Summary of carbohydrate metabolism and enzyme in short in like 3-4 page combined for my viva exam today

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Carbohydrate Metabolism — Viva Summary

Sources: Basic Medical Biochemistry 6e, Guyton & Hall Medical Physiology, Lippincott Biochemistry 8e

1. Overview

Carbohydrate metabolism encompasses pathways for the oxidation, storage, and synthesis of glucose. The major pathways are:

Pathway	Location	Purpose
Glycolysis	Cytosol	Glucose → Pyruvate + ATP
TCA (Krebs) Cycle	Mitochondrial matrix	Acetyl-CoA → CO₂ + NADH
Oxidative Phosphorylation	Inner mitochondrial membrane	NADH/FADH₂ → ATP
Glycogenesis	Cytosol (liver, muscle)	Glucose → Glycogen (storage)
Glycogenolysis	Cytosol	Glycogen → Glucose
Gluconeogenesis	Liver (mainly)	Non-carbohydrate → Glucose
Pentose Phosphate Pathway	Cytosol	Glucose-6-P → NADPH + Ribose-5-P

2. Glycolysis

Location: Cytosol | Net yield: 2 ATP (anaerobic), more via aerobic continuation

Glucose is the major carbohydrate fuel — provides ≥50% of calories in most diets.

Two Phases:

Preparative phase (energy investment): 2 ATP consumed

Glucose → Glucose-6-phosphate → Fructose-6-phosphate → Fructose-1,6-bisphosphate → 2× Glyceraldehyde-3-phosphate (G3P)

ATP-generating phase (energy payoff): 4 ATP produced

G3P → 1,3-bisphosphoglycerate → 3-phosphoglycerate → 2-phosphoglycerate → Phosphoenolpyruvate (PEP) → Pyruvate

Net per glucose: 2 ATP + 2 NADH + 2 Pyruvate

Key Regulated Enzymes (Irreversible Steps):

Step	Enzyme	Regulators
Glucose → Glucose-6-P	Hexokinase (muscle) / Glucokinase (liver)	Hexokinase: inhibited by G-6-P; Glucokinase: high Km, not inhibited by product, induced by insulin
Fructose-6-P → Fructose-1,6-bisP	Phosphofructokinase-1 (PFK-1) — rate-limiting	Activated by AMP, fructose-2,6-bisP; inhibited by ATP, citrate
PEP → Pyruvate	Pyruvate Kinase	Activated by fructose-1,6-bisP; inhibited by ATP, alanine

3. Fate of Pyruvate

Condition	Fate	Enzyme
Aerobic	Pyruvate → Acetyl-CoA (enters TCA)	Pyruvate dehydrogenase complex (PDC)
Anaerobic	Pyruvate → Lactate (regenerates NAD⁺)	Lactate dehydrogenase (LDH)
Gluconeogenesis	Pyruvate → Oxaloacetate	Pyruvate carboxylase
Lipogenesis	Pyruvate → Acetyl-CoA → Fatty acids	PDC + fatty acid synthase

Pyruvate Dehydrogenase Complex (PDC):

Cofactors: Thiamine (B₁), Lipoic acid, FAD (B₂), NAD⁺ (B₃), CoA (B₅)
Mnemonic: "The Lovely Foolish Nancy Comes"
Inhibited by: NADH, Acetyl-CoA, ATP
Activated by: NAD⁺, CoA, AMP, Ca²⁺

4. TCA Cycle (Krebs / Citric Acid Cycle)

Location: Mitochondrial matrix For each Acetyl-CoA (×2 per glucose): 3 NADH + 1 FADH₂ + 1 GTP + 2 CO₂

Steps and Enzymes:

Reaction	Enzyme	Notes
Oxaloacetate + Acetyl-CoA → Citrate	Citrate synthase	Condensation; inhibited by ATP, NADH
Citrate → Isocitrate	Aconitase	Via cis-aconitate
Isocitrate → α-Ketoglutarate + CO₂	Isocitrate dehydrogenase	1st NADH; activated by ADP, Ca²⁺; rate-limiting
α-Ketoglutarate → Succinyl-CoA + CO₂	α-Ketoglutarate dehydrogenase	2nd NADH; requires same cofactors as PDC
Succinyl-CoA → Succinate	Succinyl-CoA synthetase	GTP produced (substrate-level phosphorylation)
Succinate → Fumarate	Succinate dehydrogenase	FADH₂ produced; inhibited by malonate
Fumarate → Malate	Fumarase	Hydration
Malate → Oxaloacetate	Malate dehydrogenase	3rd NADH; regenerates OAA

Per glucose (×2 cycles): 6 NADH + 2 FADH₂ + 2 GTP

5. Oxidative Phosphorylation

Location: Inner mitochondrial membrane

Each NADH → 2.5 ATP
Each FADH₂ → 1.5 ATP

Total ATP yield per glucose (aerobic):

Glycolysis: 2 ATP + 2 NADH (≈5 ATP via shuttle) = ~7
PDC: 2 NADH = ~5 ATP
TCA: 6 NADH (15) + 2 FADH₂ (3) + 2 GTP = 20 ATP
Grand total: ~30–32 ATP

6. Glycogenesis (Glycogen Synthesis)

Activated by: Insulin (fed state) | Location: Liver (glucose buffer) + Muscle (local fuel)

Steps:

Glucose → Glucose-6-P — Glucokinase/Hexokinase
Glucose-6-P → Glucose-1-P — Phosphoglucomutase
Glucose-1-P + UTP → UDP-Glucose + PPi — UDP-glucose pyrophosphorylase
UDP-Glucose added to glycogen chain (α-1,4 bonds) — Glycogen synthase (key enzyme)
Branching at every 8–10 residues (α-1,6 bonds) — Branching enzyme (amylo-4,6-transglucosidase)

Glycogen synthase: Active (dephosphorylated, insulin); Inactive (phosphorylated, glucagon/adrenaline)

7. Glycogenolysis (Glycogen Breakdown)

Activated by: Glucagon (liver), Epinephrine (liver + muscle) — via cAMP → PKA cascade

Steps:

Glycogen → Glucose-1-P — Glycogen phosphorylase (cleaves α-1,4 bonds) — key enzyme
Debrancher enzyme removes α-1,6 branches → free glucose + glucose-1-P
Glucose-1-P → Glucose-6-P — Phosphoglucomutase
Liver only: Glucose-6-P → Free glucose — Glucose-6-phosphatase (releases glucose into blood)
- Muscle lacks glucose-6-phosphatase → glucose-6-P enters glycolysis directly

Glycogen phosphorylase: Activated by phosphorylation (glucagon/adrenaline), AMP; Inhibited by glucose-6-P, ATP

8. Gluconeogenesis

Location: Mainly liver; also kidney cortex Precursors: Lactate, amino acids (alanine, glutamine), glycerol, odd-chain fatty acids (propionyl-CoA) Activated by: Glucagon, cortisol (fasting/stress)

Gluconeogenesis is essentially the reverse of glycolysis, except at 3 irreversible steps bypassed by unique enzymes:

Glycolysis (irreversible)	Bypass Enzyme(s) in Gluconeogenesis
Pyruvate → PEP (pyruvate kinase)	Pyruvate carboxylase (Pyruvate → OAA) + PEPCK (OAA → PEP)
Fructose-6-P → Fructose-1,6-bisP (PFK-1)	Fructose-1,6-bisphosphatase (FBPase-1)
Glucose → Glucose-6-P (hexokinase)	Glucose-6-phosphatase (liver only)

Pyruvate carboxylase requires biotin (CO₂ carrier) and is activated by acetyl-CoA
PEPCK requires GTP

9. Pentose Phosphate Pathway (PPP)

Location: Cytosol | Tissue: Liver, adipose, RBCs, adrenal cortex

Functions:

Generates NADPH (for lipid synthesis, antioxidant defense via glutathione)
Generates Ribose-5-phosphate (for nucleotide synthesis)
Accounts for 5–10% of hepatic glucose metabolism

Key enzyme: Glucose-6-phosphate dehydrogenase (G6PD)

Irreversible, rate-limiting
Deficiency → hemolytic anemia on oxidant exposure (primaquine, fava beans)

10. Hormonal Regulation at a Glance

Hormone	State	Effect
Insulin	Fed (↑ glucose)	↑ Glycolysis, ↑ Glycogenesis, ↑ PPP; ↓ Gluconeogenesis, ↓ Glycogenolysis
Glucagon	Fasting (↓ glucose)	↑ Glycogenolysis, ↑ Gluconeogenesis; ↓ Glycolysis, ↓ Glycogenesis
Epinephrine	Stress	↑ Glycogenolysis (liver + muscle), ↑ Glycolysis (muscle)
Cortisol	Prolonged stress	↑ Gluconeogenesis (induces PEPCK), ↑ amino acid release from muscle

11. Quick-Reference: Key Enzymes Summary

Enzyme	Pathway	Regulation
Glucokinase / Hexokinase	Glycolysis	Glucokinase: induced by insulin; high Km
PFK-1 (rate-limiting)	Glycolysis	↑ AMP, fructose-2,6-bisP; ↓ ATP, citrate
Pyruvate Kinase	Glycolysis	↑ F-1,6-bisP; ↓ ATP, alanine
Pyruvate Dehydrogenase	Pyruvate → Acetyl-CoA	↑ AMP, Ca²⁺; ↓ NADH, Acetyl-CoA
Citrate Synthase	TCA	↓ ATP, NADH
Isocitrate Dehydrogenase (rate-limiting)	TCA	↑ ADP, Ca²⁺; ↓ NADH, ATP
α-KG Dehydrogenase	TCA	↓ Succinyl-CoA, NADH
Glycogen Synthase	Glycogenesis	Active when dephosphorylated (insulin)
Glycogen Phosphorylase	Glycogenolysis	Active when phosphorylated (glucagon/adrenaline)
Pyruvate Carboxylase	Gluconeogenesis	↑ Acetyl-CoA (biotin-dependent)
PEPCK	Gluconeogenesis	Induced by glucagon/cortisol
FBPase-1	Gluconeogenesis	↓ AMP, fructose-2,6-bisP
G6PD	Pentose Phosphate	Rate-limiting; NADP⁺ activates

12. Clinically Important Points for Viva

Lactate acidosis: From anaerobic glycolysis; e.g., shock, metformin overdose
G6PD deficiency: X-linked; hemolysis with oxidant drugs (primaquine, dapsone)
Von Gierke disease: Glucose-6-phosphatase deficiency → can't release glucose from liver → severe fasting hypoglycemia
McArdle disease: Muscle glycogen phosphorylase deficiency → muscle cramps on exercise
Pompe disease: Lysosomal acid maltase (α-1,4 glucosidase) deficiency → glycogen accumulation in all organs
Pyruvate dehydrogenase deficiency: Lactic acidosis + neurological signs; treat with thiamine
Fructose-2,6-bisphosphate: Master regulator — stimulates PFK-1 (glycolysis), inhibits FBPase-1 (gluconeogenesis); made by PFK-2, activated by insulin

Basic Medical Biochemistry: A Clinical Approach, 6e | Guyton & Hall Medical Physiology | Lippincott Illustrated Reviews: Biochemistry, 8e

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Enzymes — Viva Summary

Sources: Lippincott Illustrated Reviews: Biochemistry 8e, Harper's Illustrated Biochemistry 32e, Basic Medical Biochemistry 6e

1. Definition & General Properties

Enzymes are biological catalysts — mostly proteins (except ribozymes, which are RNA)
They accelerate reaction rates by lowering the activation energy (ΔG‡) without being consumed
They do not alter the equilibrium constant (Keq) or the standard free energy change (ΔG°) of a reaction
They are highly specific — substrate specificity and reaction specificity
They emerge unchanged at the end of the reaction

2. Classification (IUB System — 6 Classes)

Class	Type	Action	Example
1. Oxidoreductases	Redox reactions	Transfer e⁻ / H	LDH, G6PD, Cytochrome oxidase
2. Transferases	Group transfer	Move functional groups	Aminotransferases (ALT, AST), Kinases
3. Hydrolases	Hydrolysis	Break bonds with H₂O	Lipase, Amylase, Peptidase
4. Lyases	Group elimination	Add/remove groups (no H₂O)	Aldolase, Decarboxylases
5. Isomerases	Isomerization	Interconvert isomers	Phosphoglucose isomerase, Mutases
6. Ligases	Bond formation	Join molecules using ATP	Pyruvate carboxylase, Glutamine synthetase

3. Active Site & Substrate Binding

Active site: Surface cleft or pocket on enzyme where substrate binds and catalysis occurs
Contains: binding residues (hold substrate) + catalytic residues (break/make bonds)
Active site provides: proximity, orientation, acid-base catalysis, electrostatic stabilization, covalent intermediates

Two Models of Enzyme-Substrate Binding:

Model	Description
Lock & Key (Fischer)	Rigid complementarity — substrate fits pre-formed active site exactly
Induced Fit (Koshland)	Enzyme changes shape on substrate binding — active site molds around substrate (more widely accepted)

4. Enzyme Kinetics

Michaelis-Menten Equation:

$$v_i = \frac{V_{max}[S]}{K_m + [S]}$$

Term	Meaning
v (initial velocity)	Rate of reaction at a given [S]
Vmax	Maximum velocity when all enzyme is saturated with substrate
Km (Michaelis constant)	[S] at which v = Vmax/2; measure of enzyme-substrate affinity
Kcat (turnover number)	Number of substrate molecules converted per enzyme per second
Kcat/Km	Catalytic efficiency

Low Km = high affinity for substrate High Km = low affinity (needs more substrate to reach half Vmax)

Lineweaver-Burk Plot (Double Reciprocal):

$$\frac{1}{v} = \frac{K_m}{V_{max}} \cdot \frac{1}{[S]} + \frac{1}{V_{max}}$$

X-intercept = −1/Km
Y-intercept = 1/Vmax
Slope = Km/Vmax

Used to determine Km and Vmax, and to distinguish inhibition types

5. Factors Affecting Enzyme Activity

Factor	Effect
Temperature	Activity ↑ with temperature up to ~37–40°C; denatures above ~45–55°C (humans); Q10 ≈ 2
pH	Each enzyme has optimal pH (e.g., pepsin pH 2, trypsin pH 8, most enzymes ~7.4); extremes denature enzyme
Substrate concentration	Follows Michaelis-Menten; rate increases then plateaus at Vmax
Enzyme concentration	Rate directly proportional (when substrate is in excess)
Product concentration	High [P] can slow reaction (product inhibition)

6. Enzyme Inhibition

A. Reversible Inhibition

Type	Mechanism	Effect on Vmax	Effect on Km	Lineweaver-Burk
Competitive	Inhibitor binds active site (competes with substrate); overcome by ↑[S]	Unchanged	↑ (apparent)	Lines intersect at Y-axis (same Vmax, different X-intercept)
Noncompetitive	Inhibitor binds allosteric site (not active site); cannot be overcome by ↑[S]	↓	Unchanged	Lines intersect at X-axis (same Km, different Y-intercept)
Uncompetitive	Inhibitor binds only ES complex	↓	↓	Parallel lines (same slope)
Mixed	Binds both free enzyme and ES complex	↓	↑ or ↓	Lines intersect in 2nd quadrant

Clinical examples:

Competitive: Statins (HMG-CoA reductase), Methotrexate (DHFR), Aspirin (COX), Sulfonamides (DHPS)
Noncompetitive: Heavy metals (Pb, Hg) binding away from active site

B. Irreversible Inhibition

Inhibitor forms covalent bond with enzyme → permanent inactivation
Examples:
- Aspirin → irreversibly acetylates COX-1/COX-2 (cyclooxygenase)
- Organophosphates → irreversibly inhibit acetylcholinesterase (AChE)
- Lead (Pb) → reacts with –SH groups of cysteine; inhibits ferrochelatase (blocks heme synthesis)
- Penicillin → irreversibly inhibits transpeptidase (cell wall synthesis)
- Sarin (nerve gas) → irreversibly inhibits AChE

7. Regulation of Enzyme Activity

A. Allosteric Regulation

Effector binds allosteric site (away from active site) → conformational change
Positive effector (activator): ↑ enzyme activity
Negative effector (inhibitor): ↓ enzyme activity
Allosteric enzymes often show sigmoidal kinetics (not Michaelis-Menten) — described by the Hill equation
Hill coefficient (n): n > 1 = positive cooperativity; n = 1 = no cooperativity
Example: PFK-1 — activated by AMP/fructose-2,6-bisP; inhibited by ATP/citrate

B. Covalent Modification (Phosphorylation/Dephosphorylation)

Phosphorylated State	Effect
Glycogen phosphorylase	Active
Glycogen synthase	Inactive
PFK-2 (kinase domain)	Active (makes fructose-2,6-bisP → stimulates glycolysis)
FBPase-2 (phosphatase domain)	Active when phosphorylated (breaks down fructose-2,6-bisP)

Kinases add phosphate (ATP → ADP) — phosphorylation
Phosphatases remove phosphate — dephosphorylation

C. Zymogen Activation (Proteolytic Cleavage)

Enzymes secreted as inactive precursors (zymogens) → activated by cleavage
Examples:

Zymogen	Active Form	Site
Pepsinogen	Pepsin	Stomach
Trypsinogen	Trypsin	Duodenum (enterokinase)
Chymotrypsinogen	Chymotrypsin	Duodenum (trypsin)
Proelastase	Elastase	Pancreas
Prothrombin	Thrombin	Blood (coagulation)
Plasminogen	Plasmin	Blood (fibrinolysis)

D. Induction / Repression (Gene-Level)

Hormones (insulin, glucagon, cortisol) regulate gene transcription of enzymes
Example: Glucagon induces PEPCK (gluconeogenesis); Insulin induces glucokinase, fatty acid synthase

8. Cofactors & Coenzymes

Many enzymes require non-protein components for activity

Term	Description
Cofactor	Non-protein component (inorganic = metal ions; organic = coenzyme)
Coenzyme	Organic cofactor, often derived from vitamins; loosely bound
Prosthetic group	Tightly/covalently bound cofactor
Apoenzyme	Enzyme without its cofactor (inactive)
Holoenzyme	Apoenzyme + cofactor (active)

Key Coenzymes (Vitamin Derivatives):

Coenzyme	Vitamin	Function	Example Enzyme
NAD⁺/NADH	Niacin (B₃)	Electron/H⁻ carrier	LDH, Malate DH, isocitrate DH
NADP⁺/NADPH	Niacin (B₃)	Reductive biosynthesis, antioxidant	G6PD, Fatty acid synthase
FAD/FADH₂	Riboflavin (B₂)	Electron carrier	Succinate DH, α-KG DH
FMN	Riboflavin (B₂)	Electron carrier	NADH dehydrogenase (Complex I)
Thiamine pyrophosphate (TPP)	Thiamine (B₁)	Decarboxylation, transketolase	PDC, α-KG DH, Transketolase
Pyridoxal phosphate (PLP)	Pyridoxine (B₆)	Transamination, decarboxylation	ALT, AST, Amino acid decarboxylases
Coenzyme A (CoA)	Pantothenic acid (B₅)	Acyl group carrier	PDC, TCA cycle, fatty acid synthesis
Biotin	Biotin (B₇)	CO₂ carrier (carboxylation)	Pyruvate carboxylase, ACC
Tetrahydrofolate (THF)	Folic acid (B₉)	One-carbon transfer	Thymidylate synthase
Methylcobalamin	B₁₂	Methyl transfer, isomerization	Methionine synthase
Lipoic acid	(not a vitamin)	Acyl carrier	PDC, α-KG DH

Key Metal Ion Cofactors:

Metal	Enzymes
Zn²⁺	Carbonic anhydrase, Alcohol DH, Carboxypeptidase, DNA polymerase
Fe²⁺/Fe³⁺	Cytochromes, Catalase, Peroxidase, Ferrochelatase
Cu²⁺	Cytochrome c oxidase, Ceruloplasmin, Lysyl oxidase
Mg²⁺	Kinases (ATP-Mg²⁺), Enolase, Phosphatases
Mn²⁺	Arginase, Pyruvate carboxylase, SOD (mitochondria)
Se	Glutathione peroxidase

9. Isoenzymes (Isozymes)

Multiple forms of the same enzyme catalyzing the same reaction but differing in:
- Physical/chemical properties (electrophoretic mobility, Km, pH optimum)
- Tissue distribution
- Encoded by different genes or different subunit combinations

Clinically Important Isoenzymes:

Enzyme	Isoforms	Clinical Use
Lactate Dehydrogenase (LDH)	LDH1 (heart), LDH2, LDH3, LDH4, LDH5 (liver)	LDH1 > LDH2 = "flipped ratio" → MI; LDH5 ↑ in liver disease
Creatine Kinase (CK)	CK-MM (muscle), CK-MB (heart), CK-BB (brain)	CK-MB ↑ in myocardial infarction; CK-MM ↑ in muscular dystrophy
Alkaline Phosphatase (ALP)	Liver, bone, placenta, intestine	Liver ALP ↑ in cholestasis; bone ALP ↑ in Paget's disease
Amylase	Salivary (S-type), Pancreatic (P-type)	P-amylase ↑ in pancreatitis
Hexokinase vs. Glucokinase	Both phosphorylate glucose	Hexokinase: low Km (brain, RBC); Glucokinase: high Km (liver, β-cells)

10. Enzymes as Diagnostic Markers

Enzyme	↑ In
CK-MB	Myocardial infarction
Troponin I/T (not enzyme but protein)	MI (more specific)
AST, ALT	Hepatitis, liver disease (ALT more specific)
ALP	Cholestasis, bone disease, pregnancy
GGT	Alcohol abuse, cholestasis
LDH	MI, hemolysis, liver disease
Amylase, Lipase	Acute pancreatitis (Lipase more specific)
PSA (serine protease)	Prostate cancer
Acid phosphatase	Prostate cancer (now replaced by PSA)

11. Enzyme Deficiency Diseases (Viva High-Yield)

Disease	Deficient Enzyme	Result
Phenylketonuria (PKU)	Phenylalanine hydroxylase	↑ Phe → intellectual disability
Albinism	Tyrosinase	No melanin
Alkaptonuria	Homogentisate oxidase	Homogentisic acid in urine (turns black)
G6PD deficiency	Glucose-6-P dehydrogenase	Hemolytic anemia with oxidants
Von Gierke	Glucose-6-phosphatase	Fasting hypoglycemia, hepatomegaly
Gaucher's	β-Glucocerebrosidase	Glucocerebroside accumulation
Tay-Sachs	Hexosaminidase A	GM2 ganglioside accumulation → neurodegeneration
Niemann-Pick	Sphingomyelinase	Sphingomyelin accumulation
Lesch-Nyhan	HGPRT (hypoxanthine-guanine phosphoribosyl transferase)	Hyperuricemia, self-mutilation
SCID	Adenosine deaminase (ADA)	Toxic deoxyadenosine → lymphocyte death
OTC deficiency	Ornithine transcarbamylase	Hyperammonemia

12. Quick Summary — Key Regulatory Concepts

Concept	Key Point
Committed step	First irreversible step; usually the rate-limiting step; primary target of feedback inhibition
Feedback inhibition	End-product inhibits the first/committed enzyme (e.g., ATP inhibits PFK-1)
Feedforward activation	Early substrate activates a downstream enzyme (e.g., fructose-1,6-bisP activates pyruvate kinase)
Substrate concentration	Km tells you how much substrate is needed; allosteric enzymes show sigmoidal response
Cooperativity	Multiple binding sites interact; first binding facilitates subsequent binding (positive cooperativity)
Cascade amplification	One signal molecule activates many enzymes via kinase cascades (e.g., cAMP → PKA → phosphorylase kinase → glycogen phosphorylase)

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

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