---

BIOCHEMISTRY - THEORY PAPER-I | Complete Solutions

---

CASE SCENARIO 1

---

Q1. What is the diagnosis? (2 marks)

• Normal blood glucose = 70-110 mg/dL (fasting)
• 42 mg/dL is well below the threshold of 70 mg/dL, confirming hypoglycemia
• Classic Whipple's Triad is satisfied:
  1. Symptoms of hypoglycemia (sweating, tremors, confusion, LOC)
  2. Low blood glucose documented (42 mg/dL)
  3. Relief of symptoms with glucose administration

---

Q2. Which hormones help restore blood glucose during hypoglycemia? (2 marks)

Glucagon and epinephrine act within minutes; growth hormone and cortisol act over hours.

---

Q3. Explain Glycogenolysis and Gluconeogenesis (7 marks)

A. GLYCOGENOLYSIS (Breakdown of Glycogen to Release Glucose)

1. Glycogen phosphorylase (activated by glucagon/epinephrine via cAMP-PKA pathway) cleaves alpha-1,4 glycosidic bonds releasing Glucose-1-phosphate
2. Phosphoglucomutase converts Glucose-1-phosphate → Glucose-6-phosphate
3. In the liver: Glucose-6-phosphatase (absent in muscle) converts Glucose-6-phosphate → Free Glucose → released into blood
4. Branch points (alpha-1,6 bonds) are removed by Debranching enzyme (has two activities: glucan transferase + alpha-1,6 glucosidase), releasing free glucose directly

• Glucagon and epinephrine activate glycogenolysis via cAMP → PKA → phosphorylation of glycogen phosphorylase kinase → activation of phosphorylase
• Insulin inhibits this pathway

---

B. GLUCONEOGENESIS (Synthesis of New Glucose from Non-Carbohydrate Precursors)

• Lactate (from anaerobic glycolysis in RBCs, muscle) - most important
• Amino acids (esp. Alanine, Glutamine - glucogenic AAs)
• Glycerol (from lipolysis in adipose tissue)
• Propionate (from odd-chain fatty acid oxidation)

• Stimulated by: Glucagon, cortisol, glucocorticoids, fasting
• Inhibited by: Insulin, high AMP/ADP
• Cori Cycle: Lactate from muscle → liver → converted to glucose → back to muscle

---

Q4. Why does the brain become affected first during hypoglycemia? (4 marks)

1. Obligate glucose dependence: The brain normally uses glucose as its sole energy substrate (unlike muscle, liver, heart which can use fatty acids and ketones readily). Fatty acids cannot cross the blood-brain barrier.
2. No glucose storage: The brain has virtually no glycogen stores and cannot perform significant gluconeogenesis.
3. High energy demand: The brain accounts for ~20% of the body's total energy consumption (consumes ~120 g of glucose/day), despite being only 2% of body weight.
4. No GLUT4 transporters: Brain glucose transport (GLUT1 and GLUT3) is not insulin-dependent; uptake depends directly on blood glucose concentration. When blood glucose falls, cerebral glucose uptake falls proportionally.
5. Hierarchical sensitivity: As glucose falls below 60 mg/dL, cognitive impairment begins; below 50 mg/dL, confusion; below 40 mg/dL, seizures/coma - reflecting total dependence on continuous glucose supply.
6. In prolonged fasting/starvation, the brain can adapt to use ketone bodies, but this adaptation takes days and is incomplete.

---

LONG ESSAY QUESTION 1

(a) Define Enzymes and Classify Them with Suitable Examples (3 marks)

---

(b) Mechanism of Enzyme Action - Lock-and-Key and Induced-Fit Models (3 marks)

1. Lock-and-Key Model (Fischer, 1894)

• The enzyme's active site has a rigid, preformed shape complementary to the substrate
• The substrate (key) fits exactly into the active site (lock) - rigid geometric complementarity
• Limitation: Cannot explain why enzymes can bind substrate analogs (competitive inhibitors), or why the active site becomes more reactive upon substrate binding

2. Induced-Fit Model (Koshland, 1958) - Currently Accepted

• The active site is flexible, not rigid
• Upon substrate binding, the enzyme undergoes conformational change (shape change) that optimally positions catalytic residues around the substrate
• The binding energy of enzyme-substrate interaction drives this conformational change, which also contributes to catalysis
• Better explains: enzyme flexibility, binding of transition state analogs, allosteric regulation, the dynamic nature of enzyme catalysis

• Acid-base catalysis: Amino acid side chains donate/accept protons
• Covalent catalysis: Transient covalent bond with substrate (e.g., serine proteases)
• Metal ion catalysis: Metal ions (Zn²⁺, Mg²⁺) stabilize charges or activate water
• Proximity and orientation: Active site positions substrates optimally

---

(c) Factors Affecting Enzyme Activity (4 marks)

• Activity increases with temperature up to an optimum (~37°C in humans)
• Beyond optimum: denaturation of protein structure → sharp activity loss
• Q10 rule: activity roughly doubles per 10°C rise (up to optimum)

• Each enzyme has an optimal pH (e.g., pepsin: pH 1.5-2; trypsin: pH 8; most intracellular enzymes: ~7.4)
• Deviations alter ionization of active site amino acids (His, Asp, Glu, Cys) → altered binding and catalysis

• Follows Michaelis-Menten kinetics: V = Vmax[S] / (Km + [S])
• At low [S]: rate is proportional to [S] (1st order)
• At high [S]: rate plateaus at Vmax (0th order, enzyme saturated)
• Km = substrate conc at half-Vmax; indicates enzyme-substrate affinity (lower Km = higher affinity)

• At saturating [S], rate is directly proportional to [enzyme]

• Competitive inhibitor: Resembles substrate; binds active site; increases apparent Km; Vmax unchanged; reversible; overcome by excess substrate (e.g., methotrexate inhibits DHFR)
• Non-competitive inhibitor: Binds allosteric site; decreases Vmax; Km unchanged (e.g., heavy metals)
• Uncompetitive: Binds ES complex; decreases both Km and Vmax
• Irreversible inhibitors: Covalently modify enzyme (e.g., aspirin acetylates COX; organophosphates inhibit acetylcholinesterase)

• Many enzymes require cofactors: metal ions (Mg²⁺, Zn²⁺, Fe²⁺) or organic molecules (coenzymes - often B vitamins: NAD⁺, FAD, CoA)
• Apoenzyme (protein part) + cofactor = Holoenzyme (fully active)

• Regulatory molecules bind at sites other than the active site (allosteric site), causing conformational changes that alter activity (allosteric activation or inhibition)
• Example: ATP inhibits phosphofructokinase-1; AMP activates it

• Phosphorylation/dephosphorylation: activated by kinases, deactivated by phosphatases (or vice versa)

---

(d) Clinical Significance of Serum Enzymes in Diagnosis of MI, Liver Disease, and Pancreatitis (5 marks)

1. Myocardial Infarction (Heart Attack)

2. Liver Disease

3. Pancreatitis

Lipase is preferred over amylase for diagnosis of acute pancreatitis due to higher specificity and longer elevation window.

---

SHORT ESSAY QUESTIONS (5 x 10 marks each)

---

1. Wilson's Disease and Menke's Disease

Wilson's Disease (Hepatolenticular Degeneration)

• Gene: ATP7B mutation (chromosome 13)
• Mechanism: Defective biliary excretion of copper → copper accumulates in liver, brain, kidneys, cornea
• Clinical features:
  • Liver: Hepatitis, cirrhosis, fulminant hepatic failure
  • Neuropsychiatric: Dysarthria, tremor, dystonia, psychiatric changes
  • Eyes: Kayser-Fleischer rings (golden-brown copper deposits in Descemet's membrane of cornea) - pathognomonic
  • Kidneys: Fanconi syndrome (renal tubular acidosis)
  • Hemolytic anemia
• Lab: Low serum ceruloplasmin (<20 mg/dL), elevated 24-hr urinary copper, elevated liver copper on biopsy
• Treatment: D-penicillamine, Trientine (copper chelators), Zinc (blocks absorption), liver transplant

Menke's Disease (Kinky Hair Disease)

• Gene: ATP7A mutation (X-linked recessive - affects males)
• Mechanism: Defective intestinal absorption and cellular distribution of copper → copper deficiency in liver and brain, but copper accumulates in intestinal cells, kidneys
• Pathophysiology: Multiple copper-dependent enzymes are deficient: lysyl oxidase (connective tissue), cytochrome c oxidase, dopamine beta-hydroxylase, tyrosinase
• Clinical features (present in infancy):
  • Kinky/brittle, twisted hair (pili torti)
  • Severe intellectual disability, neurodegeneration
  • Hypopigmentation
  • Arterial aneurysms, bladder diverticula
  • Low body temperature
• Lab: Low serum copper, low ceruloplasmin
• Treatment: Copper histidine injections (partial benefit if started early)

---

2. Phase II Reactions of Xenobiotics (Biotransformation)

• Phase I: Functionalization (introducing/unmasking reactive groups - mainly CYP450)
• Phase II: Conjugation reactions - the drug/Phase I metabolite is conjugated with an endogenous polar molecule to make it more water-soluble for excretion

Phase II Reactions (Conjugation):

• Enzyme: UDP-glucuronosyltransferase (UGT)
• Cofactor: UDP-glucuronic acid (from glucose)
• Substrates: Bilirubin, morphine, paracetamol, chloramphenicol, steroids
• Products excreted in bile or urine

• Enzyme: Sulfotransferase
• Cofactor: PAPS (3'-phosphoadenosine-5'-phosphosulfate)
• Substrates: Steroids, thyroid hormones, dopamine, phenols
• Can activate some procarcinogens (e.g., N-OH-AAF)

• Enzyme: Glutathione S-transferase (GST)
• Cofactor: Glutathione (tripeptide: Glu-Cys-Gly)
• Neutralizes electrophilic/reactive compounds (e.g., NAPQI from paracetamol overdose)
• Final product: Mercapturic acid (excreted in urine)

• Enzyme: N-acetyltransferase (NAT1, NAT2)
• Cofactor: Acetyl-CoA
• Substrates: Isoniazid, sulfonamides, procainamide
• Fast vs slow acetylators - genetic polymorphism affecting drug efficacy and toxicity

• Enzyme: Methyltransferase
• Cofactor: S-adenosylmethionine (SAM)
• Substrates: Catecholamines (COMT), histamine, nicotinic acid

• Conjugation with glycine (bile acids, benzoate) or glutamine

Clinical significance: Phase II reactions generally produce less toxic, more water-soluble metabolites. However, exceptions exist (e.g., sulfation can activate carcinogens; acetylation of isoniazid produces hepatotoxic hydrazine).

---

3. Role of Folic Acid in Biological System

Functions of Folic Acid (as THF):

• N10-formyl-THF donates C8 and C2 of the purine ring (essential for DNA synthesis)

• N5,N10-methylene-THF donates a methyl group in the conversion of dUMP → dTMP (by thymidylate synthase) - critical for DNA synthesis

• Serine ↔ Glycine (serine hydroxymethyltransferase; generates N5,N10-methylene-THF)
• Homocysteine → Methionine (N5-methyl-THF donates methyl group; requires Vit B12 as cofactor)
• Histidine catabolism: formiminoglutamate → glutamate (FIGLU test - elevated FIGLU in urine in folate deficiency)

• Essential for methylation reactions (DNA methylation, protein methylation, phospholipid synthesis)

Deficiency Effects:

• Megaloblastic anemia (impaired DNA synthesis → large, immature RBCs)
• Neural tube defects (spina bifida, anencephaly) - due to inadequate folate in early pregnancy
• Hyperhomocysteinemia (cardiovascular risk)
• Elevated urinary FIGLU (diagnostic test)

Clinical Applications:

• Folic acid supplementation in pregnancy (400 mcg/day) to prevent neural tube defects
• Methotrexate (cancer/autoimmune drug) works by inhibiting DHFR → depletes THF → blocks DNA synthesis
• Leucovorin (folinic acid) - rescue agent after high-dose methotrexate; bypasses DHFR block

---

4. Different Types of Lipoproteins and Their Functions

• ApoB-48: Structural component of chylomicrons; no LDL receptor binding
• ApoB-100: Ligand for LDL receptor (on liver/peripheral cells)
• ApoC-II: Activates Lipoprotein Lipase (LPL) - releases fatty acids from TG-rich lipoproteins
• ApoE: Mediates remnant clearance by liver (binds LDL receptor)
• ApoA-I: Activates LCAT (Lecithin-Cholesterol Acyltransferase) - esterifies cholesterol on HDL

• Elevated LDL: Atherosclerosis, coronary artery disease
• Low HDL: Increased cardiovascular risk
• Elevated chylomicrons/VLDL: Hypertriglyceridemia, pancreatitis risk
• Familial hypercholesterolemia: Defective LDL receptor → elevated LDL → premature CAD

---

5. FAS Complex (Fatty Acid Synthase Complex)

Location and Conditions:

• Cytosol of liver, adipose tissue, mammary gland (high lipogenic activity)
• Stimulated by: High carbohydrate diet, insulin, fed state
• Inhibited by: Fasting, fat diet, AMP, long-chain acyl-CoA

Starting materials:

• Acetyl-CoA (2C starter unit, from mitochondria - transported as citrate)
• Malonyl-CoA (3C elongation unit, synthesized by Acetyl-CoA carboxylase - rate-limiting enzyme of FA synthesis)
• NADPH (reducing equivalents - from HMP shunt and malic enzyme)

FAS Complex Structure (Mammalian):

• Dimer of two identical polypeptide chains
• Each monomer has 7 enzymatic domains and an Acyl Carrier Protein (ACP) domain:
  1. Acetyl/Malonyl transferase - loads substrates onto ACP
  2. Beta-ketoacyl synthase (KS) - condensation (forms 4C from 2C+2C)
  3. Beta-ketoacyl reductase (KR) - 1st reduction (uses NADPH)
  4. Beta-hydroxyacyl dehydratase (DH) - dehydration
  5. Enoyl reductase (ER) - 2nd reduction (uses NADPH)
  6. Thioesterase (TE) - releases final product

Reaction Cycle (produces Palmitate - 16C):

1. Condensation: Acetyl-S-ACP + Malonyl-S-ACP → Acetoacetyl-S-ACP + CO2
2. 1st Reduction: Acetoacetyl-S-ACP + NADPH → D-3-Hydroxybutyryl-S-ACP
3. Dehydration: D-3-Hydroxybutyryl → Crotonyl-S-ACP + H2O
4. 2nd Reduction: Crotonyl + NADPH → Butyryl-S-ACP

---

6. GAGs (Glycosaminoglycans)

• Most GAGs are attached to core proteins to form Proteoglycans
• Hyaluronic acid is the only GAG that exists free (not as proteoglycan) and is not sulfated

• Hurler syndrome (MPS I): Alpha-L-iduronidase deficiency → dermatan + heparan sulfate accumulation
• Hunter syndrome (MPS II): Iduronate sulfatase deficiency (X-linked)
• Morquio syndrome (MPS IV): Keratan sulfate accumulation

---

7. Inhibitors of Electron Transport Chain (ETC)

A. Complex I Inhibitors (NADH dehydrogenase):

• Rotenone (plant pesticide/fish poison)
• Amytal (Amobarbital) (barbiturate)
• MPTP (neurotoxin, causes Parkinsonism)
• Effect: Block electron flow from NADH; NADH accumulates; FADH2 pathway unaffected

B. Complex II Inhibitors (Succinate dehydrogenase):

• Malonate (competitive inhibitor of succinate)
• Carboxin (fungicide)

C. Complex III Inhibitors (Ubiquinol-Cytochrome c reductase):

• Antimycin A (antibiotic/fish poison)
• Myxothiazol
• Effect: Block electron transfer from CoQH2 to cytochrome c

D. Complex IV Inhibitors (Cytochrome c oxidase):

• Cyanide (CN-) - binds ferric (Fe³⁺) form of cytochrome a3 → histotoxic hypoxia
• Carbon monoxide (CO) - binds heme of cytochrome c oxidase AND hemoglobin
• Hydrogen sulfide (H2S) - similar to cyanide
• Azide (N3-) - similar mechanism
• Effect: Block final electron transfer to O2; all upstream carriers become fully reduced; ATP synthesis stops; lactic acidosis

E. ATP Synthase (Complex V) Inhibitors:

• Oligomycin - blocks the Fo proton channel of ATP synthase
• Effect: Proton gradient builds up; ETC slows due to back-pressure

F. Uncouplers (not direct ETC inhibitors, but uncouple ATP synthesis):

• 2,4-Dinitrophenol (DNP) - carries protons across inner membrane, dissipating gradient as heat (no ATP made); used illicitly for weight loss
• Thermogenin (UCP1) - physiological uncoupler in brown adipose tissue (non-shivering thermogenesis)
• Aspirin/salicylates in overdose

---

8. Factors Affecting BMR (Basal Metabolic Rate)

---

9. Metabolism of Alcohol (Ethanol)

1. Alcohol Dehydrogenase (ADH) Pathway (Major - ~90%)

Ethanol → Acetaldehyde → Acetate → Acetyl-CoA
  (ADH)     (ALDH)        (tissues)
  NAD+ → NADH  NAD+ → NADH

• ADH: Cytosolic; Zn²⁺-containing enzyme; NAD⁺ dependent
• ALDH (Aldehyde dehydrogenase): Mitochondrial; NAD⁺ dependent; produces Acetaldehyde → Acetate
• Acetate → released into blood → metabolized in peripheral tissues to Acetyl-CoA

2. Microsomal Ethanol Oxidizing System - MEOS (CYP2E1 pathway) (~10%, inducible)

• Induced by chronic alcohol use
• Uses NADPH + O2
• Produces reactive oxygen species → oxidative stress, lipid peroxidation
• Ethanol + NADPH + O2 → Acetaldehyde + NADP+ + H2O
• Responsible for drug interactions (induces CYP2E1 which metabolizes many drugs)

3. Catalase Pathway (minor, H2O2 dependent)

• Requires H2O2 co-substrate; minor role in vivo

Metabolic Consequences of Alcohol:

1. Inhibition of gluconeogenesis (OAA and pyruvate converted to malate and lactate) → Hypoglycemia
2. Inhibition of fatty acid oxidation → fatty acids esterified → Fatty liver (steatosis)
3. Lactic acidosis (pyruvate → lactate; gluconeogenesis blocked)
4. Hyperuricemia (lactic acid competes with uric acid for excretion) → gout
5. Hypertriglyceridemia (TG synthesis increased; VLDL secretion impaired)
6. Acetaldehyde toxicity: Forms adducts with proteins, damages mitochondria, promotes inflammation → alcoholic hepatitis → cirrhosis

---

10. Difference Between Kwashiorkor and Marasmus

---