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NAMO Medical Institute - Biochemistry Paper II

Complete Answer Key

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SECTION B

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Q.2 - Long Essay Questions

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Q.2.1 - DNA Replication in Prokaryotes + Inhibitors (6+4 = 10 Marks)

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• Begins at a single fixed site called the origin of replication (oriC) - a specific sequence ~245 bp.
• DnaA protein recognizes oriC and unwinds the AT-rich region.
• Helicase (DnaB) is loaded by DnaC protein; helicase unwinds the double helix in both directions, creating two replication forks (bidirectional replication).
• Single-strand binding proteins (SSBPs) stabilize the unwound strands.
• Topoisomerase I & II (Gyrase) relieve the torsional stress ahead of the fork.

• DNA polymerase cannot initiate synthesis de novo - it requires a free 3'-OH end.
• Primase (DnaG) synthesizes a short RNA primer (~10 nucleotides) complementary to the template.

• DNA Pol III (the main replicative enzyme) adds dNTPs in 5'→3' direction using the primer as a start.
• Leading strand: synthesized continuously in the 5'→3' direction toward the replication fork.
• Lagging strand: synthesized discontinuously as Okazaki fragments (each ~1000-2000 nucleotides) in 5'→3' direction but away from the fork.

• DNA Pol I (5'→3' exonuclease activity) removes RNA primers and fills the gaps with DNA.

• DNA Ligase seals the nicks between Okazaki fragments using NAD+ as cofactor in prokaryotes.

• Occurs at the Ter sites (termination sequences) recognized by Tus protein which blocks helicase.
• Topoisomerase IV decatenates (unlinks) the two completed circular daughter chromosomes.

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Q.2.2 - Tryptophan Metabolism: Specialized Products and Inborn Errors (4+3+3 = 10 Marks)

• An aromatic, essential amino acid.
• Glucogenic + ketogenic (produces alanine → pyruvate, and acetyl-CoA).

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• Produces indole and skatole (malodorous fecal products).
• In liver: indole → indoxyl sulfate (indican) excreted in urine.

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Q.3 - Reasoning/Justification Questions (3 marks each)

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• Gout: partial deficiency or reduced activity of HGPRT (hypoxanthine-guanine phosphoribosyltransferase) - overproduction of uric acid causes hyperuricemia and crystal deposition in joints.
• Lesch-Nyhan syndrome: complete absence of HGPRT.

1. HGPRT is normally highly expressed in basal ganglia and brain neurons - these cells are uniquely dependent on the salvage pathway (they cannot efficiently synthesize purines de novo).
2. Complete HGPRT absence in neurons leads to dopamine depletion in the basal ganglia and altered dopaminergic/serotonergic neurotransmission.
3. This causes the hallmark features: choreoathetosis, spasticity, intellectual disability, self-mutilating behavior.
4. In gout, partial enzyme activity is enough to maintain neuronal purine supply; neurons are spared.

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• HbA1 (adult hemoglobin) consists of 2α + 2β chains.
• HbF (fetal hemoglobin) consists of 2α + 2γ chains.

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1. Restriction endonucleases recognize specific palindromic sequences and cleave foreign (e.g., viral/phage) DNA at these sites.
2. The bacterium's own DNA has the same recognition sequences, but it is protected by methyltransferases (the modification enzymes of the same R-M system).
3. Methyltransferases add methyl groups (-CH3) to adenine (N6 position) or cytosine (C5 or N4) within the recognition sequence - e.g., in E. coli EcoRI system, the sequence GAATTC has the inner adenine methylated.
4. Restriction endonucleases recognize and cut only unmethylated sequences; methylated DNA is refractory to cleavage.
5. After replication, newly synthesized DNA is hemimethylated (one strand methylated, one not) - the methyltransferase rapidly methylates the new strand before any restriction enzyme can cleave it.

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• Telomeres are repetitive DNA sequences (TTAGGG)n capping the ends of linear chromosomes, protecting them from degradation and end-to-end fusions.
• With each cell division, the end-replication problem causes telomeres to shorten ~50-200 bp per division (DNA Pol cannot replicate the lagging strand's 5' end fully).
• Telomerase is a ribonucleoprotein enzyme (contains TERT - reverse transcriptase + TERC - RNA template) that extends telomeres by adding TTAGGG repeats.
• In most somatic (body) cells, telomerase activity is absent or very low. So telomeres progressively shorten with each division.
• When telomeres become critically short, cells enter replicative senescence (permanent cell-cycle arrest) or undergo apoptosis - this is a molecular clock for cellular ageing.
• Shorter telomeres are associated with: premature ageing disorders (e.g., Progeria, Dyskeratosis congenita - TERC/TERT mutations), age-related diseases (cardiovascular disease, neurodegeneration).
• Telomerase is reactivated in cancer cells (~85%), conferring replicative immortality.
• Thus, telomere shortening due to insufficient telomerase is a key molecular mechanism of ageing.

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• Alpha-1-antitrypsin (A1AT) is a serine protease inhibitor (serpin) synthesized mainly in the liver and secreted into the blood.
• Its primary target is neutrophil elastase - a proteolytic enzyme released by neutrophils in the lung during inflammatory responses.
• Normally, A1AT inhibits elastase by acting as a "bait" substrate - elastase cleaves A1AT, forming a covalent inhibitory complex, inactivating the enzyme.
• In A1AT deficiency (most common mutation: PiZZ genotype - Glu342Lys substitution causing misfolding and retention in hepatocyte ER), circulating A1AT levels are very low (<15% of normal).
• Without adequate A1AT, neutrophil elastase in the lung degrades elastin in alveolar walls unchecked (protease-antiprotease imbalance).
• Elastin provides recoil to alveoli; its destruction leads to permanent enlargement of alveolar spaces (emphysema), loss of elastic recoil, and airflow obstruction (panacinar emphysema, predominantly in lower lobes).
• Smoking worsens it by: (1) recruiting more neutrophils, (2) oxidizing the Met358 residue in A1AT's active site, inactivating it.

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Q.4 - Short Notes (5 marks each)

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Q.4.1 - Formation and Fate of Bilirubin + Note on Jaundice

1. Heme catabolism (75% from senescent RBCs, 25% from myoglobin, cytochromes, ineffective erythropoiesis).
2. Aged RBCs are phagocytosed by macrophages of the RES (spleen, liver, bone marrow).
3. Heme → Biliverdin (green pigment) by heme oxygenase (releases Fe2+ and CO).
4. Biliverdin → Bilirubin (yellow pigment) by biliverdin reductase (uses NADPH).
5. This bilirubin is unconjugated (indirect) bilirubin - lipid soluble, toxic, non-polar.

• Unconjugated bilirubin is insoluble in water; transported in blood bound to albumin (2 molecules per albumin).
• Drugs (salicylates, sulfonamides) compete for albumin binding → risk of kernicterus in neonates.

• Bilirubin dissociates from albumin at hepatocyte sinusoidal membrane.
• Taken up by carrier-mediated transport (OATP1B1/OATP1B3).
• Inside hepatocyte, bound to ligandin (Y protein / glutathione-S-transferase).

• In hepatocyte smooth ER: UDP-glucuronosyltransferase (UGT1A1) conjugates bilirubin with glucuronic acid from UDP-glucuronate.
• Products: Bilirubin monoglucuronide then bilirubin diglucuronide (conjugated/direct bilirubin) - water soluble, non-toxic.

• Conjugated bilirubin secreted into bile canaliculi by MRP2 (multidrug resistance protein 2) transporter.
• Passes into intestine via bile.

• Conjugated bilirubin → urobilinogen (colorless) by intestinal bacteria (β-glucuronidase removes glucuronate, then reduction).
• Urobilinogen: 3 fates:
  1. Most oxidized → stercobilin (brown color of feces).
  2. Small amount absorbed into portal blood → liver → re-excreted in bile (enterohepatic circulation).
  3. Small amount escapes to systemic circulation → kidney → excreted as urobilin (yellow color of urine).

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Q.4.2 - Molecular Basis of Cancer

• Normally promote cell growth and proliferation.
• Mutation converts them to oncogenes (dominant - one copy affected).
• Mechanisms: point mutation (RAS - G12V; constitutively active GTPase), gene amplification (HER2/neu, N-Myc), chromosomal translocation (BCR-ABL in CML - t(9;22) Philadelphia chromosome; creates fusion tyrosine kinase).
• Examples: RAS, MYC, ERBB2 (HER2), ABL, VEGF.

• Normally inhibit cell proliferation or promote apoptosis.
• Rb gene (retinoblastoma): pRb normally binds E2F transcription factor, blocking S-phase entry; mutant pRb cannot bind E2F → uncontrolled proliferation.
• p53 gene (guardian of the genome): in DNA damage, p53 activates p21 (CDK inhibitor → G1 arrest), DNA repair, or apoptosis; mutant p53 (most common mutation in human cancer, >50%) cannot perform this → cells with damaged DNA replicate.
• Two-hit hypothesis (Knudson): both alleles of tumor suppressor must be inactivated.

• Defects lead to genomic instability.
• BRCA1/BRCA2 - breast/ovarian cancer; involved in homologous recombination repair.
• MMR genes (MLH1, MSH2) - Lynch syndrome (hereditary non-polyposis colorectal cancer); microsatellite instability.
• NER defect - Xeroderma pigmentosum - UV-induced skin cancer.

• BCL-2 overexpression (t(14;18) in follicular lymphoma) inhibits apoptosis.
• Caspase defects - escape from programmed cell death.

• Reactivated in ~85% cancers → replicative immortality.

• Sustaining proliferative signaling
• Evading growth suppressors
• Resisting cell death
• Enabling replicative immortality
• Inducing angiogenesis
• Activating invasion and metastasis

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Q.4.3 - Regulation of Gene Expression in Prokaryotes

• Structural genes: lacZ (β-galactosidase), lacY (permease), lacA (transacetylase).
• In absence of lactose: Lac repressor (encoded by lacI) binds operator → blocks transcription.
• In presence of lactose: allolactose (the actual inducer, an isomer of lactose) binds repressor → allosteric change → repressor releases operator → RNA Pol transcribes structural genes.
• Catabolite repression (positive control): When glucose is present (preferred carbon source), cAMP levels are low → CAP (catabolite activator protein) is not activated → transcription is low even if lactose is present (glucose effect/catabolite repression). When glucose is absent, cAMP ↑ → cAMP-CAP complex binds CAP site (upstream of promoter) → enhances RNA Pol binding → full induction.

• Structural genes encode enzymes for tryptophan biosynthesis.
• Aporepressor alone cannot bind operator.
• When tryptophan levels are high: tryptophan acts as corepressor → binds aporepressor → active repressor complex → binds operator → represses transcription (saves energy).
• When tryptophan is scarce: aporepressor alone is inactive → structural genes transcribed → tryptophan synthesized.
• Additional control: Attenuation - a leader sequence containing tandem Trp codons; when Trp is abundant, ribosome translates quickly, causing the mRNA to form a terminator hairpin → premature transcription termination.

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Q.4.4 - α-Helix and β-Pleated Sheet: Secondary Structure of Proteins

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• Proposed by Pauling and Corey (1951).
• A right-handed helix (in natural L-amino acids).
• 3.6 amino acids per turn; pitch = 5.4 Å (0.54 nm); rise per residue = 1.5 Å.
• H-bond formed between: -NH of residue n and -C=O of residue n+4 (intrachain, parallel to helix axis).
• R-groups project outward from the helix axis.
• Disruptors of α-helix: Proline (imino acid - N is part of ring, no H for H-bond, creates kink); charged residues in clusters; bulky R-groups; consecutive Gly (too flexible).
• Examples: Keratin (hair, nails), myosin, tropomyosin (coiled coil = two α-helices wound around each other).

• Fully extended conformation.
• H-bonds form between adjacent strands (interchain), perpendicular to strand direction.
• Parallel β-sheet: strands run in same N→C direction; H-bonds slightly angled; less stable.
• Antiparallel β-sheet: strands run in opposite directions (N→C and C→N); H-bonds are perpendicular, more stable.
• R-groups alternate above and below the plane of the sheet.
• Examples: Silk fibroin (predominantly antiparallel β-sheet); immunoglobulins; β-barrel proteins; amyloid fibrils (abnormal cross-β structure).

• Keratin (α-helical coiled coil) → tough, mechanical strength of hair, nails.
• Silk (β-sheet) → flexibility and tensile strength.
• Abnormal β-sheet aggregation → amyloid deposits (Alzheimer's - Aβ peptide; Prion diseases - PrPsc; Type 2 DM - amylin).
• Proline residues create helix-breaking turns (β-turns) connecting β-strands.

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SECTION C

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Q.5 - Clinical Cases (5 marks each)

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Q.5.1 - Myocardial Infarction: Cardiac Biomarkers + Streptokinase

• CK isoforms: CK-MM (muscle), CK-MB (cardiac, diagnostic), CK-BB (brain).
• LDH isoforms: LDH1 and LDH2 (heart); in MI, LDH1 > LDH2 = "flipped LDH" (normally LDH2 > LDH1).
  • LDH1 (HHHH - 4 H subunits), LDH2 (HHHM), LDH3 (HHMM), LDH4 (HMMM), LDH5 (MMMM - liver/skeletal muscle).

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• Streptokinase is a thrombolytic (fibrinolytic) agent derived from Group C β-hemolytic streptococci.
• Mechanism: Streptokinase combines with plasminogen to form an activator complex → this complex converts free plasminogen → plasmin (serine protease).
• Plasmin cleaves fibrin in the coronary thrombus → thrombus dissolution → restoration of coronary blood flow → salvage of ischemic myocardium.
• In STEMI, thrombus occludes the coronary artery; early reperfusion (within 12 hours, ideally <3 h) limits infarct size and reduces mortality.
• Given at 1.5 million IU IV over 60 minutes as in this case.
• Contraindications: recent surgery, stroke, active bleeding, severe hypertension.
• Note: Streptokinase is antigenic (cannot be repeated within 5 years due to antibodies); alteplase (t-PA) preferred for repeat use.

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Q.5.2 - Porphyria Case (Uroporphyrinogen Decarboxylase Deficiency)

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1. UROD deficiency → accumulation of uroporphyrinogen (and other porphyrins) in tissues (skin, liver).
2. Porphyrins absorb light at the Soret band (~400-410 nm) - in the visible spectrum (UV-A/visible light).
3. In sunlight, the accumulated porphyrins in the skin absorb photons → undergo photoexcitation.
4. Excited porphyrins transfer energy to molecular oxygen → reactive oxygen species (ROS) and singlet oxygen generated.
5. ROS damage: lipid peroxidation of cell membranes, protein oxidation, DNA damage → phototoxic skin lesions (fragile skin, bullae/blisters on sun-exposed areas, hyperpigmentation, scarring, hypertrichosis).

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Q.5.3 - Hypothyroid Patient

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1. Hypothalamus secretes TRH (Thyrotropin-Releasing Hormone) → travels via portal blood to anterior pituitary.
2. Anterior Pituitary thyrotrophs respond to TRH → secrete TSH (Thyroid-Stimulating Hormone) into systemic circulation.
3. TSH binds TSH receptors on thyroid follicular cells → stimulates:
   • Iodide uptake and organification
   • Thyroglobulin synthesis
   • T3/T4 synthesis and secretion
4. Negative feedback: T3 and T4 inhibit both TRH secretion (hypothalamus) and TSH secretion (pituitary).

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1. Iodide trapping: I- actively transported into thyroid follicular cells by Na+/I- symporter (NIS) on the basolateral membrane (driven by Na+/K+ ATPase). Pendrin transports I- apically into follicular lumen.
2. Oxidation of iodide: I- → I0 (active iodine) by thyroid peroxidase (TPO) using H2O2 (generated by Dual oxidase/DUOX2).
3. Iodination of thyroglobulin (organification): TPO iodinates tyrosyl residues of thyroglobulin (a large glycoprotein, 660 kDa, stored as colloid):
   • Tyrosine + I → Monoiodotyrosine (MIT)
   • Tyrosine + 2I → Diiodotyrosine (DIT)
4. Coupling reaction (also by TPO):
   • MIT + DIT → T3 (triiodothyronine) (3 iodines, more active)
   • DIT + DIT → T4 (thyroxine) (4 iodines, prohormone)
5. Endocytosis and proteolysis: Colloid droplets are taken up by endocytosis → lysosomes cleave thyroglobulin → release T3 and T4 into blood.
6. Peripheral conversion: In liver, kidney, muscle: T4 → T3 by 5'-deiodinase (type I/II). T3 is the metabolically active form (3-5x more potent than T4).

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Q.5.4 - Aminoacyl-tRNA Synthetase Defect (Nonsense Mutation)

• Premature termination of translation at the mutant stop codon.
• Production of a truncated, non-functional protein.
• The truncated protein may be degraded by nonsense-mediated mRNA decay (NMD) - a surveillance mechanism that degrades mRNAs with premature stop codons.
• Loss of all functional protein domains downstream of the mutation.

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1. Amino acid + ATP → Aminoacyl-AMP + PPi (amino acid activation)
2. Aminoacyl-AMP + tRNA → Aminoacyl-tRNA + AMP

• Ensures the correct amino acid is attached to the correct tRNA (second genetic code).
• aaRS recognize both the amino acid (via the active site) and the specific tRNA (anticodon loop, acceptor stem - identity elements).
• Proofreading/editing activity: aaRS have an editing (hydrolytic) site that removes mischarged amino acids, ensuring fidelity.
• Without proper charging, the ribosome cannot incorporate the correct amino acid at the codon, despite correct base pairing at the anticodon.

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1. High metabolic demand: Neurons and rapidly developing tissues (brain in fetus/infant) require very high rates of protein synthesis for growth, synaptogenesis, myelination, and receptor expression. Any impairment disproportionately affects these tissues.
2. Essential proteins affected: Many neurodevelopmental proteins (transcription factors, synaptic proteins, ion channels, structural proteins) are uniquely expressed in neurons; their truncation or absence leads to neuronal dysfunction, impaired neurite growth, synaptic failure.
3. Global proteostasis failure: aaRS defects affect ALL proteins (not just one), leading to widespread cellular dysfunction including failure of neurotrophic factor signaling, cytoskeletal organization, and myelination.
4. Irreversible developmental windows: Brain development has critical sensitive periods; protein synthesis failure during these windows causes permanent structural and functional deficits (intellectual disability, hypotonia, seizures, failure to thrive).
5. Mitochondrial energy failure: Neurons are highly energy-dependent; impaired mitochondrial protein synthesis (some aaRS are mitochondrially targeted) → reduced ATP → neuronal death.

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Q.6 - Short Notes (5 marks each)

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Q.6.1 - Hormones/Markers for Reproductive Health and Clinical Interpretation

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Q.6.2 - Role of a Physician to Society and Community

• Vaccination and immunization programs.
• Screening for communicable and non-communicable diseases (cancer screening, hypertension, diabetes).
• Health education: smoking cessation, safe sex, dietary advice.

• Mandatory reporting of notifiable diseases (tuberculosis, cholera, polio) to public health authorities.
• Participation in outbreak investigation and containment (contact tracing, quarantine).
• Role in eradication programs (WHO: smallpox, polio).

• Antenatal care, institutional delivery, immunization of children (national immunization schedule).
• Promotion of breastfeeding, micronutrient supplementation.

• Identification and reporting of occupational hazards (silicosis, asbestosis).
• Advocacy for safe water, sanitation, clean air.

• Equal access to care regardless of socioeconomic status.
• Advocacy for marginalized and vulnerable populations.
• Maintaining confidentiality while balancing public health obligations.

• Participation in clinical research ethically.
• Translating evidence into improved community health outcomes.

• Triage and emergency care in mass casualty events.
• Collaboration with government agencies in relief operations.

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Q.6.3 - Polymerase Chain Reaction (PCR): Principle, Techniques, Applications

• Template DNA (target sequence)
• Two oligonucleotide primers (flanking the target, one for each strand)
• Taq DNA polymerase (thermostable; from Thermus aquaticus, stable up to 95°C)
• dNTPs (deoxynucleoside triphosphates)
• Buffer with MgCl2 (Mg2+ is cofactor for Taq polymerase)

• Each cycle doubles the number of copies: n cycles → 2n copies (exponential amplification).
• Typically 25-35 cycles → >1 billion copies from a single template molecule.

1. Diagnosis of infectious diseases: HIV (viral load), TB (MTB detection), COVID-19, hepatitis B/C, malaria, STIs.
2. Genetic disease diagnosis: Sickle cell anemia, thalassemia, cystic fibrosis, PKU (mutation detection).
3. Prenatal diagnosis: Fetal DNA from chorionic villi/amniotic fluid.
4. Oncology: Detection of BCR-ABL fusion (CML), BRCA mutations, circulating tumor DNA (liquid biopsy).
5. Forensic medicine: DNA fingerprinting from minute biological samples (blood, semen, hair).
6. HLA typing: Organ transplantation matching.
7. Research: Cloning, sequencing, expression analysis.

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Q.6.4 - Enzyme Inhibition: Types with Clinical Examples

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• Inhibitor is structurally similar to substrate; competes for the active site.
• Can be overcome by increasing substrate concentration.
• Kinetics: Km ↑ (apparent); Vmax unchanged.
• Lineweaver-Burk: lines intersect on Y-axis (same Vmax).
• Example: Methotrexate inhibits dihydrofolate reductase (competes with DHFA) → used in cancer, psoriasis, RA.
• Example: Statins (lovastatin, atorvastatin) compete with HMG-CoA for HMG-CoA reductase → lower cholesterol.
• Example: Sulfonamides compete with PABA for dihydropteroate synthase in bacteria.

• Inhibitor binds allosteric site (not active site); can bind free enzyme or enzyme-substrate complex.
• Cannot be overcome by increasing substrate.
• Kinetics: Km unchanged; Vmax ↓.
• Lineweaver-Burk: lines intersect on X-axis (same Km).
• Example: Cyanide (CN-) inhibits cytochrome c oxidase (Complex IV of ETC) non-competitively → cellular asphyxia.
• Example: Heavy metals (Pb2+, Hg2+) inhibit various enzymes including ALA dehydratase (lead poisoning → porphyria).

• Inhibitor binds only the enzyme-substrate complex (not free enzyme).
• Kinetics: Both Km and Vmax ↓ by the same factor.
• Lineweaver-Burk: parallel lines (different intercepts).
• Example: Lithium inhibits inositol monophosphatase.

• Inhibitor binds both free enzyme and ES complex (with different affinities).
• Both Km and Vmax change.

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• Inhibitor forms permanent covalent bond with active site residue.
• Example: Organophosphates (nerve agents - sarin, VX; pesticides - malathion) → irreversible phosphorylation of Ser in the active site of acetylcholinesterase → accumulation of ACh → cholinergic crisis (SLUDGE: salivation, lacrimation, urination, defecation, GI upset, emesis).
• Example: Aspirin → irreversible acetylation of Ser530 in COX-1/COX-2 → inhibits prostaglandin and thromboxane A2 synthesis → antiplatelet effect lasting the lifetime of the platelet (7-10 days).
• Example: Penicillin → irreversible binding to transpeptidase (PBP - Penicillin Binding Proteins) → inhibits bacterial cell wall cross-linking.
• Example: 5-Fluorouracil → suicide inhibitor of thymidylate synthase → blocks dTMP synthesis → cancer chemotherapy.

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