary fat (exogenous)	LPL (activated by ApoC-II)
VLDL	Liver	Endogenous TG	ApoB-100, ApoC-II, ApoE	Transport hepatic TG	LPL
IDL	VLDL remnant	TG + CE	ApoB-100, ApoE	Intermediate; cleared by liver	LDL receptor
LDL	IDL	Cholesterol esters	ApoB-100	Deliver cholesterol to tissues	LDL receptor (deficient in FH)
HDL	Liver + intestine	Protein-rich	ApoA-I	Reverse cholesterol transport	SR-BI

Key Enzymes:

Lipoprotein Lipase (LPL): Hydrolyzes TG in chylomicrons and VLDL; activated by ApoC-II; inhibited by ApoC-III; deficiency → hypertriglyceridemia (Type I hyperlipoproteinemia)
LCAT (Lecithin-Cholesterol Acyltransferase): Esterifies cholesterol in HDL; activator = ApoA-I
CETP (Cholesterol Ester Transfer Protein): Transfers CE from HDL to VLDL/LDL in exchange for TG
Hepatic Lipase: Converts IDL → LDL; removes TG from HDL

Clinical Correlations - Cholesterol/Lipoproteins

🔴 Familial Hypercholesterolemia (FH)

LDL receptor mutation (most common: loss-of-function AR/AD)
Heterozygous FH: LDL 200-400 mg/dL; Homozygous FH: LDL >600-1000 mg/dL
Xanthomas (tendon, especially Achilles), xanthelasma, corneal arcus <45yrs
Early MI (males by 40s, females by 50s in heterozygous)
Treatment: Statins + Ezetimibe + PCSK9 inhibitors (evolocumab, alirocumab); LDL apheresis

🔴 Familial Hypertriglyceridemia

LPL deficiency (Type I) or ApoC-II deficiency: very high TG (>1000), eruptive xanthomas, lipemia retinalis, pancreatitis
Type V: combined VLDL + chylomicron elevation

🔴 Abetalipoproteinemia

MTP (microsomal triglyceride transfer protein) deficiency → cannot assemble chylomicrons or VLDL
Fat malabsorption, acanthocytes, progressive ataxia/neuropathy (Vitamin E deficiency)
Absence of ApoB-containing lipoproteins

🔴 Tangier Disease

ABCA1 transporter deficiency → cholesterol cannot be effluxed to HDL
Very low/absent HDL, orange tonsils, hepatosplenomegaly, peripheral neuropathy

Lipid Storage Disorders (Sphingolipidoses and Lipidoses)

Disorder	Deficient Enzyme	Accumulated Lipid	Key Clinical Features	Inheritance
Gaucher Disease (Type I - most common lysosomal storage disorder)	β-Glucocerebrosidase	Glucocerebroside	Gaucher cells (crinkled tissue paper cytoplasm), hepatosplenomegaly, bone pain, pancytopenia; NO CNS in Type I	AR; ERT: Imiglucerase; SRT: Miglustat
Niemann-Pick	Sphingomyelinase	Sphingomyelin	Hepatosplenomegaly, cherry-red spot (50%), foam cells; Types A (severe, early death) and B	AR
Tay-Sachs	Hexosaminidase A	GM2 ganglioside	Cherry-red spot, progressive neurodegeneration, NO organomegaly, Ashkenazi Jewish	AR
Sandhoff Disease	Hexosaminidase A + B	GM2 + globoside	Same as Tay-Sachs + organomegaly	AR
Krabbe Disease	Galactocerebrosidase	Galactocerebroside + psychosine	Peripheral neuropathy, globoid cells; infantile onset	AR
Metachromatic Leukodystrophy (MLD)	Arylsulfatase A	Sulfatide	Demyelination, peripheral neuropathy, developmental regression; metachromatic deposits	AR
Fabry Disease	α-Galactosidase A	Globotriaosylceramide (Gb3)	Angiokeratomas, neuropathic pain, corneal opacity (cornea verticillata), cardiomyopathy, renal failure	X-linked
Farber Disease	Ceramidase	Ceramide	Hoarse cry, joint swelling, subcutaneous nodules	AR
Wolman Disease	Lysosomal acid lipase	Cholesterol esters + TG	Adrenal calcification, liver failure, early death	AR
GM1 Gangliosidosis	β-Galactosidase	GM1 + keratan sulfate	Dysostosis multiplex, cherry-red spot, coarse facies	AR
Mucopolysaccharidoses (MPS)	Various glycosaminoglycan enzymes	GAGs	Coarse features, organomegaly, skeletal dysplasia, corneal clouding; see separate table	AR (except Hunter: XLR)

Mucopolysaccharidoses (MPS) - Quick Reference

Type	Eponym	Enzyme	GAG Accumulated	Key Features
I H	Hurler	α-L-Iduronidase	HS + DS	Coarse facies, corneal clouding, hepatosplenomegaly, intellectual disability, cardiac
I S	Scheie	α-L-Iduronidase	HS + DS	Mild, corneal clouding, normal intelligence
II	Hunter	Iduronate sulfatase	HS + DS	X-linked (only XLR MPS); NO corneal clouding; "hunts at night" (pebbly skin, deafness)
III	Sanfilippo	Multiple (4 subtypes)	HS	Severe intellectual disability, mild somatic features
IV	Morquio	N-acetylgalactosamine-6-sulfatase (A) or β-Galactosidase (B)	KS + CS	Short stature, atlantoaxial instability (dens hypoplasia → risk of cord compression), normal intelligence, NO corneal clouding except B
VI	Maroteaux-Lamy	Arylsulfatase B	DS	Coarse features, NO intellectual disability
VII	Sly	β-Glucuronidase	HS + DS + CS	Hydrops fetalis

MNEMONIC for Hunter Syndrome (Type II): "Hunter hunts with X (crossbow)" = X-linked; no corneal clouding (hunter needs to SEE = NO corneal haze)

PART 4: INTEGRATION OF ALL CYCLES

The Metabolic Integration Map

CARBOHYDRATES          PROTEINS            LIPIDS
    |                     |                   |
Glucose             Amino Acids         Triglycerides
    |                     |                   |
Glycolysis         Transamination       Lipolysis (HSL)
    |                     |                   |
Pyruvate -------> α-KG/OAA -------> Glycerol → Gluconeogenesis
    |                              Fatty Acids
    ↓ PDH                              |
Acetyl-CoA <---------------------- β-Oxidation
         \                              |
          \                             |
           TCA Cycle <-------- OAA ← Propionyl-CoA
          /    \                  (odd-chain FA)
    NADH/FADH₂  GTP
          |
        ETC → ATP
          |
     Ketone Bodies (liver excess acetyl-CoA)

Key Integration Points

Junction	Molecules	Connects
Acetyl-CoA	Central metabolite	Carb + Lipid + Protein all converge here
Pyruvate	Metabolic crossroads	Glycolysis → TCA (PDH), Gluconeogenesis (PC), Lactate (LDH), Alanine (ALT)
Oxaloacetate (OAA)	TCA/Gluconeogenesis	Must be replenished (anaplerosis) from amino acids; "OAA is the spark plug of TCA"
α-Ketoglutarate	TCA/Amino acid	Accepts/donates NH₂ in transamination; connects N metabolism to TCA
Succinyl-CoA	TCA/Heme/BCAA	Porphyrin synthesis, methylmalonyl-CoA, odd-chain FA
Fumarate	TCA/Urea cycle	Urea cycle feeds fumarate to TCA (Bicyclic connection)
G-6-P	Glycolysis/PPP/Glycogen	Central hub of carbohydrate metabolism
Glucose-6-phosphatase	Gluconeogenesis/Glycogenolysis	Absent in muscle; explains why muscle can't export glucose

Anaplerotic Reactions (Replenishing TCA intermediates)

Source	Reaction	Enzyme	Clinical
Pyruvate → OAA	Pyruvate + CO₂ → OAA	Pyruvate Carboxylase (Biotin)	Critical for GNG; impaired in biotin deficiency
Glutamate → α-KG	Glutamate → α-KG + NH₃	Glutamate DH	Links amino acid and TCA
Aspartate → OAA	Transamination	AST	AST reaction connects urea cycle to TCA
Propionyl-CoA → Succinyl-CoA	3 steps	Propionyl-CoA carboxylase + Methylmalonyl-CoA mutase	Requires biotin (PCC) and B12 (MCM); deficiency → organic acidemia
Odd-chain FA → Succinyl-CoA	β-oxidation + 3 steps	-	Only FA carbon that enters gluconeogenesis

MASTER COFACTOR TABLE

Cofactor	Vitamin	Key Enzymes	Deficiency Disease
TPP (Thiamine pyrophosphate)	B1	PDH, α-KG DH, BCKA DH, Transketolase	Beriberi, Wernicke-Korsakoff
FAD/FMN	B2 (Riboflavin)	Multiple DH enzymes in ETC and TCA	Glossitis, corneal vascularization, normocytic anemia
NAD⁺/NADP⁺	B3 (Niacin)	Almost all dehydrogenases	Pellagra (4Ds: Diarrhea, Dermatitis, Dementia, Death)
CoA (Coenzyme A)	B5 (Pantothenic acid)	PDH, α-KG DH, Fatty acid metabolism	Rare; burning feet syndrome
PLP (Pyridoxal Phosphate)	B6 (Pyridoxine)	ALL aminotransferases, ALA synthase, Glycogen phosphorylase, Serine DH	Sideroblastic anemia, peripheral neuropathy, convulsions; INH-induced
Biotin	B7	Pyruvate Carboxylase, ACC, Propionyl-CoA Carboxylase, MCC	Alopecia, dermatitis, neurological symptoms; raw egg whites
Folate (THF)	B9	Thymidylate synthase, purine synthesis, methionine synthesis (with B12)	Megaloblastic anemia, NTD (fetal)
Cobalamin (B12)	B12	Methionine synthase (homocysteine→methionine), Methylmalonyl-CoA mutase	Megaloblastic anemia, subacute combined degeneration, elevated MMA
Lipoic acid	-	PDH, α-KG DH, Glycine cleavage system	-
Mg²⁺	-	Hexokinase, PFK-1, Pyruvate Kinase, all ATPases	-
Mn²⁺	-	Isocitrate DH, Arginase	-

MASTER RATE-LIMITING STEPS TABLE

Pathway	Rate-Limiting Enzyme	Key Activators	Key Inhibitors
Glycolysis	PFK-1	AMP, F-2,6-BP, insulin	ATP, citrate, glucagon
Gluconeogenesis	Fructose-1,6-bisphosphatase	ATP, glucagon	AMP, F-2,6-BP, insulin
Glycogenesis	Glycogen Synthase	G-6-P, insulin, dephosphorylation	Glucagon/Epi (phosphorylation)
Glycogenolysis	Glycogen Phosphorylase	AMP, Ca²⁺, epinephrine, glucagon	G-6-P, glucose, insulin, ATP
TCA Cycle	Isocitrate Dehydrogenase	ADP, Ca²⁺, NAD⁺	ATP, NADH
Pyruvate → Acetyl-CoA	Pyruvate Dehydrogenase Complex	Ca²⁺, ADP, NAD⁺, CoA, insulin	Acetyl-CoA, NADH, ATP, PDH kinase
Pentose Phosphate Pathway	G6PD	NADP⁺	NADPH
Fatty Acid Synthesis	Acetyl-CoA Carboxylase	Citrate, insulin	Palmitoyl-CoA, glucagon, epinephrine, malonyl-CoA (feedback)
Beta-Oxidation	CPT-I (entry)	Glucagon, fasting	Malonyl-CoA
Ketogenesis	HMG-CoA Synthase (mito)	Fasting, low insulin	Fed state, insulin
Cholesterol Synthesis	HMG-CoA Reductase	Insulin, thyroxine	Glucagon, statins, cholesterol (SREBP)
Urea Cycle	CPS-I	N-Acetylglutamate	-
Fatty Acid Elongation (Mito)	Thiolase (in reverse)	-	-

INTER-CYCLE CONNECTIONS (HIGHLY TESTED)

Connection	Mechanism	Clinical Relevance
Urea Cycle + TCA	Fumarate produced in urea cycle enters TCA; OAA → aspartate for urea cycle	Hyperammonemia depletes TCA intermediates; liver failure affects both
Glycolysis + PPP	G-6-P is the branch point; competition for G-6-P	G6PD deficiency: G-6-P shunted to glycolysis only
FA Synthesis + Cholesterol	Both use Acetyl-CoA + NADPH + HMG-CoA	Statins inhibit only cholesterol synthesis step (HMG-CoA reductase)
β-Oxidation + Ketogenesis	FA oxidation generates acetyl-CoA → ketone bodies when oxaloacetate is limiting	Starvation/DM1: OAA used for gluconeogenesis → acetyl-CoA cannot enter TCA → ketogenesis
Glycolysis + FA Synthesis	Acetyl-CoA for FA synthesis comes from glucose via PDH	High carb diet → ↑ lipogenesis; role of citrate shuttle
Cori Cycle	Lactate (muscle) → glucose (liver)	Shuttles energy; requires gluconeogenesis in liver
Methionine Cycle + Folate Cycle	Methionine synthase (B12) uses MTHF to regenerate methionine from Hcy	B12 deficiency = folate trap; both → megaloblastic anemia
Propionyl-CoA + TCA	Odd-chain FA/BCAA → Propionyl-CoA → Methylmalonyl-CoA → Succinyl-CoA	B12 deficiency blocks this step → methylmalonic acidemia
Glucose-Alanine Cycle	Muscle sends alanine → liver → glucose returned	Reduces NH₃ toxicity; maintains glucose during fasting

CLINICAL CROSS-CORRELATION TABLE (MULTI-SUBJECT - NEET PG / USMLE)

Metabolic Defect	Biochemistry	Medicine/Pediatrics	Surgery/Obs	Pharmacology
PDH Deficiency / B1 deficiency	Pyruvate → acetyl-CoA blocked	Wernicke-Korsakoff, beriberi	Bariatric surgery → B1 deficiency	Give IV thiamine before glucose in Wernicke's
G6PD Deficiency	PPP impaired, ↓ NADPH	Hemolytic anemia triggers: infections, drugs	-	Avoid primaquine, dapsone, sulfonamides, nitrofurantoin
HMG-CoA Reductase (statins)	↓ cholesterol synthesis	Familial hypercholesterolemia, statin myopathy	Pre-op lipid management	Statin myopathy: ↑ CK, rhabdomyolysis; monitor LFTs
Glycogen storage (Von Gierke)	Glucose-6-phosphatase	Fasting hypoglycemia, hepatomegaly	-	Cornstarch therapy (slow glucose release)
Urea Cycle (OTC)	X-linked; NH₃ buildup	Encephalopathy in neonates	-	Sodium benzoate, sodium phenylbutyrate, rifaximin
Fatty acid oxidation (MCAD)	No ketones during fasting	Reye-like presentation, sudden death	Identified on newborn screen	Avoid fasting; carnitine supplementation debated
DKA	Insulin deficiency → ketosis	Type 1 DM management, osmotic diuresis	DKA in surgery/trauma patient	Insulin + K⁺ replacement; nitroprusside test only detects acetoacetate
Phenylketonuria	Phe hydroxylase	Intellectual disability if untreated	Maternal PKU → fetal malformations	Low-Phe diet; tetrahydrobiopterin (BH4) for responsive cases; pegvaliase
Homocystinuria	CBS deficiency	Thromboembolic events, lens dislocation	DVT risk in surgery	Pyridoxine, betaine, folate
Gaucher Disease	β-Glucocerebrosidase	Bone crises, pancytopenia, Gaucher cells	Splenomegaly requiring splenectomy	ERT (imiglucerase); SRT (miglustat)
Acute intermittent porphyria	ALA synthase dysregulation	Episodic neurovisceral attacks	Avoid surgery triggers (barbiturates, fasting)	Attacks triggered by: cytochrome P450 inducers, fasting, estrogens

STORAGE DISORDER SUMMARY TABLE - CARBOHYDRATES

Disease	Type	Enzyme	Storage Material	Organs	Treatment
Von Gierke (GSD I)	GSD	Glucose-6-phosphatase	Glycogen + Fat (liver/kidney)	Liver, Kidney	Cornstarch, allopurinol, ACE-I for nephropathy
Pompe (GSD II)	GSD (lysosomal)	Acid Maltase (α-1,4-glucosidase)	Glycogen in lysosomes	All tissues (heart, muscle)	Alglucosidase alfa (ERT)
Cori/Forbes (GSD III)	GSD	Debranching enzyme	Limit dextrin	Liver, Muscle	High-protein diet
Andersen (GSD IV)	GSD	Branching enzyme	Amylopectin-like glycogen	Liver	Liver transplant
McArdle (GSD V)	GSD	Muscle phosphorylase	Glycogen in muscle	Muscle	Sucrose before exercise, high protein diet
Galactosemia	Sugar metabolism	Gal-1-P uridyltransferase	Gal-1-P, Galactitol	Liver, Brain, Lens	Galactose-free diet (no milk)
HFI	Sugar metabolism	Aldolase B	Fructose-1-P	Liver, Kidney	Fructose/sucrose/sorbitol-free diet

STORAGE DISORDER SUMMARY TABLE - LIPIDS (Sphingolipidoses)

Disease	Enzyme	Stored Lipid	Key Feature	Inheritance	Therapy
Gaucher I	β-Glucocerebrosidase	Glucocerebroside	Crinkled-paper cells, no CNS	AR	ERT (imiglucerase), SRT (miglustat)
Niemann-Pick A/B	Sphingomyelinase	Sphingomyelin	Cherry-red spot (A), foam cells	AR	No proven ERT for A; miglustat for C
Tay-Sachs	Hexosaminidase A	GM2 ganglioside	Cherry-red spot, NO organomegaly	AR	Supportive
Sandhoff	Hex A + B	GM2 + globoside	Like Tay-Sachs + organomegaly	AR	Supportive
Krabbe	Galactocerebrosidase	Galactocerebroside	Globoid cells, peripheral neuropathy	AR	HSCT (early)
MLD	Arylsulfatase A	Sulfatide	Metachromatic deposits, demyelination	AR	HSCT, gene therapy
Fabry	α-Galactosidase A	Globotriaosylceramide	Angiokeratomas, renal failure	X-linked	ERT (agalsidase), migalastat
Farber	Ceramidase	Ceramide	Hoarse cry, joint nodules	AR	Supportive
Wolman	Lysosomal acid lipase	Cholesterol esters + TG	Adrenal calcification	AR	Sebelipase alfa

STORAGE DISORDER SUMMARY TABLE - AMINO ACID / PROTEIN

Disease	Defect	Stored/Accumulated	Key Feature	Inheritance
PKU	Phenylalanine hydroxylase	Phenylalanine, phenylpyruvate	Intellectual disability, musty odor, fair features	AR
Alcaptonuria	Homogentisate oxidase	Homogentisic acid	Black urine, ochronosis, arthritis	AR
Homocystinuria	Cystathionine β-synthase	Homocysteine	Marfanoid, lens dislocation (DOWN), thrombosis	AR
MSUD	BCKA dehydrogenase	Leu, Ile, Val + keto-acids	Maple syrup urine, neurological crisis	AR
Tyrosinemia I	FAH	Succinylacetone	Liver failure, renal tubular acidosis, ↑ AFP	AR
Methylmalonic Acidemia	Methylmalonyl-CoA mutase	Methylmalonic acid	Metabolic acidosis, hyperammonemia	AR
Propionic Acidemia	Propionyl-CoA carboxylase	Propionic acid	Severe acidosis, B12 unresponsive	AR

EXTRA HIGH-YIELD POINTS FOR NEET-PG / INICET

Unique Enzyme Locations:

PDH complex: inner mitochondrial membrane (matrix side)
Fatty acid synthase: cytoplasm
HMG-CoA synthase: cytoplasm (cholesterol) vs mitochondria (ketones) - TWO separate enzymes!
Glucose-6-phosphatase: ER lumen (only in liver, kidney, intestine)
Succinate dehydrogenase: only TCA enzyme on inner mitochondrial membrane (also Complex II of ETC)

"Two HMG-CoA Synthases" - KEY DISTINCTION:

Mitochondrial HMG-CoA Synthase → Ketogenesis (in liver mito)
Cytoplasmic HMG-CoA Synthase → Cholesterol synthesis (in liver cytoplasm/ER)

Glucagon vs Insulin Effects:

Effect	Glucagon	Insulin
Blood glucose	↑	↓
Glycolysis	↓ (↓ PFK-1 via ↓ F-2,6-BP)	↑
Gluconeogenesis	↑	↓
Glycogenesis	↓	↑
Glycogenolysis	↑	↓
Fatty acid synthesis	↓	↑
Beta-oxidation	↑	↓
Ketogenesis	↑	↓
Lipolysis (HSL)	↑	↓
Protein catabolism	↑	↓

The "Well-Fed" State: Insulin dominates - glucose oxidized, glycogen formed, fatty acids synthesized, proteins synthesized

The "Fasted" State: Glucagon dominates - glycogenolysis, gluconeogenesis, fatty acid oxidation, ketogenesis, protein catabolism for gluconeogenic substrates

Sources: Harper's Illustrated Biochemistry 32nd Ed, Lippincott's Biochemistry 8th Ed, Harrison's Principles of Internal Medicine 22nd Ed, Guyton & Hall Medical Physiology, Adams & Victor's Neurology 12th Ed, Basic Medical Biochemistry - Lieberman & Ricer 6th Ed, Mulholland & Greenfield's Surgery 7th EdYour credits are exhausted. Your daily credits reset tomorrow, or you can purchase a top-up for more.

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