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📚 Biochemistry Paper 2 - Complete Exam Guide

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PRIORITY ORDER (by marks):

• 🔴 13 marks each: Protein & AA Metabolism, Molecular Biology, Molecular Biology Techniques, Heme Metabolism, Organ Function Tests
• 🟠 10 marks each: Nucleotide Metabolism, Biochemistry of Cancer
• 🟡 6 marks each: Protein Chemistry, Nucleic Acid Chemistry, Free Radicals & Antioxidants
• 🟢 5 marks each: Plasma Proteins, Immunology, Clinical Chemistry
• 🔵 3 marks: Xenobiotics & Detoxification

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🔴 TOPIC 1: Protein & Amino Acid Metabolism (13 marks)

Most Important Questions:

• Urea cycle occurs in liver (both mitochondria + cytoplasm)
• Converts toxic NH₃ → urea (excreted in urine)
• Steps:
  1. NH₃ + CO₂ + 2ATP → Carbamoyl phosphate (enzyme: CPS-I, in mitochondria)
  2. Carbamoyl phosphate + Ornithine → Citrulline (OTC enzyme)
  3. Citrulline → Argininosuccinate (+ Aspartate, ATP)
  4. Argininosuccinate → Arginine + Fumarate
  5. Arginine → Ornithine + Urea (arginase)
• Disorders: Hyperammonemia - ornithine transcarbamylase deficiency (most common, X-linked)

• Deficiency of phenylalanine hydroxylase (PAH)
• Phenylalanine accumulates → phenylpyruvate, phenyllactate in urine
• Features: intellectual disability, fair skin/hair/eyes, musty odor, seizures
• Treatment: Low-phenylalanine diet, tetrahydrobiopterin (BH4)
• Screening: Guthrie test (heel-prick blood spot)

• Transamination: Transfer of -NH₂ group from AA to α-keto acid (enzyme: transaminases/aminotransferases, coenzyme: PLP/Pyridoxal phosphate)
  • ALT: Alanine + α-KG ↔ Pyruvate + Glutamate
  • AST: Aspartate + α-KG ↔ Oxaloacetate + Glutamate
• Oxidative deamination: Glutamate → α-KG + NH₃ (enzyme: Glutamate dehydrogenase, GDH)

• Deficiency of branched-chain α-keto acid dehydrogenase
• Branched chain AAs accumulate: Leucine, Isoleucine, Valine
• Features: Sweet maple syrup odor of urine, encephalopathy, death if untreated
• Treatment: Diet restricted in branched-chain AAs

• SAM (S-Adenosylmethionine) = universal methyl donor
• Homocysteine → Methionine (requires folate + Vit B12)
• Homocystinuria: CBS enzyme deficiency → homocysteine accumulates → thromboembolism, Marfanoid habitus, lens dislocation, intellectual disability

• Tyrosine → DOPA → Dopamine → Norepinephrine → Epinephrine
• Rate-limiting enzyme: Tyrosine hydroxylase
• Degradation by MAO (monoamine oxidase) and COMT

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🔴 TOPIC 2: Molecular Biology (13 marks)

Most Important Questions:

• Semi-conservative replication (Watson-Crick)
• Key enzymes:

  Enzyme	Function
  Helicase	Unwinds double helix
  Primase	Synthesizes RNA primer
  DNA Pol III	Main replicating enzyme (prokaryotes)
  DNA Pol I	Removes primers, fills gaps
  DNA Ligase	Joins Okazaki fragments
  Topoisomerase	Relieves torsional stress

• Leading strand: continuous synthesis
• Lagging strand: discontinuous (Okazaki fragments)

• Template strand = antisense strand; coding strand = sense strand
• RNA Polymerase (no primer needed)
• 3 types of RNA:
  • mRNA - carries genetic info
  • tRNA - anticodon, carries AA (clover-leaf structure)
  • rRNA - structural component of ribosome (most abundant)
• Post-transcriptional modifications of mRNA: 5' capping, 3' poly-A tail, splicing of introns

• Codons: triplet code; AUG = start codon (Met)
• Stop codons: UAA, UAG, UGA
• Ribosomes: A site (aminoacyl), P site (peptidyl), E site (exit)
• Initiation → Elongation → Termination
• Wobble hypothesis (Crick): 3rd base of codon can mismatch

• Point mutations: Missense (wrong AA), Nonsense (stop codon), Silent (same AA)
• Frameshift: insertion/deletion → shifts reading frame
• Transition: purine↔purine or pyrimidine↔pyrimidine
• Transversion: purine↔pyrimidine
• Clinical: Sickle cell anemia = missense mutation (Glu→Val, codon 6, β-globin)

• Lac operon (prokaryote model): Repressor model
  • Structural genes: lacZ (β-galactosidase), lacY (permease), lacA (transacetylase)
  • When lactose present: inducer binds repressor → repressor leaves operator → genes expressed
• Eukaryotes: Enhancers, silencers, transcription factors, methylation, histones

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🔴 TOPIC 3: Molecular Biology Techniques (13 marks)

Most Important Questions:

• Amplifies specific DNA sequences
• 3 steps (repeated ~30 cycles):
  1. Denaturation (~94°C): strands separate
  2. Annealing (~55°C): primers bind template
  3. Extension (~72°C): Taq polymerase extends
• Result: Exponential amplification (2ⁿ copies)
• Applications: Diagnosis of infections (HIV, TB), genetic diseases, forensics, paternity testing, COVID-19 testing

• General steps: Gel electrophoresis → Transfer to nitrocellulose membrane → Hybridization with probe → Detection

• Restriction endonucleases cut DNA at specific palindromic sequences
• Differences in fragment lengths = genetic polymorphisms
• Used in: Genetic fingerprinting, diagnosis of sickle cell disease, paternity testing

• Uses chain-terminating ddNTPs (dideoxynucleotides)
• Four reactions with ddATP, ddTTP, ddGTP, ddCTP
• Gel electrophoresis separates fragments → sequence read

• Steps: Isolation of gene → Restriction enzyme cuts → Ligation into vector (plasmid) → Transformation into host → Selection → Expression
• Vectors: Plasmids, bacteriophages, cosmids, YACs
• Applications: Insulin production (1st recombinant protein), HGH, erythropoietin, Hepatitis B vaccine

• Gene editing tool: guide RNA directs Cas9 nuclease to specific DNA sequence → cuts DNA
• Used for gene therapy, cancer research

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🔴 TOPIC 4: Heme Metabolism (13 marks)

Most Important Questions:

• Occurs in: Liver (for cytochromes) and Bone marrow (for Hb)
• Steps:
  1. Glycine + Succinyl CoA → δ-ALA (enzyme: ALA synthase - rate-limiting, requires PLP/B6)
  2. 2 ALA → Porphobilinogen (PBG) - ALA dehydratase (inhibited by lead)
  3. 4 PBG → Hydroxymethylbilane → Uroporphyrinogen III → Coproporphyrinogen III → Protoporphyrin IX
  4. Protoporphyrin IX + Fe²⁺ → Heme (enzyme: Ferrochelatase, inhibited by lead)

• Acute Intermittent Porphyria (AIP): PBG deaminase deficiency; abdominal pain, neuropsychiatric symptoms, port-wine urine; No photosensitivity
• Porphyria Cutanea Tarda (PCT): Uroporphyrinogen decarboxylase deficiency; most common porphyria; photosensitivity, blistering skin
• Congenital Erythropoietic Porphyria: Uroporphyrinogen III cosynthase deficiency; red urine, severe photosensitivity, hemolytic anemia

• RBC destruction → Heme → Biliverdin (by Heme oxygenase) → Unconjugated bilirubin (UCB)
• UCB is insoluble, carried in blood bound to albumin
• In liver: UCB + glucuronic acid (UGT enzyme) → Conjugated bilirubin (CB) = direct bilirubin
• CB excreted in bile → intestine → Urobilinogen → Stercobilin (feces, brown color) + Urobilin (urine, yellow)

• Physiological: Days 2-4 due to immature liver UGT
• Hemolytic disease: Rh incompatibility → severe UCB
• Kernicterus: UCB deposits in basal ganglia → neurological damage
• Treatment: Phototherapy (converts UCB to lumirubin, water-soluble)

• Inhibits: ALA dehydratase and Ferrochelatase
• ALA and Coproporphyrin III accumulate in urine
• Features: Basophilic stippling of RBCs, microcytic hypochromic anemia, Burton's lines (gums), encephalopathy
• Treatment: Chelation therapy (DMSA, EDTA)

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🔴 TOPIC 5: Organ Function Tests (13 marks)

Most Important Questions:

• Blood Urea Nitrogen (BUN): 7-20 mg/dL; ↑ in renal failure, dehydration
• Serum Creatinine: 0.6-1.2 mg/dL; best index of GFR; not affected by diet
• GFR: Normal ~125 mL/min; CKD if <60 mL/min for >3 months
• Uric acid: End product of purine catabolism; ↑ in gout
• Creatinine clearance = [U × V] / P (gold standard for GFR estimation)
• Urine: Proteinuria, hematuria, casts in nephritis

• TSH: Most sensitive test (0.4-4.0 mIU/L)
• Free T4 (fT4) and Free T3 (fT3)
• Hypothyroid: ↑TSH, ↓fT4 | Hyperthyroid: ↓TSH, ↑fT4
• T3 is metabolically more active, T4 is converted to T3 in tissues

• Troponin = most specific and sensitive for myocardial damage

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🟠 TOPIC 6: Nucleotide Metabolism (10 marks)

Most Important Questions:

• Purine ring is built atom by atom on PRPP (phosphoribosyl pyrophosphate)
• Key reactions: 10 steps; end product = IMP → AMP or GMP
• Rate-limiting enzyme: PRPP amidotransferase (inhibited by AMP, GMP - feedback)
• Key donors: Glutamine (N), glycine, formate (C10-THF), CO2, aspartate

• Recycles free purine bases → nucleotides
• HGPRT (Hypoxanthine-Guanine Phosphoribosyl Transferase) - converts hypoxanthine → IMP, guanine → GMP
• Lesch-Nyhan syndrome: HGPRT deficiency → hypoxanthine not salvaged → excess uric acid production → gout + neurological features (self-mutilation, choreoathetosis, intellectual disability)

• Purines → Hypoxanthine → Xanthine → Uric Acid (enzyme: Xanthine oxidase)
• Gout: Uric acid crystals deposit in joints (esp. big toe = podagra), tophi, kidney stones
• Treatment: Allopurinol (xanthine oxidase inhibitor); Colchicine (acute attack)

• Ring is first synthesized, then attached to ribose
• CAD enzyme (Carbamoyl phosphate synthetase II, Aspartate transcarbamylase, Dihydroorotase) - rate-limiting complex
• End product: UMP → UDP → UTP → CTP
• Orotic aciduria: Deficiency of UMP synthase → orotic acid in urine, megaloblastic anemia

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🟠 TOPIC 7: Biochemistry of Cancer (10 marks)

Most Important Questions:

• Proto-oncogenes → Oncogenes when mutated (accelerators of cell growth)
  • Examples: ras (most commonly mutated in cancer), myc, her2/neu, bcr-abl
  • bcr-abl: Philadelphia chromosome in CML; target of Imatinib
• Tumor suppressor genes = brakes for cell growth
  • p53 (guardian of the genome): mutated in >50% of cancers; triggers apoptosis at G1
  • Rb (retinoblastoma protein): mutated in retinoblastoma, osteosarcoma

• Cancer cells preferentially use aerobic glycolysis (glucose → lactate) even when O2 is available
• Reason: Need for biosynthetic precursors (pentose phosphate pathway, lipid synthesis)
• PET scan exploits this: ¹⁸FDG (glucose analog) taken up more by cancer cells

• G1 → S → G2 → M phases
• Cyclins and CDKs drive progression
• Checkpoints: G1/S (Rb-E2F), G2/M (p53)
• Apoptosis: Intrinsic (Bcl-2/Bax, mitochondria) and extrinsic (death receptor, FasL) pathways

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🟡 TOPIC 8: Protein Chemistry (6 marks)

Most Important Questions:

• Primary: AA sequence (peptide bonds)
• Secondary: α-helix (H-bonds within same chain, 3.6 AA/turn), β-pleated sheet (H-bonds between adjacent chains)
• Tertiary: 3D folding; stabilized by disulfide bonds, hydrophobic interactions, H-bonds, ionic bonds
• Quaternary: Multiple subunits (e.g., Hb = 2α + 2β)

• Loss of 3D structure without breaking peptide bonds
• Agents: Heat, strong acids/bases, organic solvents, heavy metals, urea
• Renaturation = reversible denaturation

• Missense mutation: codon 6 of β-globin: GAG → GTG (Glu → Val)
• In deoxygenated state: HbS polymerizes → sickle-shaped RBCs
• Features: Chronic hemolytic anemia, vaso-occlusive crises, splenomegaly, stroke
• HbS in sickle cell: Hydrophobic valine creates sticky patch

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🟡 TOPIC 9: Nucleic Acid Chemistry (6 marks)

Most Important Questions:

• Double helix, antiparallel strands
• B-DNA: right-handed, 10 bp/turn, most common physiological form
• Base pairing: A=T (2 H-bonds), G≡C (3 H-bonds)
• Backbone: Sugar (deoxyribose) - Phosphate
• Purines: Adenine, Guanine | Pyrimidines: Cytosine, Thymine (DNA), Uracil (RNA)
• Chargaff's rules: A=T, G=C; %A+G = %T+C (50% purines, 50% pyrimidines)

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🟡 TOPIC 10: Free Radicals and Antioxidants (6 marks)

Most Important Questions:

• Reactive Oxygen Species (ROS): Superoxide (O₂•⁻), Hydroxyl (•OH), Peroxyl, H₂O₂
• Generated by: Mitochondrial electron transport, phagocytosis, radiation, smoking
• Damage: Lipid peroxidation (membrane damage), DNA strand breaks, protein oxidation

• Enzymatic:
  • SOD (Superoxide dismutase): O₂•⁻ → H₂O₂ (contains Mn, Cu, Zn)
  • Catalase: H₂O₂ → H₂O + O₂ (in peroxisomes)
  • Glutathione peroxidase: H₂O₂ + GSH → GSSG (requires Se)
• Non-enzymatic: Vit C, Vit E (lipid-soluble, most important), β-carotene, uric acid, albumin

• ROS → atherosclerosis (LDL oxidation), aging, cancer, reperfusion injury, diabetes complications

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🟢 TOPIC 11: Plasma Proteins (5 marks)

Most Important Questions:

• Albumin (most abundant, 60%): Transport (bilirubin, FFA, drugs, Ca²⁺), maintain colloid osmotic pressure, buffer
• Globulins: α1, α2, β, γ
  • α1: α1-antitrypsin (protease inhibitor), orosomucoid
  • α2: Haptoglobin (binds free Hb), ceruloplasmin (copper transport), α2-macroglobulin
  • β: Transferrin (iron transport), fibronectin, complement
  • γ: Immunoglobulins (IgG, IgA, IgM, IgD, IgE)
• Fibrinogen: Coagulation; converted to fibrin by thrombin

• Rise rapidly in inflammation/infection
• Positive: CRP (best marker), fibrinogen, ferritin, haptoglobin, α1-antitrypsin, SAA
• Negative: Albumin, transferrin ↓ (diverted resources to synthesis of positive APP)
• CRP uses: monitor inflammation, infection, assess response to therapy

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🟢 TOPIC 12: Immunology (5 marks)

Most Important Questions:

• Basic unit: 2 heavy chains + 2 light chains (κ or λ) linked by disulfide bonds
• Fab region: antigen binding | Fc region: complement activation, receptor binding | Ig | Special feature | |----|----------------| | IgG | Most abundant; only one crossing placenta; secondary response | | IgM | Pentamer; first produced in primary response; largest | | IgA | Secretory (saliva, tears, breast milk, gut); dimer | | IgE | Allergy, helminth defense; triggers mast cell degranulation | | IgD | B cell surface receptor |

• Classical pathway: Activated by Ag-Ab complex (IgM/IgG)
• Alternate pathway: Activated by bacterial surfaces, LPS
• Lectin pathway: MBL binds mannose on pathogens
• Final: MAC (Membrane Attack Complex) C5b-9 → lyses cells

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🟢 TOPIC 13: Clinical Chemistry (5 marks)

Most Important Questions:

• Fasting blood glucose: 70-100 mg/dL normal
• Impaired fasting: 100-125 mg/dL; Diabetes: ≥126 mg/dL (fasting, confirmed twice)
• 2-hr OGTT: ≥200 mg/dL = diabetes
• HbA1c: Best measure of long-term glycemic control (3 months); ≥6.5% = diabetes
• Glycosylation: Glucose attaches to N-terminal valine of Hb non-enzymatically

• Total cholesterol: <200 mg/dL desirable
• LDL ("bad"): <100 mg/dL optimal; ↑ risk of atherosclerosis
• HDL ("good"): >60 mg/dL protective
• Triglycerides: <150 mg/dL normal
• Friedewald formula: LDL = Total cholesterol - HDL - (TG/5)

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🔵 TOPIC 14: Xenobiotics and Detoxification (3 marks)

Most Important Questions:

• Phase I (Functionalization): Oxidation, reduction, hydrolysis → adds/exposes functional groups (-OH, -NH₂)
  • Enzymes: Cytochrome P450 (CYP) enzymes in ER of liver
  • Products: May be more or less toxic
• Phase II (Conjugation): Adds polar group → makes highly water-soluble for excretion
  • Glucuronidation (UDP-glucuronosyltransferase - most important)
  • Sulfation, acetylation, methylation, glutathione conjugation
• Phase III: Transporters (efflux pumps, P-glycoprotein) export conjugated products

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📝 RAPID REVISION - KEY MNEMONICS

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⭐ HIGHEST-YIELD 1-LINERS FOR TOMORROW

1. Most common porphyria: Porphyria Cutanea Tarda (PCT)
2. Rate-limiting enzyme of heme synthesis: ALA synthase
3. Rate-limiting enzyme of urea cycle: CPS-I (Carbamoyl Phosphate Synthetase-I)
4. Universal methyl donor: SAM (S-Adenosylmethionine)
5. Lesch-Nyhan = HGPRT deficiency → Self-mutilation + gout
6. Sickle cell = Glu → Val (codon 6, β-globin)
7. Most specific cardiac marker: Troponin I/T
8. Most common oncogene mutated in cancer: ras
9. Guardian of the genome: p53
10. Most abundant plasma protein: Albumin
11. Phototherapy treats neonatal jaundice by converting UCB → lumirubin
12. Only Ig crossing placenta: IgG
13. PCR = Taq polymerase (heat-stable, from Thermus aquaticus)
14. Guthrie test = newborn screening for PKU
15. Allopurinol = xanthine oxidase inhibitor → treats gout

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1. Heme metabolism (full pathway + porphyrias + jaundice table)
2. Molecular biology (central dogma + lac operon)
3. Molecular biology techniques (PCR + blotting table)
4. Protein/AA metabolism (urea cycle + PKU + amino acidopathies)
5. Organ function tests (LFTs + cardiac markers)