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Khulna Medical University — 1st Professional MBBS Examination, May-2025

Biochemistry (Paper I) — Subject Code 115

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GROUP A

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Q1. Define pH and Buffer. Name 3 body buffers with conjugate bases. Most important buffer and why? (2+1+2)

1. It is present in large quantities in blood (normal [HCO₃⁻] = 24 mEq/L)
2. It is an open system — CO₂ is regulated by the lungs (respiratory compensation) and HCO₃⁻ is regulated by the kidneys (renal compensation)
3. These two independent regulatory organs provide precise and rapid pH control

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Q2. Functional classification of proteins with examples. Denaturation and denaturing agents (3+1+1)

• Physical agents: Heat, UV radiation, X-rays, agitation
• Chemical agents:
  • Strong acids/alkalis (alter charge)
  • Organic solvents (acetone, alcohol — disrupt hydrophobic interactions)
  • Urea and guanidinium HCl (disrupt hydrogen bonds)
  • Detergents (SDS — disrupts hydrophobic interactions)
  • Heavy metal salts (Hg²⁺, Pb²⁺ — disrupt disulfide bonds)
  • Reducing agents (β-mercaptoethanol — cleave disulfide bonds)

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Q3. Define enzyme, coenzyme, and isoenzyme. Factors affecting enzyme activity (3+2)

• NAD⁺/NADH (carries electrons)
• FAD/FADH₂ (carries electrons)
• Coenzyme A (carries acyl groups)
• TPP, PLP, Biotin, THF

1. Substrate concentration [S]: Activity increases with [S] up to Vmax (Michaelis-Menten kinetics); Km indicates affinity.
2. Temperature: Activity increases with temperature up to an optimum (~37°C in humans); beyond that, denaturation occurs.
3. pH: Each enzyme has an optimal pH (e.g., pepsin = pH 2, trypsin = pH 8, salivary amylase = pH 7); extremes denature enzymes.
4. Enzyme concentration: Activity increases proportionally if substrate is not limiting.
5. Inhibitors:
   • Competitive: Structurally similar to substrate; compete for active site; reversible; increases apparent Km.
   • Non-competitive: Bind to allosteric site; reduce Vmax; don't change Km.
   • Irreversible: Covalently bind and permanently inhibit (e.g., organophosphates inhibit acetylcholinesterase).
6. Activators and cofactors: Metal ions (Mg²⁺, Zn²⁺, Ca²⁺) are required for many enzymes.
7. Allosteric regulation: Binding of modulators to allosteric sites changes enzyme conformation and activity.

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Q4. Biochemical tests for diagnosis of diabetes mellitus with interpretation. Procedure of OGTT (3+2)

1. Patient fasts for 8–14 hours (water allowed)
2. Baseline fasting blood glucose is drawn
3. Patient drinks 75 g anhydrous glucose dissolved in 250–300 mL water within 5 minutes
4. Blood glucose is measured at 30, 60, 90, and 120 minutes (the 2-hour value is most important)
5. Patient remains sedentary; no food, smoking, or exercise during the test

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Q5. Renal function tests. Procedure of creatinine clearance test (3+2)

• Serum creatinine (normal: 0.6–1.2 mg/dL)
• Blood urea nitrogen (BUN) (normal: 8–20 mg/dL)
• Serum uric acid
• GFR (calculated or measured)
• Serum electrolytes (Na⁺, K⁺, Cl⁻, HCO₃⁻)
• Serum cystatin C

• Urinalysis (protein, glucose, casts, RBCs)
• Urine osmolality and specific gravity
• Urine protein:creatinine ratio
• 24-hour urine protein
• Creatinine clearance (CCr)

1. On day 1 at a fixed time (e.g., 8 AM), patient empties bladder and discards this urine
2. All urine is collected for exactly 24 hours, stored in a refrigerated container
3. At the end of 24 hours, the last sample is collected and the total volume is measured
4. A blood sample is drawn (at any point during collection) for serum creatinine
5. Urine creatinine concentration is measured

CCr (mL/min) = (Urine creatinine × Urine volume per minute) / Serum creatinine

• Normal CCr: Males ~97–137 mL/min; Females ~88–128 mL/min
• Reduced CCr indicates decreased GFR and renal dysfunction

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Q6. (Compulsory) 58-year-old man, severe chest pain radiating to left arm, 3 hours, ECG: ST-elevation

a. Most probable diagnosis (3 marks):

• Age and sex (middle-aged male, high cardiovascular risk)
• Severe chest pain for >3 hours
• Radiation to the left arm
• ST-segment elevation on ECG

b. Biochemical tests to establish the diagnosis (2 marks):

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Q7. (Compulsory — Choose ONE)

Option A: Carbohydrates — Definition, Classification, Importance, Mucopolysaccharides, Starch vs Glycogen (3+2+3+2)

• Sucrose = glucose + fructose
• Lactose = glucose + galactose
• Maltose = glucose + glucose

• Homopolysaccharides: Starch, glycogen, cellulose, dextrin
• Heteropolysaccharides (mucopolysaccharides/glycosaminoglycans): Hyaluronic acid, chondroitin sulfate, heparin, keratan sulfate

1. Energy source — glucose is the primary fuel; 1g CHO = 4 kcal
2. Obligatory fuel for brain, RBCs, and renal medulla (glucose-dependent)
3. Glycogen — energy storage in liver (maintains blood glucose) and muscle
4. Structural role — cellulose in plants; ribose/deoxyribose in RNA/DNA
5. Precursors — for synthesis of amino acids, fatty acids, nucleotides
6. Cell recognition and signaling — glycoproteins and glycolipids on cell surfaces
7. Protein sparing — adequate CHO intake prevents protein catabolism

1. Hyaluronic acid — lubricant in synovial fluid and vitreous humor; fills extracellular space
2. Chondroitin sulfate — structural component of cartilage, bone, tendons
3. Heparin — anticoagulant (activates antithrombin III); stored in mast cells
4. Keratan sulfate — found in cornea, cartilage
5. Dermatan sulfate — in skin, blood vessels, heart valves
6. Heparan sulfate — on cell surfaces, acts as co-receptor

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Option B (OR): Define and classify jaundice. Liver function assessed by bilirubin. Bilirubin conjugation and excretion. Types of jaundice (tabulated) (2+1+4+3)

1. Pre-hepatic (hemolytic)
2. Hepatic (hepatocellular)
3. Post-hepatic (obstructive/cholestatic)

• Serum total bilirubin, direct (conjugated) bilirubin, and indirect (unconjugated) bilirubin
• Elevated direct bilirubin → hepatocellular or obstructive disease
• Elevated indirect bilirubin → hemolysis or neonatal jaundice
• The ratio helps diagnose the type of jaundice

1. RBCs are broken down → heme released → converted to biliverdin by heme oxygenase → then to unconjugated bilirubin (fat-soluble, bound to albumin in blood)
2. In the liver: bilirubin is taken up, and conjugated with glucuronic acid by UDP-glucuronosyltransferase → conjugated (direct) bilirubin (water-soluble)
3. Conjugated bilirubin is secreted into bile canaliculi → bile ducts → intestine
4. Gut bacteria convert it to urobilinogen → some reabsorbed (enterohepatic circulation), excreted in urine as urobilin; the rest is oxidized to stercobilin (gives stool its brown color)

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GROUP B

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Q8. Draw and label Electron Transport Chain (ETC). Name the inhibitors of ETC (4+1)

NADH → Complex I → CoQ → Complex III → Cytochrome c → Complex IV → O₂ → H₂O
FADH₂ → Complex II → CoQ ↗

• Complex I (NADH-CoQ oxidoreductase): Accepts electrons from NADH; pumps 4H⁺
• Complex II (Succinate-CoQ oxidoreductase): Accepts electrons from FADH₂ (from succinate); does NOT pump H⁺
• Coenzyme Q (Ubiquinone): Mobile electron carrier between complexes I/II and III
• Complex III (CoQ-cytochrome c oxidoreductase): Pumps 4H⁺ (Q cycle)
• Cytochrome c: Mobile carrier between complexes III and IV
• Complex IV (Cytochrome c oxidase): Transfers electrons to O₂, forming H₂O; pumps 2H⁺
• Complex V (ATP synthase/F₀F₁-ATPase): Uses the H⁺ gradient (chemiosmosis) to synthesize ATP

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Q9. Sources and fates of pyruvate. Why TCA cycle is called the common metabolic pathway (2.5+2.5)

1. Glycolysis — the primary source (glucose → 2 pyruvate)
2. Transamination of alanine (alanine + α-ketoglutarate → pyruvate + glutamate)
3. Degradation of certain amino acids: Serine, glycine, cysteine, threonine, tryptophan (glucogenic)
4. From lactate via lactate dehydrogenase (in the Cori cycle)
5. From malate by malic enzyme

1. Carbohydrates → Glucose → Pyruvate → Acetyl-CoA → enters TCA
2. Fats → Fatty acids → β-oxidation → Acetyl-CoA → enters TCA; glycerol → DHAP → glucose → pyruvate
3. Proteins → Amino acids → various intermediates (pyruvate, acetyl-CoA, α-ketoglutarate, succinyl-CoA, fumarate, oxaloacetate) → enter TCA at multiple points

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Q10. Substrates of gluconeogenesis. Importance of HMP shunt pathway (2+3)

1. Lactate — most important; from RBCs, exercising muscle (Cori cycle)
2. Pyruvate — from alanine (glucose-alanine cycle) and glycolysis
3. Alanine — most important gluconeogenic amino acid (from muscle)
4. Glutamine — from muscle; enters as α-ketoglutarate
5. Other glucogenic amino acids — aspartate, serine, threonine, valine, etc. (all except leucine and lysine)
6. Glycerol — from fat hydrolysis (triglycerides) → DHAP → glucose
7. Oxaloacetate (OAA) — from TCA cycle intermediates
8. Propionate — from odd-chain fatty acid oxidation → succinyl-CoA → OAA

1. NADPH production (the major function):
   • Essential for reductive biosynthesis: Fatty acid synthesis, cholesterol synthesis, steroid hormone synthesis
   • Needed to regenerate reduced glutathione (GSH) — protects RBCs from oxidative hemolysis (deficiency → G6PD deficiency and hemolytic anemia)
   • Required for the respiratory burst in neutrophils (NADPH oxidase → superoxide radical → kills bacteria)
   • Used by cytochrome P450 for drug detoxification in the liver
2. Ribose-5-phosphate production:
   • Precursor for nucleotide synthesis (RNA, DNA, ATP, NAD⁺, FAD, CoA)
3. Interconversion of sugars:
   • Converts between 3-, 4-, 5-, 6-, and 7-carbon sugars via transketolase and transaldolase

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Q11. Ketone bodies. Mechanism of ketoacidosis in uncontrolled diabetes mellitus (1+4)

1. Acetoacetate
2. β-Hydroxybutyrate (D-3-hydroxybutyrate)
3. Acetone (volatile; exhaled — "fruity breath")

1. Insulin deficiency → cells cannot take up glucose → hyperglycemia
2. ↑ Glucagon/insulin ratio → activation of hormone-sensitive lipase in adipose tissue → massive lipolysis → release of free fatty acids (FFAs) into blood
3. FFAs transported to liver → enter β-oxidation → massive production of acetyl-CoA
4. Simultaneously, insulin deficiency → reduced oxaloacetate (OAA) (OAA is diverted to gluconeogenesis)
5. Excess acetyl-CoA cannot enter TCA (insufficient OAA) → diverted to ketogenesis
6. Large amounts of acetoacetate and β-hydroxybutyrate (ketoacids) accumulate
7. These are strong organic acids → dissociate → release H⁺ → metabolic acidosis
8. Blood pH falls below 7.35 → diabetic ketoacidosis (DKA)
9. Compensatory hyperventilation (Kussmaul breathing) to blow off CO₂
10. Ketonuria, glycosuria, polyuria, dehydration, electrolyte imbalance

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Q12. Lipoproteins. HDL and cholesterol. Control of high blood cholesterol (1+2+2)

• Prevents cholesterol accumulation in arterial walls
• Reduces formation of atherosclerotic plaques
• High HDL (>60 mg/dL) is protective against coronary heart disease

• Low saturated fat, low cholesterol diet
• Increase dietary fiber (binds bile acids)
• Weight reduction
• Aerobic exercise (raises HDL)
• Cessation of smoking

1. Statins (lovastatin, atorvastatin) — inhibit HMG-CoA reductase (rate-limiting enzyme of cholesterol synthesis) — most effective
2. Bile acid sequestrants (cholestyramine) — bind bile acids in gut, prevent reabsorption → liver uses cholesterol to make more bile acids
3. Niacin (nicotinic acid) — inhibits lipolysis in adipose tissue → reduces VLDL; raises HDL
4. Fibrates (gemfibrozil) — activate PPAR-α → reduce TG; raise HDL
5. Ezetimibe — inhibits intestinal cholesterol absorption (NPC1L1 transporter)
6. PCSK9 inhibitors (evolocumab) — increase LDL receptor expression

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Q13. (Compulsory) 30-year-old woman, menstrual irregularity, weight gain, cold intolerance, decreased T3 & T4, increased TSH

a. Diagnosis (3 marks):

• Cold intolerance, weight gain, menstrual irregularity → classic hypothyroid symptoms
• ↓ T3 & T4 → confirms reduced thyroid hormone production
• ↑ TSH → pituitary compensates by increasing TSH (negative feedback lost) → confirms primary origin (thyroid gland itself is the problem)

b. Other tests for evaluation (2 marks):

1. Anti-TPO antibodies (anti-thyroid peroxidase) — most sensitive test for Hashimoto's thyroiditis
2. Anti-thyroglobulin antibodies — also confirms autoimmune thyroiditis
3. Thyroid ultrasound — assess gland size, echogenicity, nodules
4. Free T3 and Free T4 — for accurate assessment of active hormone levels
5. Thyroid scan (radioactive iodine uptake) — assess gland function
6. CBC — hypothyroidism can cause anemia
7. Lipid profile — hypothyroidism raises LDL cholesterol
8. Serum prolactin — ↑TRH → ↑TSH and also ↑prolactin → contributes to menstrual irregularity

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Q14. (Compulsory — Choose ONE)

Option A: Lipolytic enzymes. Dietary lipids and digestion. Emulsification. End products absorbed from GIT (1+2+3+4)

1. Lingual lipase (mouth) — partial digestion of triglycerides
2. Gastric lipase (stomach) — acts on short-chain TGs (important in infants)
3. Pancreatic lipase (major enzyme) — hydrolyzes TG at sn-1 and sn-3 positions → 2 fatty acids + 2-monoglyceride
4. Colipase — cofactor that anchors pancreatic lipase to lipid surface at the bile salt interface
5. Phospholipase A₂ — digests phospholipids (requires bile salts)
6. Cholesterol ester hydrolase — digests dietary cholesterol esters → free cholesterol + fatty acid
7. Lipoprotein lipase (LPL) — on capillary endothelium; hydrolyzes TG from chylomicrons and VLDL in blood
8. Hormone-sensitive lipase (HSL) — in adipose tissue; releases stored fat during fasting/stress

• Bile salts (synthesized in liver from cholesterol; stored in gallbladder) are amphipathic: hydrophobic steroid core + hydrophilic hydroxyl/conjugated groups
• They emulsify large fat droplets in the duodenum, increasing surface area for lipase action
• They form mixed micelles with fatty acids, monoglycerides, cholesterol, and phospholipids, allowing their transport to the intestinal brush border for absorption

1. Free fatty acids and 2-monoglycerides from mixed micelles are absorbed by enterocytes (passive diffusion + protein-facilitated)
2. Inside enterocytes, they are re-esterified to TG in the smooth ER
3. TG + cholesterol + phospholipids + Apo B-48 → packaged into chylomicrons
4. Chylomicrons are secreted into lymphatics (lacteals) → thoracic duct → blood
5. Short and medium chain fatty acids (C<12) bypass lymphatics → absorbed directly into portal blood → bound to albumin → liver

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Option B (OR): Transamination and deamination. Sources of ammonia. Toxicity → urea. Ammonia intoxication. Conditions causing ammonia intoxication (2+1+4+4+2+1)

Alanine + α-ketoglutarate ⇌ Pyruvate + Glutamate (by ALT) Aspartate + α-ketoglutarate ⇌ OAA + Glutamate (by AST)

Glutamate + NAD⁺ → α-ketoglutarate + NH₄⁺ + NADH

1. Amino acid catabolism (deamination) — major source
2. Glutamine hydrolysis in the kidney (by glutaminase) — acid-base regulation
3. Intestinal bacteria — urea → NH₃ (by urease); major clinical source in liver disease
4. Adenosine/AMP deamination (purine catabolism)
5. Biogenic amine catabolism (by MAO)

1. NH₄⁺ + HCO₃⁻ + 2ATP → Carbamoyl phosphate (by CPS-I, rate-limiting; activated by N-acetylglutamate)
2. Carbamoyl phosphate + Ornithine → Citrulline (by OTC; exits to cytosol)
3. Citrulline + Aspartate + ATP → Argininosuccinate (by argininosuccinate synthetase)
4. Argininosuccinate → Arginine + Fumarate (by argininosuccinate lyase)
5. Arginine + H₂O → Ornithine + Urea (by arginase; ornithine re-enters mitochondria)

• Blood ammonia normally <35 μmol/L
• Toxic effects: NH₃ crosses blood-brain barrier → reacts with α-ketoglutarate → depletes TCA cycle intermediates → impairs energy production → astrocyte swelling → cerebral edema → encephalopathy
• Features: Asterixis (flapping tremor), confusion, lethargy, coma

1. Liver cirrhosis / liver failure — liver cannot run urea cycle efficiently; gut bacteria produce NH₃
2. Portal hypertension with portosystemic shunts — gut-derived NH₃ bypasses liver
3. Urea cycle enzyme defects (inherited): CPS-I, OTC (X-linked, most common), argininosuccinate synthetase (citrullinemia), etc. — present in neonates
4. Reye's syndrome — mitochondrial dysfunction → inhibition of CPS-I
5. High protein diet or GI bleeding — excess amino acid/protein load overwhelms urea cycle
6. Valproate toxicity — inhibits CPS-I
7. Urinary tract infection with urease-producing organisms (e.g., Proteus) — urea → NH₃

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