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MBBS 1st Year - Biochemistry Exam: Complete Answers

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SECTION A / SHORT ANSWER TYPE

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1. Oncogenes (6 marks)

• Growth factors (e.g., sis - PDGF)
• Growth factor receptors (e.g., erb-B - EGF receptor)
• Signal transducers (e.g., ras - GTP-binding protein)
• Nuclear transcription factors (e.g., myc, fos, jun)
• Cell cycle regulators (e.g., cyclins)

1. Point mutation (e.g., ras mutation - single amino acid change)
2. Gene amplification (e.g., N-myc in neuroblastoma)
3. Chromosomal translocation (e.g., bcr-abl in CML - Philadelphia chromosome)
4. Promoter insertion (retroviral insertion near proto-oncogene)

• ras oncogene: encodes p21 protein, GTPase activity lost → persistent signaling
• myc: transcription factor, overexpressed in Burkitt's lymphoma
• erb-B2/HER2: amplified in breast cancer
• bcl-2: inhibits apoptosis

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2. Lac Operon (6 marks)

• Promoter (P) - RNA polymerase binding site
• Operator (O) - repressor binding site
• Structural genes: lacZ (β-galactosidase), lacY (permease), lacA (transacetylase)
• Regulator gene (lacI) - codes for repressor protein

• Repressor protein binds operator → transcription blocked → no enzyme synthesis

• Lactose → allolactose (true inducer) by β-galactosidase
• Allolactose binds repressor → conformational change → repressor cannot bind operator → transcription proceeds → enzymes produced

• When glucose is present, cAMP levels are low → cAMP-CAP complex cannot form → poor transcription
• When glucose is absent, cAMP rises → cAMP binds CAP → CAP-cAMP binds upstream of promoter → enhances RNA polymerase binding → maximal transcription

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3. Biochemical Roles of Thiamine Pyrophosphate (TPP) (6 marks)

• Dry beriberi: peripheral neuropathy (nerve cells depend heavily on glucose)
• Wet beriberi: cardiac failure, edema
• Wernicke-Korsakoff syndrome (in alcoholics): confusion, ataxia, ophthalmoplegia; due to transketolase failure in neural tissue

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4. Blotting Techniques and Applications (6 marks)

• DNA → restriction digestion → gel electrophoresis → transfer to nitrocellulose membrane → hybridize with labeled probe
• Applications: gene mapping, RFLP analysis, forensic DNA fingerprinting, diagnosis of sickle cell anemia

• RNA → gel electrophoresis → transfer to membrane → hybridize with probe
• Applications: mRNA expression studies, viral RNA detection

• Proteins → SDS-PAGE → transfer to PVDF membrane → incubate with primary antibody → secondary antibody (enzyme-linked) → detect bands
• Applications: HIV diagnosis (confirmatory), protein identification, detection of antibodies

• Detects post-translational modifications (lipids, phosphorylation)

• Detects DNA-binding proteins

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5. Importance of Doctor-Patient Relationship (6 marks)

1. Trust and Communication: Patients disclose sensitive information only when they trust their physician, leading to accurate diagnosis.
2. Compliance/Adherence: A good relationship improves patient adherence to treatment plans by 50-70%.
3. Therapeutic effect: The relationship itself has healing value - placebo effect, reduced anxiety, better outcomes.
4. Informed consent: Effective communication ensures patients understand their condition, treatment options, risks.
5. Confidentiality: Foundation of medical ethics - patient information kept private, encouraging honest disclosure.
6. Holistic care: Good relationship allows the physician to address psychological, social, and economic factors alongside biological disease.
7. Prevention: Patients with good doctor relationships are more likely to undergo preventive screenings.

• Paternalistic model (doctor decides)
• Autonomous model (patient decides)
• Shared decision-making model (current preferred model)

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6. PCR and Its Applications (6 marks)

• Template DNA
• Two flanking primers (oligonucleotides)
• Taq DNA polymerase (thermostable, from Thermus aquaticus)
• dNTPs (dATP, dGTP, dCTP, dTTP)
• Buffer with Mg²⁺

1. Denaturation (94-96°C): DNA double strand separates
2. Annealing (50-65°C): Primers bind to complementary sequences
3. Extension (72°C): Taq polymerase extends from primers

• Diagnosis of infectious diseases (HIV, TB, COVID-19, Hepatitis)
• Genetic disease diagnosis (cystic fibrosis, sickle cell disease)
• Prenatal diagnosis
• Forensic medicine and crime investigation
• Cancer diagnosis (oncogene detection)
• Paternity testing
• Sequencing (RT-PCR for RNA viruses)

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MECHANISM OF ACTION OF STEROID HORMONES (5 marks)

1. Steroid hormone diffuses freely across plasma membrane (lipophilic)
2. Binds to specific intracellular receptor protein in cytoplasm or nucleus
3. Binding causes conformational change → receptor activated → heat shock proteins (HSP70, HSP90) dissociate
4. Hormone-receptor complex dimerizes and translocates to nucleus (if cytoplasmic)
5. Complex binds to specific DNA sequences called Hormone Response Elements (HREs)
6. Acts as transcription factor → alters gene transcription (increase or decrease)
7. New mRNA produced → new protein synthesized → biological effect

• Slow onset of action (hours - days) due to protein synthesis required
• Prolonged duration of action
• No second messenger required
• Genomic effects (gene regulation)
• Some steroids also have rapid non-genomic effects via membrane receptors

• Cortisol → gluconeogenesis genes upregulated
• Aldosterone → Na⁺/K⁺ ATPase and ENaC upregulated in renal tubules
• Estrogen → uterine proliferation genes activated

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LIVER FUNCTION TESTS (LFTs) AND ASSOCIATED DISORDERS (5 marks)

• Serum albumin (N: 3.5-5 g/dL) - low in chronic liver disease
• Prothrombin time (PT)/INR - clotting factors synthesized in liver
• Serum fibrinogen

• Serum bilirubin (N: 0.3-1.0 mg/dL) - total, direct, indirect
• Urine bilirubin and urobilinogen
• Serum bile acids

• ALT (SGPT) - most specific for hepatocyte damage (N: <40 IU/L)
• AST (SGOT) - hepatocyte damage but also muscle (N: <40 IU/L)
• ALP (Alkaline phosphatase) - cholestasis, bone disease (N: 40-120 IU/L)
• GGT - alcoholic liver disease, cholestasis
• LDH - non-specific, hepatic necrosis

• BSP (Bromsulphthalein) retention test
• ICG clearance

• Hepatitis: ALT/AST markedly elevated (ALT > AST); bilirubin elevated
• Obstructive jaundice: ALP markedly elevated; direct bilirubin elevated; conjugated bilirubin in urine
• Alcoholic liver disease: AST:ALT ratio > 2:1; GGT elevated
• Cirrhosis: Low albumin, prolonged PT, elevated bilirubin
• Liver failure: All parameters severely deranged

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MUCOSAL BLOCK THEORY OF IRON ABSORPTION (5 marks)

1. Dietary iron (Fe³⁺) is reduced to Fe²⁺ by gastric acid and ascorbic acid
2. Fe²⁺ enters mucosal cell via DMT-1 (Divalent Metal Transporter-1)
3. Inside the cell, iron binds to apoferritin to form ferritin (storage form)
4. Iron is released from ferritin and binds to transferrin in plasma for transport
5. Mucosal block: When body iron stores are high → ferritin in mucosal cells is already saturated → newly absorbed iron cannot leave the cell → when mucosal cells shed (every 3-4 days), iron is lost in feces → acts as a regulatory block

• Hepcidin (liver peptide): high iron stores → high hepcidin → degrades ferroportin → blocks iron export from enterocytes
• IRP (Iron Regulatory Protein): regulates transferrin receptor and ferritin mRNA

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METABOLIC ACIDOSIS (5 marks)

• Methanol poisoning
• Uremia
• Diabetic ketoacidosis
• Propylene glycol
• Isoniazid/Iron
• Lactic acidosis
• Ethanol/Ethylene glycol
• Salicylates

• Diarrhea (loss of HCO₃⁻)
• Renal tubular acidosis
• Ammonium chloride administration

• Blood pH <7.35
• Plasma HCO₃⁻ decreased
• pCO₂ decreased (compensatory hyperventilation - Kussmaul breathing)
• Plasma chloride may increase (in normal AG type)

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TRANSPORT MECHANISMS ACROSS CELL MEMBRANE (5 marks)

1. Simple diffusion: Movement down concentration gradient. Small nonpolar molecules (O₂, CO₂, N₂, urea, ethanol). Rate proportional to concentration gradient.
2. Facilitated diffusion: Carrier protein or channel protein, still down concentration gradient, no ATP. Examples: glucose transport in RBCs (GLUT-1), water (aquaporins - channel proteins).
3. Osmosis: Water movement across semipermeable membrane from low to high solute concentration.
4. Filtration: Movement due to hydrostatic pressure (glomerular filtration).

1. Primary active transport: Direct use of ATP. Examples:
   • Na⁺/K⁺ ATPase: 3 Na⁺ out, 2 K⁺ in per ATP (establishes membrane potential)
   • Ca²⁺ ATPase pump
2. Secondary active transport: Uses Na⁺ gradient established by Na⁺/K⁺ ATPase (indirect ATP):
   • Cotransport (symport): Na⁺-glucose transporter (SGLT) in intestine and kidney
   • Countertransport (antiport): Na⁺/H⁺ exchanger

• Endocytosis: phagocytosis, pinocytosis, receptor-mediated endocytosis (LDL uptake)
• Exocytosis: secretion of hormones, neurotransmitters

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PROFESSIONAL QUALITIES AND ROLES OF A PHYSICIAN (5 marks)

1. Medical competence: Up-to-date knowledge and clinical skills
2. Integrity and honesty: Truthfulness with patients and colleagues
3. Empathy and compassion: Understanding patient suffering
4. Responsibility and accountability: For patient outcomes
5. Commitment to learning: Lifelong medical education
6. Respect for patients: Dignity, privacy, autonomy
7. Teamwork: Collaboration with nurses, specialists, paramedics
8. Ethical conduct: Adherence to medical ethics and law

1. Clinician: Diagnosis and treatment of disease
2. Teacher: Medical education, patient education
3. Researcher: Advancing medical knowledge
4. Manager/Administrator: Hospital management, health policy
5. Community health advocate: Preventive medicine, public health
6. Counselor: Psychosocial support to patients and families
7. Patient advocate: Ensuring patient rights and access to care

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VITAMIN A - BIOCHEMICAL FUNCTION, RDA, AND DEFICIENCY (5 marks)

• Retinal: visual function
• Retinoic acid: gene regulation, growth and differentiation
• Retinol: reproduction, storage form

1. Vision (Visual Cycle):
   • 11-cis retinal + Opsin → Rhodopsin (rod cells for night vision)
   • Light → rhodopsin → all-trans retinal + opsin → nerve impulse → vision
   • Deficiency → Night blindness (nyctalopia)
2. Epithelial differentiation: Retinoic acid acts on nuclear receptors (RAR, RXR) → regulates genes for epithelial cell differentiation; prevents squamous metaplasia.
3. Immune function: Maintains mucosal barrier integrity; essential for lymphocyte function.
4. Growth: Required for normal bone growth (modulates osteoblast/osteoclast activity).
5. Antioxidant (β-carotene): Quenches singlet oxygen and free radicals.
6. Reproduction: Required for spermatogenesis and ovarian function.

• Night blindness (earliest sign)
• Xerophthalmia (dry eye), Bitot's spots (conjunctival plaques)
• Keratomalacia (corneal softening) → blindness
• Follicular hyperkeratosis (toad skin)
• Increased susceptibility to infections
• Impaired growth and reproductive failure

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HMP SHUNT (Hexose Monophosphate Pathway) AND ITS SIGNIFICANCE (5 marks)

• Glucose-6-phosphate → 6-phosphoglucono-δ-lactone (G6PD, rate-limiting) → Ribulose-5-phosphate
• Products: 2 NADPH, 1 CO₂ per G-6-P

• Interconversion of sugars (3C, 4C, 5C, 6C, 7C)
• Transketolase (TPP-dependent) and transaldolase reactions
• Products: Fructose-6-phosphate and Glyceraldehyde-3-phosphate (enter glycolysis)

1. NADPH generation:
   • Fatty acid and cholesterol synthesis (requires NADPH)
   • Steroid hormone synthesis
   • Reduced glutathione maintenance in RBCs (protects against oxidative hemolysis)
   • Cytochrome P450 reactions
2. Ribose-5-phosphate synthesis:
   • Required for nucleotide and nucleic acid synthesis (DNA, RNA, ATP, coenzymes)
3. Interconversion of sugars: Provides metabolic flexibility

• NADPH deficiency → oxidized glutathione accumulates → Heinz bodies in RBCs → hemolytic anemia
• Triggered by oxidant drugs (primaquine, dapsone), infections, fava beans
• X-linked disorder, most common enzyme deficiency worldwide

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GLYCOGEN STORAGE DISEASES (5 marks)

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FOLATE TRAP (5 marks)

• Folate (folic acid) enters cells as 5-methyltetrahydrofolate (5-methyl-THF)
• 5-methyl-THF must donate its methyl group to homocysteine to form methionine (via methionine synthase, which requires Vitamin B12 as cofactor)
• This reaction regenerates THF (active form)

• In Vitamin B12 deficiency, methionine synthase cannot function
• 5-methyl-THF cannot be converted to THF
• Folate remains "trapped" as 5-methyl-THF (inactive for DNA synthesis)
• THF is depleted → no thymidylate synthesis → DNA synthesis impaired
• Result: Megaloblastic anemia (identical to folate deficiency) even when serum folate levels are normal or elevated

• Explains why Vitamin B12 deficiency causes megaloblastic anemia via folate trapping
• Serum folate: high in B12 deficiency (trapped as 5-methyl-THF)
• RBC folate: low (reflects true intracellular folate deficiency)
• Giving folic acid alone in B12 deficiency corrects anemia but does NOT prevent neurological damage (subacute combined degeneration of spinal cord) - hence always check B12 before treating with folate alone

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PUFA AND THEIR SIGNIFICANCE (5 marks)

• ω-6 (n-6) series: Linoleic acid (18:2, ω-6) - essential; arachidonic acid (20:4, ω-6)
• ω-3 (n-3) series: α-Linolenic acid (18:3, ω-3) - essential; EPA (20:5, ω-3); DHA (22:6, ω-3)

1. Structural: Major components of cell membranes and myelin (maintain fluidity)
2. Eicosanoid precursors:
   • Arachidonic acid → prostaglandins, thromboxanes, leukotrienes (inflammation, pain, fever, platelet aggregation)
   • EPA → series-3 prostaglandins (less inflammatory)
3. Cardiovascular protection (ω-3):
   • Reduce triglycerides
   • Antiarrhythmic
   • Anti-inflammatory
   • Reduce platelet aggregation
   • DHA: brain development and retinal function
4. EFA deficiency: Scaly dermatitis, growth retardation, increased susceptibility to infection, impaired wound healing

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COMPETITIVE ENZYME INHIBITION - CLINICAL IMPORTANCE (5 marks)

1. Methotrexate inhibits dihydrofolate reductase (DHFR) → competitively inhibits dihydrofolate → THF cannot be synthesized → DNA synthesis blocked → used in cancer, rheumatoid arthritis.
2. Sulfonamides competitively inhibit dihydropteroate synthase in bacteria (compete with PABA) → bacterial folate synthesis blocked → bacteriostatic.
3. Allopurinol inhibits xanthine oxidase (competitive then irreversible) → reduces uric acid synthesis → used in gout.
4. Statin drugs (Lovastatin, Simvastatin) competitively inhibit HMG-CoA reductase → reduces cholesterol synthesis → used in hypercholesterolemia.
5. Neostigmine competitively inhibits acetylcholinesterase → increases acetylcholine at synapse → used in myasthenia gravis.
6. Captopril/Enalapril inhibit Angiotensin Converting Enzyme (ACE) → reduce angiotensin II → antihypertensive.
7. Aspirin - irreversible (non-competitive) inhibitor of COX → important to distinguish.

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DIABETIC KETOACIDOSIS (DKA) (5 marks)

1. Insulin deficiency + glucagon excess:
   • Glucose cannot enter cells → hyperglycemia
   • Gluconeogenesis and glycogenolysis increased
   • Lipolysis massively increased → free fatty acids released → liver β-oxidation → excess acetyl CoA → ketone body synthesis (acetoacetate, β-hydroxybutyrate, acetone)
   • Ketone bodies are acids → metabolic acidosis (high anion gap)
2. Electrolyte disturbances:
   • Osmotic diuresis → dehydration, Na⁺, K⁺ loss
   • Total body K⁺ depleted (despite initial normal/high serum K⁺ due to acidosis)
   • Treatment with insulin causes K⁺ to shift into cells → hypokalemia risk

• Blood glucose >250 mg/dL
• Arterial pH <7.3, HCO₃⁻ <15 mEq/L
• Urine/serum ketones positive

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VITAMIN B-COMPLEX AS CO-ENZYME (5 marks)

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DNA REPAIR MECHANISMS (5 marks)

1. Base Excision Repair (BER):
   • Single damaged base removed by DNA glycosylase
   • AP endonuclease cleaves backbone
   • DNA polymerase fills gap, ligase seals
   • Corrects: deamination, oxidation (8-oxoguanine), alkylation
2. Nucleotide Excision Repair (NER):
   • Removes bulky adducts and UV-induced thymine dimers
   • Excises a 25-30 nucleotide oligomer
   • Defective in Xeroderma pigmentosum → extreme UV sensitivity, skin cancers
3. Mismatch Repair (MMR):
   • Corrects base-pair mismatches after replication
   • Defective in Hereditary Non-Polyposis Colorectal Cancer (HNPCC/Lynch syndrome)
4. Homologous Recombination: Repairs double-strand breaks using sister chromatid as template (accurate). Involves BRCA1, BRCA2. Defective in hereditary breast/ovarian cancer.
5. Non-Homologous End Joining (NHEJ): Repair of double-strand breaks - error prone. Main pathway in G1 phase.
6. Direct repair: Photolyase (not in humans), methyltransferase (MGMT).

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KREBS-HENSELEIT (UREA) CYCLE (5 marks)

1. NH₃ + CO₂ + 2ATP → Carbamoyl phosphate (mitochondria; CPS-I, rate-limiting; N-acetylglutamate is allosteric activator)
2. Carbamoyl phosphate + Ornithine → Citrulline (OTC enzyme)
3. Citrulline enters cytoplasm
4. Citrulline + Aspartate + ATP → Argininosuccinate (argininosuccinate synthetase)
5. Argininosuccinate → Arginine + Fumarate (argininosuccinate lyase)
6. Arginine + H₂O → Urea + Ornithine (arginase)
7. Ornithine returns to mitochondria

• CPS-I deficiency: high ammonia
• OTC deficiency (X-linked): most common; high ammonia, high orotic acid
• Argininosuccinate lyase deficiency: argininosuccinicaciduria

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BIOCHEMICAL CHANGES IN DIFFERENT TYPES OF JAUNDICE (5 marks)

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BIOLOGICALLY IMPORTANT PEPTIDES (5 marks)

1. Glutathione (γ-Glu-Cys-Gly): Tripeptide; antioxidant; protects RBCs from oxidative hemolysis; detoxification (conjugation with xenobiotics via GST).
2. Oxytocin (9 AA): Uterine contraction during labor; milk ejection; social bonding.
3. Vasopressin/ADH (9 AA): Water reabsorption in collecting duct; vasoconstriction at high doses.
4. Bradykinin (9 AA): Vasodilation, pain, inflammation; kallikrein-kinin system.
5. Angiotensin II (8 AA): Vasoconstriction; aldosterone stimulation; BP regulation.
6. Insulin (51 AA): Blood glucose lowering; anabolic hormone.
7. Enkephalins (5 AA): Met-enkephalin, Leu-enkephalin; endogenous opioids; pain modulation.
8. TRH (Thyrotropin-releasing hormone) (3 AA): pGlu-His-Pro; stimulates TSH and prolactin release.
9. Carnosine (β-alanyl-histidine): Antioxidant in muscle; buffer.
10. Anserine and Homocarnosine: Related dipeptides in muscle and brain.

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FREE RADICALS - DEFINITION AND PHYSIOLOGICAL FUNCTIONS (5 marks)

1. Antimicrobial defense (Respiratory burst):
   • Neutrophils and macrophages use NADPH oxidase to generate superoxide → H₂O₂ → HOCl (myeloperoxidase) → kills ingested bacteria
   • Essential for defense against Staphylococcus, Aspergillus
   • Defective in Chronic Granulomatous Disease (CGD)
2. Nitric Oxide (NO) as signaling molecule:
   • Endothelial NO (NO•) diffuses to smooth muscle → activates guanylate cyclase → cGMP → vasodilation
   • In nervous system: NO acts as retrograde neurotransmitter
   • In macrophages: high-output NO kills intracellular pathogens (Mycobacterium, Leishmania)
3. Prostaglandin synthesis:
   • Arachidonic acid oxidation involves free radical intermediates (COX reaction: lipoxygenase pathway)
   • Prostaglandins regulate inflammation, fever, pain, platelet aggregation, gastric mucosal protection

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GOUT (5 marks)

• Uric acid = final product of purine catabolism in humans (unlike most mammals that have uricase)
• Normal serum uric acid: <7 mg/dL (men), <6 mg/dL (women)
• Hyperuricemia → supersaturation → crystal deposition

• Primary gout: Overproduction (HGPRT deficiency - Lesch-Nyhan; PRPP synthetase overactivity) or underexcretion (idiopathic, 90% of cases)
• Secondary gout: Increased cell turnover (leukemia, psoriasis), renal failure, drugs (thiazides, aspirin low dose)

• Acute gouty arthritis: Podagra (big toe, 1st MTP joint), intense pain, warmth, redness - often nocturnal
• Tophi: Urate deposits in soft tissue (ear pinna, around joints)
• Gouty nephropathy, uric acid nephrolithiasis

• Acute: Colchicine, NSAIDs, corticosteroids
• Chronic: Allopurinol (xanthine oxidase inhibitor) - reduces uric acid synthesis; Febuxostat; Uricosuric agents (probenecid)

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P53 GENE (5 marks)

• Tumor suppressor gene
• Encodes a transcription factor protein (p53) - 393 amino acids

1. Cell cycle arrest: p53 activates p21 (CDK inhibitor) → blocks G1/S transition → time for DNA repair
2. DNA repair: Transcribes DNA repair genes (e.g., GADD45)
3. Apoptosis: If damage is irreparable → p53 activates pro-apoptotic genes (BAX, PUMA) → programmed cell death → prevents propagation of damaged DNA
4. Senescence: Induces permanent cell cycle arrest in pre-cancerous cells
5. Angiogenesis inhibition: Suppresses VEGF expression

• Li-Fraumeni syndrome: Germline p53 mutation → multiple early-onset cancers (sarcomas, breast cancer, leukemias)
• Somatic mutations: Found in colorectal, lung, breast, bladder cancers

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PEM - PROTEIN ENERGY MALNUTRITION (5 marks)

• Typically in children weaned onto carbohydrate-rich, protein-poor diet
• Features: Edema (hypoalbuminemia), "flaky paint" dermatosis, hair changes (flag sign, hypopigmentation), moon face, fatty liver (VLDL synthesis impaired), apathy, irritability
• Serum albumin markedly low

• Severe overall starvation
• Features: Extreme wasting, "old man" appearance, prominent ribs, no edema, no fatty liver
• Serum albumin relatively maintained (catabolizes muscle protein)

• ↓ serum albumin (<2.8 g/dL)
• ↓ transferrin, ↓ retinol-binding protein
• ↓ serum amino acids (essential AAs)
• Fatty liver (excess fat accumulation)
• Normal or high serum glucose
• Anemia (iron, folate, B12 deficiency)

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LESCH-NYHAN SYNDROME (5 marks)

• HGPRT normally salvages hypoxanthine and guanine → IMP and GMP (purine salvage pathway)
• Without HGPRT: hypoxanthine and guanine cannot be salvaged → degraded to uric acid → severe hyperuricemia
• PRPP (phosphoribosyl pyrophosphate) accumulates (normally used by HGPRT) → stimulates de novo purine synthesis → more purines → more uric acid

• Severe gout (hyperuricemia, tophi, nephropathy)
• Self-mutilation (biting lips, fingers) - pathognomonic
• Choreoathetosis (basal ganglia dysfunction)
• Mental retardation (IQ <50)
• Spasticity, opisthotonus

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ONE-CARBON METABOLISM (5 marks)

• N⁵-methyl-THF: methyl group (-CH₃) - most reduced
• N⁵,N¹⁰-methylene-THF: methylene (-CH₂-)
• N⁵,N¹⁰-methenyl-THF: methenyl (-CH=)
• N¹⁰-formyl-THF: formyl (-CHO) - most oxidized
• N⁵-formimino-THF: formimino (-CH=NH)

1. Thymidylate synthesis: N⁵,N¹⁰-methylene-THF + dUMP → dTMP (thymidylate synthase) → DNA synthesis
2. Purine synthesis: N¹⁰-formyl-THF donates carbon-2 and carbon-8 of purine ring
3. Methionine synthesis: N⁵-methyl-THF + Homocysteine → Methionine (methionine synthase, B12-dependent)
4. Serine ↔ Glycine interconversion: N⁵,N¹⁰-methylene-THF involved (serine hydroxymethyltransferase)

• Folate/B12 deficiency → impaired one-carbon metabolism → megaloblastic anemia
• Methotrexate blocks DHFR → depletes THF → impairs one-carbon metabolism
• Homocystinuria: methionine synthase deficiency → homocysteine accumulates

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MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) (5 marks)

• Expressed on ALL nucleated cells + platelets
• Present endogenous (intracellular) peptides (9-mer) to CD8⁺ T cells (cytotoxic T lymphocytes)
• Structure: α-chain + β₂-microglobulin

• Expressed on APCs (dendritic cells, macrophages, B cells)
• Present exogenous (extracellular) peptides (13-18 mer) to CD4⁺ T helper cells
• Structure: α + β chains (both MHC-encoded)

• Transplantation: HLA matching reduces rejection risk
• Disease associations: HLA-B27 → ankylosing spondylitis, Reiter's syndrome; HLA-DR3/DR4 → Type 1 diabetes, rheumatoid arthritis; HLA-DQ2/DQ8 → Celiac disease
• Blood transfusion: HLA antibodies cause febrile non-hemolytic transfusion reactions
• Antigen presentation is fundamental to adaptive immunity

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ACUTE INTERMITTENT PORPHYRIA (AIP) (5 marks)

• Heme synthesis is impaired → less heme → negative feedback on ALA synthase (ALAS1) is removed → ALAS1 increases → more ALA and PBG accumulate (neurotoxic)
• Triggers: Drugs (barbiturates, estrogens, sulfonamides, rifampicin), fasting, infection, stress, alcohol

• Severe colicky abdominal pain (most common)
• Peripheral neuropathy (motor > sensory)
• Psychiatric symptoms (anxiety, psychosis, depression)
• Autonomic dysfunction: tachycardia, hypertension, urinary retention
• Dark red/brown urine (on standing, PBG polymerizes to porphobilin)
• NO SKIN PHOTOSENSITIVITY (distinguishes from other porphyrias)

• Urine PBG elevated (Watson-Schwartz test: positive - cherry red with Ehrlich's reagent)
• Urine ALA elevated
• Genetic testing

• Acute: IV hemin (glucose loading to suppress ALAS1), high carbohydrate, IV glucose (glucose effect)
• Avoid triggers

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ENZYME INHIBITION AND ITS TYPES (5 marks)

• Inhibitor structurally similar to substrate; competes for active site
• Km increases, Vmax unchanged (increased substrate overcomes inhibition)
• Lineweaver-Burk: lines meet on y-axis (same Vmax); x-intercept changes
• Example: Malonate inhibits succinate dehydrogenase; statins inhibit HMG-CoA reductase

• Inhibitor binds allosteric site (not active site); can bind with or without substrate
• Km unchanged, Vmax decreases (substrate cannot overcome inhibition)
• Lineweaver-Burk: lines meet on x-axis; slope changes
• Example: Cyanide inhibits cytochrome oxidase

• Inhibitor binds only to enzyme-substrate (ES) complex
• Both Km and Vmax decrease (apparent Km decreases)
• Lineweaver-Burk: parallel lines
• Example: Lithium inhibits inositol phosphatase

• Inhibitor can bind enzyme or ES complex (with different affinities)
• Km may increase or decrease; Vmax decreases

• Inhibitor forms covalent bond with enzyme → permanent inactivation
• Examples: Organophosphates inhibit acetylcholinesterase; Aspirin irreversibly inhibits COX; DFP inhibits serine proteases

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COLLAGEN - STRUCTURE, MATURATION, AND FUNCTIONS (5 marks)

• Basic unit: Tropocollagen - triple helix of 3 α-chains; 300 nm long, 1.5 nm wide
• Each α-chain: repeating (Gly-X-Y)ₙ sequence (Gly at every 3rd position is essential; Gly is smallest AA - fits inside helix)
• X is often proline, Y is often hydroxyproline or hydroxylysine (stabilize triple helix)
• 3 α-chains coil into right-handed triple helix

1. Hydroxylation of Pro → Hydroxyproline; Lys → Hydroxylysine (requires Vitamin C + O₂ + Fe²⁺; enzyme: prolyl hydroxylase)
2. Glycosylation of hydroxylysine (Glc-Gal added)
3. Signal peptide cleaved; pro-peptides removed extracellularly by procollagen peptidase → tropocollagen
4. Cross-linking: Hydroxylysine → allysine (lysyl oxidase, requires Cu²⁺) → Schiff base and aldol condensation → covalent cross-links between adjacent tropocollagen molecules → collagen fibrils → collagen fibers

• Structural support (bone, tendon, ligament, skin, cartilage)
• Wound healing
• Basement membrane (Type IV)

• Scurvy (Vit C deficiency): Impaired hydroxylation → unstable collagen → bleeding gums, perifollicular hemorrhage, poor wound healing
• Osteogenesis imperfecta: Defective Type I collagen → brittle bones, blue sclerae
• Ehlers-Danlos syndrome: Defective cross-linking → hyperextensible skin, hypermobile joints
• Marfan syndrome: Fibrillin-1 defect (associated connective tissue protein)

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STRUCTURE AND ROLE OF HEMOGLOBIN AS A PH BUFFER (5 marks)

• Hemoglobin has numerous histidine residues (pKa ~6.0; ideal for physiological pH buffering at 7.4)
• Imidazole group of histidine can accept or donate H⁺:
  • His-H⁺ (protonated, acidic form) ⇌ His (free base) + H⁺

• In tissues: CO₂ produced → enters RBC → carbonic anhydrase → H₂CO₃ → H⁺ + HCO₃⁻
• H⁺ ions bind to deoxyhemoglobin (Hb is a better proton acceptor than OxyHb)
• OxyHb releases O₂ (Bohr effect) to tissues
• HCO₃⁻ exchanges with Cl⁻ (chloride shift)

• O₂ binds Hb → HbO₂ releases H⁺ → H⁺ + HCO₃⁻ → H₂CO₃ → CO₂ expired

• Hb contributes ~80% of non-bicarbonate buffering capacity of blood
• Deoxygenated Hb is a weaker acid than oxyHb → better proton buffer in tissues

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OXIDATION OF VERY LONG CHAIN FATTY ACIDS (5 marks)

1. VLCFA activated to VLCFA-CoA (outer peroxisomal membrane)
2. First oxidation step: VLCFA-CoA → trans-enoyl-CoA by acyl-CoA oxidase (FAD-linked but electrons passed to O₂ → H₂O₂, NOT to ETC; H₂O₂ destroyed by catalase)
3. Hydration, oxidation (NAD⁺-linked), thiolysis - same as mitochondrial β-oxidation
4. Chain is shortened to medium/short chain fatty acids (C8:0 octanoyl-CoA)
5. Medium chain acyl-CoA exits peroxisome → enters mitochondria for complete oxidation

• First step oxidation by FAD-linked oxidase → H₂O₂ (not FADH₂ → ETC)
• No ATP produced from first oxidation step (less efficient)
• Cannot completely oxidize to acetyl-CoA; the remainder goes to mitochondria

• Zellweger syndrome (Cerebrohepatorenal syndrome): Absent peroxisomes → VLCFA accumulate → brain, liver, kidney damage
• X-linked adrenoleukodystrophy (ALD): Defective VLCFA transporter (ABCD1) → VLCFA accumulate in CNS and adrenal cortex → demyelination, adrenal insufficiency (Lorenzo's Oil disease)

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PHASE II REACTIONS OF BIOTRANSFORMATION (5 marks)

• Phase I: Functionalization (oxidation, reduction, hydrolysis - introduces -OH, -NH₂, -COOH groups) - mainly CYP450
• Phase II: Conjugation reactions - attaches polar groups to Phase I products → more water-soluble → easier excretion

• NAT polymorphism: Slow acetylators → higher isoniazid plasma levels → neuropathy risk; fast acetylators → need higher doses
• UGT1A1 polymorphism → Gilbert's syndrome, neonatal jaundice

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TUMOR MARKERS IN DIAGNOSIS AND PROGNOSIS (5 marks)

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PROTEIN TARGETING AND SORTING (5 marks)

1. Cytosolic proteins: No signal sequence → remain in cytoplasm or directed to nucleus/mitochondria
2. Secretory/Membrane proteins (Secretory pathway):
   • N-terminal signal sequence → Signal Recognition Particle (SRP) recognizes it
   • Ribosome-SRP complex docks on ER membrane → protein enters ER lumen co-translationally
   • → Golgi (modification, packaging) → secretory vesicles → plasma membrane/secretion
3. Nuclear proteins:
   • Nuclear localization signal (NLS) - basic amino acid-rich sequence (Lys, Arg)
   • Importins recognize NLS → transport through nuclear pore complex
4. Mitochondrial proteins:
   • N-terminal amphipathic helix signal (matrix targeting sequence)
   • Translocases TOM (outer) and TIM (inner) import proteins
5. Peroxisomal proteins:
   • PTS1: C-terminal tripeptide (Ser-Lys-Leu)
   • PTS2: N-terminal signal
   • Peroxin (PEX) proteins mediate import
6. Lysosomal proteins (Mannose-6-phosphate tagging):
   • Glycoproteins in Golgi receive mannose-6-phosphate tag
   • Mannose-6-P receptor directs them to lysosomes
   • Defect: I-cell disease (mucolipidosis II) → lysosomal enzymes secreted instead of targeted → accumulation

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SHORT ANSWER TYPE QUESTIONS

Ammonia Intoxication - Why Life-Threatening? (5 marks)

1. Depletion of α-ketoglutarate (α-KG):
   • NH₃ + α-KG → Glutamate (glutamate dehydrogenase)
   • NH₃ + Glutamate → Glutamine (glutamine synthetase)
   • α-KG depleted → TCA cycle inhibited → ATP production falls → brain cells (which are highly aerobic) fail → cerebral edema, coma
2. Glutamine accumulation in astrocytes:
   • Astrocytes convert NH₃ to glutamine
   • Glutamine is osmotically active → astrocyte swelling → cerebral edema (major cause of death)
   • Glutamine also osmotically disturbs mitochondria
3. Altered neurotransmission:
   • Excess glutamate → NMDA receptor overstimulation → excitotoxicity
   • Disruption of GABA (inhibitory) and glutamate (excitatory) balance
   • False neurotransmitters may accumulate

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Q Cycle (5 marks)

• QH₂ (ubiquinol) binds outer Qo site
• One electron → Rieske FeS → Cyt c₁ → Cyt c (reduced)
• Other electron → Cyt bL → Cyt bH → Q at Qi site → semiquinone Q•⁻
• 2H⁺ released to intermembrane space (IMS)

• Another QH₂ at Qo site, same bifurcation
• Second electron reaches Q•⁻ at Qi site → QH₂ (takes 2H⁺ from matrix)
• Another 2H⁺ to IMS; another Cyt c reduced

• 2 QH₂ oxidized, 1 QH₂ regenerated at inner surface
• 2 Cyt c reduced
• 4H⁺ pumped into IMS per 2 electrons (increases P/O ratio)

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Hartnup's Disease (5 marks)

• Tryptophan normally → Niacin (via kynurenine pathway) and serotonin
• Failure to absorb tryptophan → colonic bacteria convert tryptophan → indole compounds → absorbed → neurological/skin symptoms
• Functional niacin deficiency → pellagra-like features

• Dermatitis: Photosensitive skin rash (exposed areas)
• Diarrhea
• Dementia/Cerebellar ataxia (neuropsychiatric features)

• Aminoaciduria: Generalized neutral aminoaciduria (tryptophan, alanine, serine, threonine, etc. in urine)
• Normal plasma amino acids (transport defect, not metabolic defect)
• Elevated urine indole compounds (indican, indole acetic acid)

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Ketogenic Diet (5 marks)

• Carbohydrate restriction → low insulin, high glucagon
• Increased lipolysis → increased free fatty acids → β-oxidation → excess acetyl-CoA
• Acetyl-CoA exceeds TCA cycle capacity → ketone body synthesis (acetoacetate, β-hydroxybutyrate)
• Brain adapts to use ketone bodies as fuel instead of glucose

1. Epilepsy (main indication): Ketone bodies raise seizure threshold; GABA production increases; reduces neuronal excitability; effective in drug-resistant epilepsy, especially in children
2. Type 2 diabetes and insulin resistance (weight loss, improved glycemic control)
3. Obesity management
4. GLUT-1 deficiency syndrome (brain cannot use glucose → ketones bypass the defect)
5. Pyruvate dehydrogenase deficiency

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Mechanism of Action of ADH (Antidiuretic Hormone/Vasopressin) (5 marks)

1. ADH binds to V2 receptor on basolateral membrane of collecting duct principal cells
2. V2 receptor is Gs-coupled → activates adenylyl cyclase → ↑ cAMP
3. cAMP activates Protein Kinase A (PKA)
4. PKA phosphorylates aquaporin-2 (AQP2) vesicles
5. AQP2 vesicles insert into apical membrane (via exocytosis)
6. Apical membrane becomes permeable to water
7. Water moves from tubular lumen → cell → interstitium (via AQP3 and AQP4 on basolateral side) → concentrated urine
8. On withdrawal of ADH → AQP2 endocytosed back → dilute urine

• Diabetes insipidus: Lack of ADH (central DI) or lack of V2 receptor/AQP2 response (nephrogenic DI) → dilute polyuria
• SIADH: Excess ADH → water retention → hyponatremia

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XENOBIOTICS AND CYTOCHROME P450 (5 marks)

• Superfamily of heme-containing monooxygenases (mixed function oxidases)
• Located mainly in ER of hepatocytes; also intestine, lung, adrenal
• Named for characteristic absorption at 450 nm when bound to CO

1. Xenobiotic (RH) binds CYP450 (Fe³⁺)
2. Molecular O₂ binds → NADPH reduces Fe³⁺ to Fe²⁺ → O₂ activated
3. One oxygen atom → substrate (adds -OH group: hydroxylation)
4. Other oxygen atom → H₂O
5. Net reaction: RH + O₂ + NADPH → ROH + H₂O + NADP⁺

• CYP3A4: metabolizes ~50% of drugs (statins, midazolam, cyclosporine)
• CYP2D6: codeine → morphine; polymorphism affects opioid response
• CYP1A2: caffeine, theophylline, paracetamol
• CYP2E1: ethanol, acetaminophen (toxic metabolite NAPQI)

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VITAMIN D - BIOSYNTHESIS, ACTIVATION, MECHANISM, DEFICIENCY (5 marks)

1. Skin: 7-dehydrocholesterol + UV radiation (UVB) → Cholecalciferol (Vitamin D₃)
2. Liver: D₃ + 25-hydroxylase → 25(OH)D₃ (calcidiol) - major circulating form; storage form; measured clinically
3. Kidney: 25(OH)D₃ + 1α-hydroxylase (PTH-stimulated, phosphate-low) → 1,25(OH)₂D₃ (calcitriol) - active form; 1,25-dihydroxycholecalciferol (also called 1,25-dihydroxyvitamin D or the question image notes "1,25-dicalciferol")

• Increases intestinal Ca²⁺ and phosphate absorption
• Increases renal Ca²⁺ reabsorption
• Stimulates bone mineralization
• With PTH: bone resorption (increases serum Ca²⁺)
• Immunomodulation; anti-proliferative effects

• Children: Rickets - soft, deformable bones; bowleg (genu varum), knock-knee (genu valgum), Harrison's groove, pigeon chest (pectus carinatum), Rachitic rosary, frontal bossing, delayed dentition, hypocalcemic tetany/convulsions
• Adults: Osteomalacia - bone pain, tenderness, muscle weakness, proximal myopathy, Looser's zones (pseudofractures on X-ray)
• Both: Hypocalcemia, hypophosphatemia, elevated ALP, elevated PTH (secondary hyperparathyroidism)

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HEMOGLOBINOPATHIES (5 marks)

• Mutation: β-chain, codon 6 → Glu replaced by Val (GAG → GTG)
• HbS polymerizes when deoxygenated → sickling → hemolysis, vaso-occlusion
• Features: Hemolytic anemia, vaso-occlusive crises (bone pain, stroke, acute chest syndrome), splenic sequestration, dactylitis, increased infection risk
• Diagnosis: HPLC, sickling test, electrophoresis
• Treatment: Hydroxyurea (increases HbF), exchange transfusion, stem cell transplant

• Deletion of α-globin genes (4 genes total on chr 16)
• 1 gene deleted: silent carrier; 2: α-thalassemia trait; 3: HbH disease (β₄ tetramers); 4: Hb Bart's (γ₄) - hydrops fetalis, incompatible with life

• Mutations in β-globin gene (chr 11); β⁺ or β⁰ mutations
• β-thalassemia major (Cooley's anemia): severe hemolytic anemia, splenomegaly, bone deformities, iron overload, transfusion-dependent
• β-thalassemia minor: mild microcytic anemia, usually asymptomatic

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COMPLEMENT SYSTEM PATHWAYS AND THEIR ROLES (5 marks)

• Activated by: Antigen-antibody complexes (IgG or IgM bound to antigen)
• Initiated by C1q binding to Fc region of antibody
• C1q → C1r, C1s activated → C4, C2 → C3 convertase (C4b2a)

• Activated by: Mannose-binding lectin (MBL) or ficolins binding to microbial carbohydrates (mannose, fucose)
• MASP-1, MASP-2 activated → C4, C2 → C3 convertase (same as classical)

• Continuous low-level C3 hydrolysis ("tick-over")
• Amplified on foreign surfaces (bacteria, fungi) lacking complement regulatory proteins
• Factor B, Factor D, Properdin → C3 convertase (C3bBb)

• Opsonization: C3b coats bacteria → phagocytosis (CR1 on macrophages)
• Lysis (MAC): Direct killing of gram-negative bacteria
• Anaphylatoxins: C3a, C4a, C5a → mast cell degranulation, inflammation, chemotaxis (C5a most potent)
• Immune complex clearance: C3b facilitates clearance via RBCs and spleen
• Enhancement of antibody response

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BMR - DEFINITION, FACTORS, CLINICAL SIGNIFICANCE (5 marks)

1. Thyroid hormones (T₃, T₄) - most important regulator - increase Na⁺/K⁺ ATPase activity, uncoupling
2. Fever (13% increase per 1°C rise)
3. Growth hormone, testosterone
4. Sympathetic stimulation (adrenaline/noradrenaline)
5. Pregnancy and lactation
6. Tall, thin body build (high surface area)
7. Young age (children have higher BMR)
8. Male sex
9. Cold environment

1. Hypothyroidism
2. Malnutrition, starvation
3. Obesity (lower surface area to mass ratio)
4. Old age
5. Sleep
6. Female sex

• Hypothyroidism: BMR decreased by 30-40%
• Hyperthyroidism: BMR increased by 50-100%
• Historically: BMR used to diagnose thyroid disorders (now replaced by TSH, T₃, T₄)
• Nutritional planning: Calculating caloric requirements for hospitalized patients, athletes
• Obesity management: Understanding energy balance

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COMPLETE LIST OF QUESTIONS ANSWERED

1. Oncogenes
2. Lac operon
3. Biochemical roles of Thiamine pyrophosphate
4. Blotting techniques and applications
5. Importance of doctor-patient relationship
6. PCR and its application

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• For 5-mark questions: Write ~400-500 words with a clear heading, subheadings, and a table or diagram where possible.
• For 6-mark questions: Write ~500-600 words with more detail.
• Always mention clinical correlations - examiners love practical applications.
• Draw simple diagrams for: lac operon, urea cycle, HMP shunt, collagen structure, ADH mechanism.
• Use mnemonics you see here (MUDPILES for metabolic acidosis, SNoW DRoP for blotting).