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GAZI MEDICAL COLLEGE — 2nd Term Final Examination

Biochemistry: Metabolism & Body Fluids

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GROUP-A

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Q1. Main Constituents of Gastric Juice; Stomach Protection Against High Acidity; HCl Secretion (2+1+2)

Main Constituents of Gastric Juice and Their Functions

Protection Against High Gastric Acidity (Gastric Mucosal Barrier)

1. Mucus layer — thick, viscous gel forms a physical barrier over epithelium
2. Bicarbonate secretion — mucous cells secrete HCO₃⁻, neutralizing acid at the epithelial surface
3. Tight junctions — between epithelial cells prevent back-diffusion of H⁺
4. Rapid cell turnover — damaged cells replaced every 3–5 days
5. Prostaglandins (E2, I2) — stimulate mucus and HCO₃⁻ secretion, maintain mucosal blood flow

How HCl is Secreted (Parietal Cell Mechanism)

• Carbonic anhydrase converts CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻
• H⁺/K⁺-ATPase (proton pump) on luminal surface actively pumps H⁺ into stomach lumen in exchange for K⁺
• Cl⁻ exits via apical Cl⁻ channels into lumen alongside H⁺ → forming HCl
• HCO₃⁻ exchanges for Cl⁻ on basolateral side (the "alkaline tide")
• Stimulated by: histamine (H₂ receptors), gastrin, acetylcholine (vagus)
• Inhibited by: omeprazole (blocks proton pump), ranitidine (H₂ blocker), somatostatin

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Q2. Digestion of Protein; Absorption of End Products (3+2)

Digestion of Protein

• HCl denatures proteins (unfolding)
• Pepsinogen → pepsin (by HCl and autocatalysis)
• Pepsin: endopeptidase, cleaves peptide bonds at aromatic amino acids (Phe, Tyr, Trp) → polypeptides

• Aminopeptidases — remove N-terminal amino acids from peptides
• Dipeptidases — cleave dipeptides → free amino acids
• End products: free amino acids, dipeptides, tripeptides

Absorption of End Products

• Free amino acids: absorbed by Na⁺-dependent active transport (co-transport with Na⁺) via specific carriers (neutral, basic, acidic, imino acid transporters) on brush border → enter portal blood
• Dipeptides and tripeptides: absorbed via PepT1 transporter (H⁺-dependent) → hydrolyzed to amino acids inside enterocytes
• Amino acids enter the portal vein → liver for metabolism
• Some amino acids (Lys, Arg) share transporters (e.g., cystinuria results from defective basic amino acid transporter)

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Q3. Components and Arrangements of ETC (Electron Transport Chain); Importance and Inhibitors (3+2)

Components and Arrangement of ETC

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

Importance of ETC

• Major source of ATP: NADH yields ~2.5 ATP; FADH₂ yields ~1.5 ATP
• Drives oxidative phosphorylation — the cell's primary energy currency production
• Creates proton motive force (electrochemical gradient) across inner mitochondrial membrane
• Generates heat (thermogenesis), especially uncoupled in brown adipose tissue
• Essential for aerobic life; without it, only anaerobic glycolysis available (2 ATP/glucose vs ~30–32 ATP)

Inhibitors of ETC

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Q4. HMP Shunt: Definition, Importance, Clinical Importances of Shunt Pathway (1+3+1)

What is HMP Shunt?

• Rate-limiting enzyme: Glucose-6-phosphate dehydrogenase (G6PD)
• Two phases: Oxidative (irreversible, generates NADPH + CO₂) and Non-oxidative (reversible, interconverts sugars)

Importance of HMP Shunt

1. NADPH generation — essential for:
   • Reductive biosynthesis (fatty acid synthesis, cholesterol synthesis, steroid synthesis)
   • Maintaining reduced glutathione (GSH) — protects RBCs from oxidative hemolysis
   • Cytochrome P450 reactions (drug detoxification in liver)
   • NADPH oxidase in phagocytes (respiratory burst — killing bacteria)
   • Nitric oxide synthesis
2. Ribose-5-phosphate production — precursor for nucleotide and nucleic acid synthesis (DNA, RNA, ATP, NAD, FAD, CoA)
3. Interconversion of sugars — non-oxidative phase provides 3C, 4C, 5C, 6C, 7C sugars for glycolysis and other pathways
4. Active in tissues requiring high NADPH — liver, adrenal cortex (steroidogenesis), mammary gland (lactation), RBCs, phagocytes

Clinical Importances

1. G6PD Deficiency — most common enzyme deficiency in humans (X-linked). Without NADPH, RBCs cannot regenerate GSH → oxidative damage → hemolytic anemia triggered by drugs (primaquine, dapsone), infections, fava beans
2. Favism — hemolysis in G6PD-deficient individuals after eating fava beans
3. Chronic Granulomatous Disease (CGD) — NADPH oxidase deficiency → phagocytes cannot produce superoxide → recurrent severe bacterial/fungal infections
4. Pentosuria — defect in non-oxidative phase → xylulose-5-phosphate excreted in urine (benign condition)

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Q5. Transamination, Oxidative Deamination, Importance of Each, Ammonia Intoxication (2+2+1)

Transamination

Aspartate + α-Ketoglutarate ⇌ Oxaloacetate + Glutamate (catalyzed by AST — Aspartate aminotransferase)

Alanine + α-Ketoglutarate ⇌ Pyruvate + Glutamate (catalyzed by ALT — Alanine aminotransferase)

• Channels amino groups into glutamate (central amino acid)
• Provides non-essential amino acids by synthesis
• Links amino acid and carbohydrate metabolism
• ALT and AST are important liver function markers in clinical diagnosis

Oxidative Deamination

Glutamate + NAD⁺ (or NADP⁺) → α-Ketoglutarate + NH₃ + NADH

• Primary mechanism of free ammonia generation in the body
• Linked to transamination via glutamate (transamination–deamination tandem)
• NH₃ is then converted to urea (in liver) for excretion
• Regenerates α-ketoglutarate for TCA cycle

Ammonia Intoxication

• Normally, blood NH₃ < 35 μmol/L; urea cycle in liver detoxifies it
• Causes of elevated NH₃: liver failure, urea cycle enzyme defects, portosystemic shunts, urease-producing bacteria
• Mechanism of toxicity:
  • NH₃ combines with α-ketoglutarate → glutamate → depletes TCA cycle intermediate (α-KG), impairing brain energy (ATP) production
  • Excess glutamate → glutamine (trapping α-KG) → astrocyte swelling (cerebral edema)
  • Altered neurotransmitter balance (GABA, glutamate)
• Features: asterixis (flapping tremor), confusion, stupor, coma — hepatic encephalopathy

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Q6. 2-Year-Old with Diarrhea, Abdominal Discomfort, Flatulence After Milk — Causes and Enzyme (4+1)

Possible Causes (Differential Diagnosis)

1. Primary (Congenital) Lactase Deficiency — rare, presents at birth
2. Secondary Lactase Deficiency — most common in this age group; follows gastroenteritis (rotavirus), celiac disease, Giardia infection, or any mucosal injury that destroys brush border enzymes
3. Congenital sucrase-isomaltase deficiency — if sucrose also causes symptoms
4. Cow's milk protein allergy — immune-mediated; causes vomiting, diarrhea, blood in stool
5. Galactosemia — enzyme defect preventing galactose metabolism (jaundice, liver disease in neonates)

Most Likely Diagnosis: Lactose Intolerance (Lactase Deficiency)

• Normally, lactase (β-galactosidase) on intestinal brush border cleaves lactose → glucose + galactose
• In deficiency, undigested lactose passes to colon
• Colonic bacteria ferment lactose → short-chain fatty acids + CO₂ + H₂ and lactic acid
• Results in: osmotic diarrhea (lactose osmotically draws water), flatulence (gas from fermentation), bloating/abdominal discomfort

Responsible Enzyme

• Location: brush border of small intestinal enterocytes (jejunum)
• Normally highest at birth, declines after weaning in most populations (lactase non-persistence)
• Secondary lactase deficiency: reduced after mucosal damage

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Q7. Ketone Bodies, Ketoacidosis in DM, β-Oxidation of Fatty Acids, ATP from 18-C Saturated FA (2+1+2+1+2+2)

Ketone Bodies — Definition and Use in the Body

1. Acetoacetate — primary KB; can be used directly
2. β-hydroxybutyrate (3-hydroxybutyrate) — reduced form; predominates
3. Acetone — spontaneous decarboxylation of acetoacetate; exhaled

• Transported in blood to extrahepatic tissues (brain, heart, skeletal muscle, kidney cortex)
• Converted back to Acetyl-CoA via succinyl-CoA transferase (thiophorase) → enter TCA cycle
• The liver cannot use its own ketone bodies (lacks thiophorase)
• Important fuel during prolonged fasting, starvation, and uncontrolled diabetes

Ketoacidosis in Uncontrolled Diabetes Mellitus

• In uncontrolled T1DM: absolute insulin deficiency → cells cannot take up glucose
• Glucagon/insulin ratio rises → lipolysis ↑ → massive FFAs released from adipose tissue → liver
• Liver β-oxidizes excess FFAs → huge Acetyl-CoA accumulation
• Oxaloacetate (OAA) is diverted to gluconeogenesis → TCA cycle cannot accept Acetyl-CoA
• HMG-CoA pathway overwhelmed → excess ketone bodies → ketonemia
• Ketones are acidic (pKa ~3.5) → metabolic acidosis (pH ↓, HCO₃⁻ ↓)
• Features: Kussmaul breathing, fruity breath (acetone), hyperglycemia, polyuria, dehydration, anion gap metabolic acidosis

β-Oxidation of Fatty Acids

ATP Calculation for 18-Carbon Saturated FA (Stearic Acid)

• 18C FA → 9 Acetyl-CoA units
• Cycles needed = 9 − 1 = 8 cycles

• 8 FADH₂ × 1.5 ATP = 12 ATP
• 8 NADH × 2.5 ATP = 20 ATP

• 9 Acetyl-CoA × 10 ATP = 90 ATP

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Q8. Tubular Load, Renal Threshold, Transport Maximum; Peculiarities of Renal Blood Flow (3+2)

Tubular Load

Tubular Load = GFR × Plasma concentration of substance

Renal Threshold

• For glucose: ~180 mg/dL (the glucose threshold — below this, all filtered glucose is reabsorbed)
• Threshold may vary among nephrons (explains why some glucose appears in urine before Tm is reached — "splay" in titration curve)

Transport Maximum (Tm)

• TmG (glucose) = ~375 mg/min (male) — above this, all additional filtered glucose appears in urine
• Once Tm is exceeded, reabsorption plateaus and urinary excretion increases linearly with plasma concentration

Peculiarities of Renal Blood Flow

1. Two capillary beds in series: glomerular capillaries (high pressure ~60 mmHg for filtration) → efferent arteriole → peritubular capillaries (low pressure for reabsorption)
2. Autoregulation: RBF and GFR are maintained constant over mean arterial pressure 80–180 mmHg via myogenic mechanism and tubuloglomerular feedback (macula densa)
3. High blood flow relative to organ size: kidneys receive ~20–25% of cardiac output (1200 mL/min) but represent only 0.5% of body weight
4. High O₂ delivery but modest extraction: renal venous blood still O₂-rich (kidneys don't extract much O₂ per volume)
5. Vasa recta in medulla: hairpin loop structure maintains medullary osmotic gradient for concentration of urine (countercurrent exchange)
6. Juxtaglomerular apparatus: specialized region controlling renin secretion and GFR

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Q9. Daily Water Intake and Output Chart; Water Turnover; Calculation (3+2)

Daily Water Intake and Output Chart (Adult, ~70 kg)

Obligatory Water Loss

Water Turnover

1. Give a known dose of D₂O orally
2. Allow equilibration (~3–4 hours)
3. Measure the concentration of D₂O in plasma/urine
4. Total body water (TBW) = Dose administered / Concentration at equilibrium
5. TBW ≈ 60% of body weight in males (42 L in 70 kg man)
6. Water turnover rate = daily intake or output / TBW × 100%

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Q10. Electrolyte Profile with Reference Ranges; Common Causes of Hyponatremia; Consequences (2+2+1)

Electrolyte Profile with Reference Ranges

Common Causes of Hyponatremia (Na⁺ < 135 mmol/L)

• Vomiting, diarrhea, burns
• Adrenal insufficiency (Addison's disease)
• Diuretic use (thiazides especially)

• SIADH (Syndrome of Inappropriate ADH secretion) — most common cause
• Hypothyroidism
• Psychogenic polydipsia

• Congestive heart failure
• Nephrotic syndrome
• Liver cirrhosis

Consequences of Hyponatremia

• Cellular edema — osmotic water shift into cells (low ECF osmolality → water moves into cells)
• Brain: cerebral edema → headache, nausea, confusion, seizures, coma, herniation (life-threatening)
• Muscle weakness and fatigue
• Severe (Na⁺ < 120 mmol/L) → seizures, respiratory arrest, death
• ⚠️ Rapid correction → Central Pontine Myelinolysis (osmotic demyelination syndrome)

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Q11. Plasma Anion Gap; Measured & Unmeasured Cations and Anions; Causes of Increased Anion Gap (1+2+2)

Plasma Anion Gap (AG)

AG = Na⁺ − (Cl⁻ + HCO₃⁻) Normal AG = 8–12 mEq/L (or up to 16 if K⁺ excluded)

Measured and Unmeasured Ions

Causes of Increased Anion Gap (High AG Metabolic Acidosis)

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Q12. Classification of Primary Acid-Base Disorders; Causes and Compensation of Respiratory Acidosis (2+3)

Classification of Primary Acid-Base Disorders

Respiratory Acidosis

• Opiates, sedatives, barbiturates, anesthetics
• Brainstem lesions, trauma

• Guillain-Barré syndrome, myasthenia gravis
• Muscular dystrophy, diaphragm paralysis

• COPD (most common chronic cause)
• Severe asthma, status asthmaticus
• Pneumonia, pulmonary edema, pneumothorax
• Upper airway obstruction (foreign body, laryngospasm)

• Kyphoscoliosis
• Flail chest

Compensation for Respiratory Acidosis

• CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻
• H⁺ buffered by hemoglobin and plasma proteins
• Modest rise in HCO₃⁻: ~1 mEq/L per 10 mmHg rise in PaCO₂ (acute)

• Kidney proximal tubule: ↑ H⁺ secretion → ↑ HCO₃⁻ reabsorption
• Kidney distal tubule: ↑ NH₄⁺ excretion, ↑ titratable acid
• HCO₃⁻ rises ~3.5 mEq/L per 10 mmHg rise in PaCO₂ (chronic)
• Expected HCO₃⁻ in chronic resp. acidosis = 24 + 3.5 × (PaCO₂ − 40)/10

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Q13. Clinical Case: pH 7.52, HCO₃⁻ 32 mmol/L, PCO₂ 48 mmHg in Woman with Persistent Vomiting (2+3)

Step 1 — Identify the Acid-Base Disorder

Diagnosis: Metabolic Alkalosis with Respiratory Compensation

• pH is alkalotic + primary rise in HCO₃⁻ → Metabolic Alkalosis
• PCO₂ elevated (48 mmHg) = appropriate respiratory compensation (hypoventilation to retain CO₂)
• Check compensation: Expected PCO₂ = 40 + 0.7 × (HCO₃⁻ − 24) = 40 + 0.7 × 8 = 45.6 mmHg ≈ matches (48 close to expected — appropriate compensation, not a mixed disorder)

Other Causes of Metabolic Alkalosis

• Vomiting / nasogastric suction (most common) — loss of HCl
• Diuretic use (loop, thiazide) — loss of Cl⁻ and K⁺
• Post-hypercapnic alkalosis
• Villous adenoma of colon (Cl⁻ losing diarrhea)

• Primary hyperaldosteronism (Conn's syndrome) — ↑ H⁺ and K⁺ excretion
• Cushing's syndrome (excess cortisol with mineralocorticoid activity)
• Severe hypokalemia (K⁺ shifts into cell, H⁺ shifts out)
• Bartter's/Gitelman's syndrome (hereditary renal tubular disorders)
• Exogenous alkali (excessive NaHCO₃, antacid ingestion)
• Milk-alkali syndrome

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Q14. Hormones Acting on Kidney; Acidification of Urine (PCT vs DCT); Glycosuria in DM; Limiting pH of Urine (3+3+2+2)

Hormones Acting on the Kidney and Their Functions

Acidification of Urine: PCT vs DCT

• Reabsorbs ~80–85% of filtered HCO₃⁻
• H⁺ secreted via Na⁺/H⁺ exchanger (NHE3) — antiporter (secondary active)
• H⁺ + HCO₃⁻ (luminal) → H₂CO₃ → CO₂ + H₂O (catalyzed by carbonic anhydrase IV on brush border)
• CO₂ diffuses into cell; regenerated to HCO₃⁻ intracellularly (by CA II); HCO₃⁻ exits basolaterally
• Cannot lower urine pH below 6.0 — buffer system prevents extreme acidification
• Major role: HCO₃⁻ reclamation, not net acid excretion

• Type A (α) intercalated cells secrete H⁺ via H⁺-ATPase (primary active) and H⁺/K⁺-ATPase on luminal surface
• This is net acid secretion (not just HCO₃⁻ reclamation)
• H⁺ is buffered in lumen by:
  • Phosphate buffer (H₂PO₄⁻ → titratable acid)
  • NH₃ (secreted by tubular cells) + H⁺ → NH₄⁺ (trapped and excreted)
• Can acidify urine to pH 4.5–5.0
• Aldosterone stimulates H⁺ secretion here
• Type B intercalated cells secrete HCO₃⁻ (for alkalinization when needed)

Why Glycosuria Occurs in Diabetes Mellitus

• Glucose is normally filtered at glomerulus and completely reabsorbed in PCT via SGLT2 (sodium-glucose cotransporter 2) and SGLT1
• Renal threshold for glucose = ~180 mg/dL
• In DM, blood glucose >> 180 mg/dL → tubular load exceeds Tm (~375 mg/min)
• Excess filtered glucose cannot be reabsorbed → spills into urine (glycosuria)
• Glycosuria causes osmotic diuresis (glucose osmotically retains water in tubule) → polyuria → polydipsia

Limiting pH of Urine

• Maximum urinary acidification = pH 4.5 (minimum pH)
• Maximum urinary alkalinization = pH ~8.0–8.5
• The lower limit (pH 4.5) is set by the maximum H⁺ gradient the H⁺-ATPase in intercalated cells can maintain against the concentration gradient
• Below pH 4.5, the back-diffusion of H⁺ equals secretion — no further net acidification possible

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MCQ PAPER — True/False + SBA (20 marks, 30 minutes)

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Questions 1–10: True (T) / False (F)

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Q1. Cholesterol —

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Q2. Salient Features of HMP Shunt —

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Q3. Ketone Bodies —

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Q4. Proteolytic Enzymes —

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Q5. Favoring Forces of GFR —

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Q6. Transcellular Fluid Includes —

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Q7. Plasma Potassium —

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Q8. Positive Nitrogen Balance Occurs In —

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Q9. TAG-Rich Lipoproteins Are —

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Q10. Causes of Hypotonic Hypervolemia —

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Quick Reference Summary — MCQ T/F Answers

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Note: The SBA questions 11–20 were not visible in the image provided. If you can share those, I'll answer them too. All answers above are based on standard biochemistry and physiology (Harper's Biochemistry, Ganong's Physiology, Lehninger's Principles of Biochemistry).