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Biochemistry - University Exam Master Guide

Topics Arranged by Importance with Clinical Correlations

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🔴 TIER 1 - MOST FREQUENTLY EXAMINED (High Priority)

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1. CARBOHYDRATE METABOLISM

A. Glycolysis (Embden-Meyerhof Pathway)

• Steps 1-10, enzymes at each step
• ATP yield (net 2 ATP from glucose)
• Rate-limiting enzyme: Phosphofructokinase-1 (PFK-1)
• Regulation: AMP activates, ATP/citrate inhibit PFK-1
• Clinical: Pyruvate kinase deficiency → hemolytic anemia; Hexokinase deficiency

B. Citric Acid Cycle (TCA / Krebs Cycle)

• 8 steps, enzymes, cofactors (NAD+, FAD, CoA, thiamine)
• Products per turn: 3 NADH, 1 FADH2, 1 GTP, 2 CO2
• Rate-limiting: isocitrate dehydrogenase
• Clinical: Thiamine (B1) deficiency → inhibits pyruvate dehydrogenase, α-ketoglutarate dehydrogenase → Wernicke's encephalopathy, beriberi

C. Oxidative Phosphorylation & ETC

• Complexes I-V, proton gradient, chemiosmosis
• ATP yield: 30-32 ATP per glucose
• Inhibitors (cyanide, CO, rotenone) and uncouplers (2,4-DNP, thermogenin)
• Clinical: Cyanide poisoning (Complex IV); MELAS, MERRF (mitochondrial disease)

D. Glycogenesis & Glycogenolysis

• Glycogen synthase (activated by insulin) vs. glycogen phosphorylase (activated by glucagon/epinephrine)
• Branching enzyme (glycogenesis); debranching enzyme (glycogenolysis)
• Clinical: Glycogen Storage Diseases (GSDs):
  • Type I (Von Gierke) - glucose-6-phosphatase deficiency → hypoglycemia, hepatomegaly, lactic acidosis
  • Type II (Pompe) - lysosomal acid maltase (α-1,4-glucosidase) → cardiomegaly, muscle weakness
  • Type III (Cori) - debranching enzyme
  • Type V (McArdle) - muscle phosphorylase → exercise intolerance, myoglobinuria
  • Type VI (Hers) - liver phosphorylase

E. Gluconeogenesis

• Substrates: lactate, alanine, glycerol, oxaloacetate
• Key enzymes (bypass glycolysis): pyruvate carboxylase, PEPCK, fructose-1,6-bisphosphatase, glucose-6-phosphatase
• Clinical: Gluconeogenesis inhibited by alcohol; metformin inhibits hepatic gluconeogenesis

F. Pentose Phosphate Pathway (HMP Shunt)

• Produces NADPH (for reductive biosynthesis, antioxidant defense) and ribose-5-phosphate (nucleotide synthesis)
• G6PD as rate-limiting enzyme
• Clinical: G6PD deficiency (most common enzyme deficiency, X-linked) → Heinz bodies, bite cells, hemolytic anemia triggered by oxidants (primaquine, dapsone, fava beans)

G. Galactose & Fructose Metabolism

• Clinical:
  • Galactosemia - GALT deficiency → cataracts, jaundice, liver damage, E. coli sepsis in neonates
  • Essential fructosuria - fructokinase deficiency (benign)
  • Hereditary fructose intolerance - aldolase B deficiency → hypoglycemia, liver failure

H. Blood Glucose Regulation

• Insulin vs. glucagon actions
• Fed state vs. fasting state metabolism
• Clinical: Diabetes mellitus (Type 1 & 2), hypoglycemia

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2. LIPID METABOLISM

A. Fatty Acid Synthesis

• Occurs in cytoplasm; acetyl-CoA carboxylase (ACC) is rate-limiting (activated by citrate, inhibited by palmitoyl-CoA)
• Malonyl-CoA as key intermediate; FAS complex; NADPH required
• Clinical: Biotin deficiency impairs carboxylase reactions

B. Beta-Oxidation of Fatty Acids

• Occurs in mitochondria; carnitine shuttle required (carnitine acyltransferase I, II)
• ATP yield: palmitate (C16) → 106 ATP net
• Odd-chain FA → propionyl-CoA → succinyl-CoA (requires B12, biotin)
• Clinical:
  • Carnitine deficiency → muscle weakness, hypoketotic hypoglycemia
  • MCAD deficiency (medium-chain acyl-CoA dehydrogenase) → most common FA oxidation disorder, hypoketotic hypoglycemia, sudden infant death

C. Ketogenesis & Ketolysis

• Occurs in liver (production); utilized in extrahepatic tissues
• HMG-CoA synthase → HMG-CoA → acetoacetate → β-hydroxybutyrate
• Starvation and uncontrolled T1DM → ketoacidosis
• Clinical: Diabetic ketoacidosis (DKA) - fruity breath, Kussmaul breathing, anion gap metabolic acidosis; β-hydroxybutyrate elevated

D. Cholesterol Metabolism

• Rate-limiting: HMG-CoA reductase (inhibited by statins)
• Mevalonate pathway → cholesterol → bile acids, steroid hormones, Vit D
• Clinical:
  • Familial hypercholesterolemia - LDL receptor deficiency (autosomal dominant) → xanthomas, premature CAD
  • Statins (competitive inhibitors of HMG-CoA reductase)
  • Bile acid sequestrants, PCSK9 inhibitors

E. Lipoproteins

• Structure: apolipoproteins, phospholipid shell
• Classes: chylomicrons → VLDL → IDL → LDL → HDL
• Apolipoproteins: ApoB-48 (chylomicrons), ApoB-100 (VLDL/LDL), ApoA-I (HDL), ApoC-II (activates LPL), ApoE (remnant uptake)
• Clinical:
  • Type I hyperlipoproteinemia - LPL deficiency (ApoC-II deficiency) → hypertriglyceridemia, pancreatitis
  • Abetalipoproteinemia - no ApoB → fat malabsorption, acanthocytosis, retinitis pigmentosa

F. Phospholipid & Sphingolipid Metabolism

• Sphingomyelin, cerebrosides, gangliosides
• Clinical (Lysosomal Storage Diseases):
  • Gaucher disease - β-glucocerebrosidase deficiency → glucocerebroside accumulates → hepatosplenomegaly, Gaucher cells (wrinkled tissue paper cytoplasm)
  • Niemann-Pick - sphingomyelinase deficiency → sphingomyelin → cherry-red macula, hepatosplenomegaly
  • Tay-Sachs - hexosaminidase A deficiency → GM2 ganglioside → cherry-red macula, no hepatosplenomegaly, neurodegeneration
  • Fabry disease - α-galactosidase A deficiency → ceramide trihexoside → X-linked, angiokeratomas, renal failure

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3. PROTEIN & AMINO ACID METABOLISM

A. Protein Structure

• Primary (peptide bonds), secondary (α-helix, β-sheet), tertiary, quaternary
• Chaperones (HSP70) prevent misfolding
• Clinical: Prion diseases - misfolded proteins (PrPSc); Alzheimer's (amyloid plaques, tau tangles)

B. Amino Acid Classification

• Essential (10): Phe, Val, Thr, Trp, Ile, Met, His, Arg, Leu, Lys (mnemonic: PVT TIM HaLL)
• Glucogenic, ketogenic, or both
• Branched-chain AAs (Leu, Ile, Val): metabolized in muscle

C. Transamination & Urea Cycle

• Transaminases (ALT, AST) require pyridoxal phosphate (B6)
• Urea cycle: carbamoyl phosphate synthetase I (CPS-I) → citrulline → argininosuccinate → fumarate + arginine → urea
• Occurs in liver (hepatocytes)
• Clinical:
  • Urea cycle defects → hyperammonemia → encephalopathy, vomiting
  • OTC deficiency (most common urea cycle disorder, X-linked) → high ammonia, orotic aciduria
  • ALT/AST elevated in hepatocellular damage (ALT more specific for liver)

D. Amino Acid-Derived Products

E. Phenylketonuria (PKU) - Detailed

• Phenylalanine hydroxylase (PAH) deficiency; requires tetrahydrobiopterin (BH4)
• Accumulation of phenylpyruvate, phenylacetate, phenyllactate
• Newborn screening (Guthrie test)
• Treatment: low-phenylalanine diet, BH4 (sapropterin) for BH4-responsive PKU
• Maternal PKU: untreated mother → fetal microcephaly, cardiac defects

F. Homocystinuria

• Cystathionine beta-synthase (CBS) deficiency (B6-dependent)
• Remethylation defect: methionine synthase (needs B12 + folate)
• Clinical: Marfanoid habitus, ectopia lentis (downward), intellectual disability, thromboembolism
• Distinguish from Marfan (lens dislocates upward)

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4. HEMOGLOBIN (Hb) CHEMISTRY & METABOLISM

A. Hb Structure & Types

• Tetramer: 2α + 2β subunits (HbA, adult); 2α + 2γ (HbF, fetal); 2α + 2δ (HbA2)
• Heme group: protoporphyrin IX + Fe²⁺ (must stay ferrous for O2 binding)
• HbF has higher O2 affinity (binds 2,3-BPG less) → useful for fetal O2 extraction

B. Oxygen Dissociation Curve

• Sigmoidal shape = cooperative binding (Hill coefficient ~2.8)
• Right shift (↓O2 affinity): ↑CO2, ↑H+ (Bohr effect), ↑2,3-BPG, ↑temperature
• Left shift (↑O2 affinity): ↓CO2, ↓H+, ↓2,3-BPG, ↓temp, CO, HbF, methemoglobin
• Clinical: At altitude: ↑2,3-BPG to compensate; CO poisoning: left shift + decreased capacity

C. Hemoglobin Derivatives

D. Hemoglobin Synthesis (Heme Biosynthesis)

• ALA synthase (rate-limiting, in mitochondria, requires B6/pyridoxal phosphate)
• ALA dehydratase (Zn²⁺-dependent; inhibited by lead)
• Ferrochelatase adds Fe²⁺ (inhibited by lead)
• Clinical: PORPHYRIAS
  • AIP (Acute Intermittent Porphyria) - PBG deaminase deficiency → ↑ALA, PBG in urine → abdominal pain, neuropsychiatric symptoms, autonomic dysfunction; urine turns dark on standing; Rx: heme arginate + glucose (suppress ALA synthase)
  • Porphyria Cutanea Tarda (PCT) - uroporphyrinogen decarboxylase → skin blistering, photosensitivity; associated with HCV, alcohol, iron overload
  • Lead poisoning - inhibits ALA dehydratase and ferrochelatase → ↑ALA, ↑free erythrocyte protoporphyrin (FEP), basophilic stippling, microcytic anemia

E. Hemoglobinopathies

• Sickle Cell Disease (HbS): Glu→Val at position 6 of β-chain; HbS polymerizes when deoxygenated → sickling
  • Autosomal recessive; HbSS = sickle cell disease; HbAS = sickle cell trait
  • Complications: vaso-occlusive crises, acute chest syndrome, stroke, splenic sequestration, aplastic crisis (parvovirus B19), autosplenectomy → encapsulated organism infections (Salmonella osteomyelitis, pneumococcal sepsis)
  • Diagnosis: Hb electrophoresis; Sickledex test
  • Treatment: hydroxyurea (↑HbF), transfusions, bone marrow transplant, penicillin prophylaxis, vaccines
• HbC: Glu→Lys at position 6 β-chain; target cells, mild hemolysis
• Thalassemias:
  • α-Thalassemia: deletion of α-globin genes (chromosome 16)
    • 1 gene deleted: silent carrier; 2: α-thal trait; 3: HbH disease (β4 tetramers); 4: Hb Barts (γ4) → hydrops fetalis (fatal)
  • β-Thalassemia: point mutations in β-globin gene (chromosome 11)
    • β-thal minor (trait): mild microcytic anemia; β-thal major (Cooley anemia): severe, transfusion-dependent, "hair on end" X-ray, iron overload, splenomegaly; HbA2 ↑
  • Treatment: transfusions, iron chelation (deferoxamine), bone marrow transplant

F. Hemoglobin Catabolism (Bilirubin Metabolism)

• RBC lysis → heme → biliverdin → unconjugated (indirect) bilirubin (lipid soluble, bound to albumin)
• Liver: UDP-glucuronosyltransferase conjugates → conjugated (direct) bilirubin (water soluble)
• Excreted in bile → urobilinogen → urobilin (urine) / stercobilin (feces)
• Clinical:
  • Pre-hepatic jaundice: ↑unconjugated Bil (hemolysis); urine normal
  • Hepatic jaundice: ↑both (hepatitis, cirrhosis)
  • Post-hepatic (obstructive) jaundice: ↑conjugated Bil; dark urine (bilirubinuria), pale stools
  • Crigler-Najjar Type I: complete absence of UGT → severe unconjugated hyperbilirubinemia → kernicterus; Rx: liver transplant
  • Gilbert syndrome: mild UGT deficiency → benign unconjugated hyperbilirubinemia triggered by fasting/stress
  • Dubin-Johnson: defective MRP2 transporter → conjugated hyperbilirubinemia; black liver pigment

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5. ENZYMES

A. Enzyme Kinetics

• Michaelis-Menten: v = Vmax[S] / (Km + [S])
• Km = substrate concentration at ½Vmax; low Km = high affinity
• Vmax = maximum velocity (proportional to [E]total)
• Lineweaver-Burk plot (double reciprocal)

B. Types of Inhibition

C. Allosteric Regulation

• Sigmoidal kinetics (not Michaelis-Menten)
• Positive/negative effectors alter enzyme conformation
• Examples: PFK-1, ATCase, hemoglobin

D. Enzyme Cofactors & Coenzymes

E. Isoenzymes (Clinically Important)

• LDH: LDH-1 (heart) > LDH-2 in MI; LDH-5 (liver/muscle)
• CK: CK-MB (cardiac muscle) → used in MI diagnosis; CK-MM (skeletal muscle); CK-BB (brain)
• Alkaline phosphatase: liver (biliary), bone (osteoblasts), placenta → elevated in cholestasis, Paget's, pregnancy
• ALT (SGPT): more liver-specific; ↑↑ in viral hepatitis
• AST (SGOT): liver + heart + muscle; AST:ALT > 2:1 suggests alcoholic hepatitis
• GGT: most sensitive for alcohol use; elevated in cholestasis

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6. BIOLOGICAL OXIDATION

A. Electron Transport Chain (ETC)

• Complex I (NADH dehydrogenase): NADH → CoQ; inhibited by rotenone, amytal
• Complex II (Succinate dehydrogenase): FADH2 → CoQ; contains heme b; no proton pumping
• Complex III (Cytochrome bc1): CoQ → Cyt C; inhibited by antimycin A
• Complex IV (Cytochrome c oxidase): Cyt C → O2 → H2O; inhibited by CN⁻, CO, H2S, azide
• Complex V (ATP synthase): H+ gradient → ATP; inhibited by oligomycin; uncoupled by 2,4-DNP

B. Oxidative Phosphorylation

• Chemiosmotic theory (Mitchell): proton gradient drives ATP synthase
• P/O ratio: NADH → ~2.5 ATP; FADH2 → ~1.5 ATP
• Uncouplers: thermogenin (brown fat, newborns), 2,4-DNP → ↑O2 consumption, ↑heat, ↓ATP
• Clinical: Mitochondrial diseases (maternal inheritance); MELAS, Leber's optic neuropathy

C. Reactive Oxygen Species (ROS) & Antioxidants

• Superoxide (O2•⁻) → H2O2 (by SOD) → H2O (by catalase/GPx)
• GPx requires reduced glutathione (GSH); GSH regenerated by glutathione reductase (needs NADPH from G6PD/HMP shunt)
• Clinical: G6PD deficiency → ↓NADPH → ↓GSH → Heinz bodies; ischemia-reperfusion injury (burst of ROS)

D. High-Energy Compounds

• ATP, GTP, phosphocreatine, acetyl-CoA, 1,3-BPG
• Phosphorylation potential: ΔG' of ATP hydrolysis (-30.5 kJ/mol)
• Substrate-level vs. oxidative phosphorylation

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7. NUCLEOTIDE METABOLISM

A. Purine Synthesis

• De novo: starts with ribose-5-phosphate (from HMP shunt); first purine nucleotide = IMP; rate-limiting step: PRPP amidotransferase (glutamine-PRPP amidotransferase)
• Purines: Adenine, Guanine
• Salvage pathway: HGPRT enzyme (hypoxanthine-guanine phosphoribosyltransferase) recycles hypoxanthine and guanine
• Clinical:
  • Gout: ↑uric acid (end product of purine catabolism in humans) → monosodium urate crystals → acute arthritis (podagra), tophi, urate nephropathy
  • Lesch-Nyhan syndrome: HGPRT deficiency (X-linked) → ↑uric acid, hyperuricemia, intellectual disability, self-mutilation, choreoathetosis
  • Allopurinol/febuxostat inhibit xanthine oxidase (last step of purine catabolism)

B. Pyrimidine Synthesis

• De novo: starts with carbamoyl phosphate (cytoplasmic) + aspartate; rate-limiting: CAD complex (CPS-II)
• Pyrimidines: Cytosine, Uracil (RNA), Thymine (DNA)
• dTMP synthesis: dUMP + N5,N10-methylene-THF → dTMP (thymidylate synthase); regeneration of THF requires DHFR
• Clinical:
  • Methotrexate/trimethoprim: DHFR inhibitors → block dTMP synthesis (anti-cancer/anti-bacterial)
  • 5-Fluorouracil (5-FU): suicide inhibitor of thymidylate synthase → ↓dTMP → cancer treatment
  • Orotic aciduria: UMP synthase deficiency (de novo pyrimidine defect) → orotic acid in urine, megaloblastic anemia; NOT hyperammonemia (distinguishes from OTC deficiency, which also has orotic aciduria but with ↑NH3)

C. Nucleotide Degradation

• Purines → xanthine → uric acid (xanthine oxidase)
• Pyrimidines → malonyl-CoA, methylmalonyl-CoA (soluble products, not gout)
• Clinical:
  • ADA (adenosine deaminase) deficiency → ↑dATP → inhibits ribonucleotide reductase → no dNTPs → lymphocyte apoptosis → SCID (severe combined immunodeficiency)
  • Gene therapy success story: ADA-SCID first successfully treated with gene therapy

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8. VITAMINS

Water-Soluble Vitamins (B-complex + C)

Fat-Soluble Vitamins (A, D, E, K)

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9. MINERALS

Macrominerals

Trace Elements

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10. CELL BIOLOGY (Biochemical Aspects)

A. Cell Membrane

• Fluid mosaic model: phospholipid bilayer + cholesterol (regulates fluidity)
• Integral vs. peripheral proteins
• Clinical: Hereditary spherocytosis (spectrin/ankyrin defect) → osmotic fragility; Paroxysmal nocturnal hemoglobinuria (PNH) - GPI anchor deficiency → complement-mediated hemolysis

B. Signal Transduction

• G-protein coupled receptors (GPCRs):
  • Gs → ↑adenylyl cyclase → ↑cAMP → PKA activation (glucagon, epinephrine, β-receptors, PTH, TSH)
  • Gi → ↓cAMP (α2 receptors, M2/M4 muscarinic)
  • Gq → ↑phospholipase C → ↑IP3 (Ca²⁺ release) + DAG (PKC activation) (α1, M1/M3, H1)
• Receptor tyrosine kinases (RTKs): insulin, IGF-1, EGF, PDGF → autophosphorylation → Ras-MAPK cascade
• Clinical: Cholera toxin (ADP-ribosylates Gs → permanent ↑cAMP → Cl⁻/H2O secretion → rice-water diarrhea); Pertussis toxin (ADP-ribosylates Gi → ↑cAMP in respiratory epithelium)

C. Cell Organelles

D. Cell Cycle & DNA Replication

• G1 → S → G2 → M phases
• Cyclins + CDKs control progression; checkpoints (p53 at G1/S)
• Clinical: p53 mutations (most common in human cancer - "guardian of genome"); BRCA1/2 (DNA repair) → breast/ovarian cancer risk; cyclins dysregulated in cancer

E. Apoptosis

• Intrinsic (mitochondrial): Bax/Bak (pro-apoptotic) vs. Bcl-2 (anti-apoptotic) → cytochrome c release → caspase cascade
• Extrinsic (death receptor): FasL/TRAIL → Fas/DR → DISC → caspase-8
• Clinical: Bcl-2 overexpression in follicular lymphoma (t14;18); caspase-3 defects in autoimmunity

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🟡 TIER 2 - HIGH YIELD CLINICAL CORRELATIONS SUMMARY

Inborn Errors of Metabolism - Quick Reference

Key Laboratory Tests

• HbA1c: glycated Hb → reflects glycemic control over 2-3 months (RBC lifespan)
• Serum electrophoresis: identifies M-spike (multiple myeloma), abnormal Hb bands
• Urine organic acids: methylmalonate (B12 deficiency/MMA), homocysteine
• Enzyme assays in WBCs/fibroblasts: for lysosomal storage diseases
• Newborn screening (tandem MS/MS): PKU, MSUD, organic acidemias, fatty acid oxidation defects, amino acidopathies

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🔵 TIER 3 - INTEGRATION POINTS (Frequently Asked in Long Questions)

Metabolic Integration by Fed/Fasted State

Important Exam "Links"

1. Thiamine connects: TCA cycle + glycolysis + HMP shunt (transketolase)
2. Niacin (NAD+/NADP+) connects: all redox reactions across all pathways
3. CoA connects: FA metabolism + TCA cycle + cholesterol synthesis
4. NADPH connects: FA synthesis + cholesterol synthesis + antioxidant defense (GPx/GSH) + P450 reactions + HMP shunt
5. Acetyl-CoA is the central hub: from glycolysis (PDH) → TCA / ketogenesis / cholesterol / FA synthesis
6. Urea cycle links to TCA: fumarate produced in urea cycle enters TCA; argininosuccinate → fumarate + arginine

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EXAM TIPS

• Most MCQs test deficiency diseases (vitamins, minerals, enzymes) and clinical presentations
• Always link enzyme defects to accumulated substrate (the toxic molecule, not the missing product)
• For kinetics questions: remember competitive inhibition = ↑Km, same Vmax
• For Hb questions: know all causes of left shift vs. right shift of ODC
• For porphyrias: AIP = abdominal pain + neuropsychiatric; PCT = skin blistering
• Lead poisoning affects 2 enzymes: ALA dehydratase AND ferrochelatase → both inhibited
• Distinguish B12 vs. folate deficiency by methylmalonate: elevated only in B12 deficiency
• For jaundice: always determine pre-hepatic/hepatic/post-hepatic using conjugated vs. unconjugated bilirubin

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