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BIOCHEMISTRY PAPER II - COMPLETE STUDY GUIDE

Based on: Harper's Illustrated Biochemistry (32e), Basic Medical Biochemistry (6e), Lippincott Illustrated Reviews (8e)

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SECTION A: CARBOHYDRATES

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1. DEFINITION AND CLASSIFICATION OF CARBOHYDRATES

A. Monosaccharides (cannot be hydrolyzed further)

• Trioses (3C): Glyceraldehyde, Dihydroxyacetone
• Pentoses (5C): Ribose (in RNA), Deoxyribose (in DNA), Ribulose, Xylulose
• Hexoses (6C): Glucose, Fructose, Galactose, Mannose

B. Disaccharides (2 monosaccharides joined by glycosidic bond)

C. Oligosaccharides (3-10 monosaccharides)

• Found on cell surface glycoproteins and glycolipids; role in cell recognition, blood group antigens.

D. Polysaccharides (>10 monosaccharides)

• Homopolysaccharides: Same monomer units
  • Starch (amylose + amylopectin) - storage in plants
  • Glycogen - storage in liver and muscle
  • Cellulose - structural in plants
  • Dextran - storage in bacteria
• Heteropolysaccharides: Different monomer units
  • Glycosaminoglycans (see Topic 4)

• Lactose intolerance: deficiency of intestinal lactase → undigested lactose fermented by bacteria → gas, bloating, osmotic diarrhea.
• Von Gierke disease: glucose-6-phosphatase deficiency → can't release free glucose from glycogen → hypoglycemia.

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2. GLYCOSIDES AND THEIR IMPORTANCE

• O-glycosides: Sugar linked to -OH (most common in nature)
• N-glycosides: Sugar linked to -NH₂ (e.g., nucleosides in DNA/RNA)
• S-glycosides: Sugar linked to -SH (rare)

• Digitalis glycosides inhibit the Na+/K+ ATPase on cardiac muscle cells → intracellular Na+ rises → Na+/Ca²+ exchanger reverses → intracellular Ca²+ rises → stronger cardiac contraction (positive inotropy). Toxicity causes arrhythmias.

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3. CELLULOSE AND STARCH

STARCH

• Main storage polysaccharide of plants.
• Two components:
  1. Amylose (~20%): Linear chain of glucose units linked by α-1,4 glycosidic bonds. Helical structure.
  2. Amylopectin (~80%): Branched; α-1,4 bonds in main chain, α-1,6 bonds at branch points (every 24-30 residues).

• Salivary α-amylase: Cleaves α-1,4 bonds → dextrins, maltose
• Pancreatic α-amylase: Continues → maltose, maltotriose, limit dextrins
• Intestinal brush border enzymes: maltase, sucrase-isomaltase, glucoamylase → free glucose

CELLULOSE

• Structural polysaccharide of plant cell walls.
• Linear polymer of glucose with β-1,4 glycosidic bonds.
• Humans lack β-glucosidase (cellulase), so cannot digest cellulose.
• Functions as dietary fiber → adds bulk to stool, prevents constipation, reduces risk of colon cancer, reduces cholesterol absorption, improves glycemic index of diet.
• Ruminants (cow, sheep) can digest cellulose via gut bacteria that produce cellulase.

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4. GLYCOSAMINOGLYCANS / HETEROPOLYSACCHARIDES

• Highly negatively charged (due to sulfate and carboxylate groups) → attract water → form a viscous gel
• Usually bound to protein cores → form proteoglycans

• Mucopolysaccharidoses (MPS): Lysosomal storage diseases due to deficiency of enzymes that degrade GAGs → GAG accumulation in lysosomes.
  • Hurler syndrome (MPS I): Deficiency of α-L-iduronidase → dermatan sulfate and heparan sulfate accumulate → coarse facies, hepatosplenomegaly, corneal clouding, mental retardation.
  • Hunter syndrome (MPS II): Iduronate-2-sulfatase deficiency; X-linked; similar but no corneal clouding.
• Heparin (a GAG) is used clinically as an anticoagulant - activates antithrombin III which inhibits thrombin and Factor Xa.
• Hyaluronic acid injections used in osteoarthritis of knee to restore lubrication.

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5. DIGESTION AND ABSORPTION OF CARBOHYDRATES

Digestion

• Salivary α-amylase (ptyalin): cleaves α-1,4 bonds in starch → dextrins and maltose.
• Inactivated in stomach by acid (pH 3.5 destroys it).

• No carbohydrate-digesting enzymes.
• Acid denatures salivary amylase.

• Pancreatic α-amylase (in duodenum): cleaves α-1,4 bonds → maltose, maltotriose, α-limit dextrins.
• Brush border enzymes (in jejunum/ileum):
  • Maltase: maltose → 2 glucose
  • Sucrase-isomaltase: sucrose → glucose + fructose; also cleaves α-1,6 bonds in limit dextrins
  • Lactase: lactose → glucose + galactose
  • Glucoamylase: removes glucose from non-reducing end of oligosaccharides

Absorption

• Absorbed by SGLT-1 (Sodium-Glucose Linked Transporter): secondary active transport. Co-transported with Na+. Energy comes from Na+ gradient maintained by Na+/K+ ATPase.
• Exit enterocyte via GLUT-2 (facilitated diffusion) into portal blood.

• Absorbed by GLUT-5 (facilitated diffusion) into enterocyte.
• Exits via GLUT-2 into portal blood.

• Oral rehydration therapy (ORS): Uses glucose + NaCl. Glucose drives Na+ absorption via SGLT-1 even in diarrhea → water follows osmotically → rehydration even when cholera toxin activates Cl- secretion.
• Lactase deficiency → lactose not absorbed → gut bacteria ferment it → H₂ gas, short-chain fatty acids, osmotic diarrhea. Diagnosed by H₂ breath test.
• Glucose-galactose malabsorption: SGLT-1 mutation → severe diarrhea in neonates; only fructose is tolerated.

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6. GLYCOLYSIS

Steps of Glycolysis (10 steps)

Regulation of Glycolysis

1. Hexokinase - inhibited by glucose-6-phosphate (product inhibition)
   • Glucokinase (liver, pancreatic β-cells): NOT inhibited by Glu-6-P; has high Km (low affinity); acts as glucose sensor in liver and pancreas
2. PFK-1 - KEY regulatory enzyme
   • Activated by: AMP, ADP, fructose-2,6-bisphosphate (most potent activator), Pi
   • Inhibited by: ATP (high energy), citrate, H+
3. Pyruvate kinase - inhibited by ATP, alanine; activated by fructose-1,6-bisP

Fate of Pyruvate

• Aerobic conditions: → Acetyl CoA (by pyruvate dehydrogenase) → TCA cycle
• Anaerobic conditions: → Lactate (by lactate dehydrogenase) - regenerates NAD+
• Other fates: → Oxaloacetate (gluconeogenesis), → Alanine (transamination), → Ethanol (yeast only)

Applied

• Pyruvate kinase deficiency (most common hereditary enzyme deficiency in RBCs): RBCs have no mitochondria, depend entirely on glycolysis for ATP. PK deficiency → less ATP → RBC membrane pump failure → hemolytic anemia.
• Arsenic poisoning: Arsenic competes with phosphate → inhibits step 6 (glyceraldehyde-3-P dehydrogenase) → depletes ATP → cell death.
• Fluoride (NaF) inhibits enolase (step 9) → used as preservative in blood glucose sample tubes to prevent glycolysis ex vivo.
• Warburg effect: Cancer cells prefer aerobic glycolysis (glycolysis + lactic acid production even in presence of O₂) → FDG-PET scan detects high glucose uptake in tumors.

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7. TCA CYCLE / KREBS CYCLE

• Multi-enzyme complex requiring: TPP (Vit B1/Thiamine), Lipoic acid, CoA (Pantothenic acid/Vit B5), FAD (Vit B2/Riboflavin), NAD+ (Vit B3/Niacin)
• Irreversible reaction
• Inhibited by: Acetyl CoA, NADH, ATP; Activated by: Pyruvate, CoA, NAD+, ADP

8 Steps of TCA Cycle

• 3 NADH × 2.5 ATP = 7.5 ATP
• 1 FADH₂ × 1.5 ATP = 1.5 ATP
• 1 GTP = 1 ATP
• Total = 10 ATP per acetyl CoA

Regulation of TCA Cycle

• Citrate synthase: Inhibited by ATP, NADH, succinyl CoA, citrate
• Isocitrate dehydrogenase: Inhibited by ATP, NADH; Activated by ADP, Ca²+
• α-KG dehydrogenase: Inhibited by succinyl CoA, NADH; Activated by Ca²+

• Catabolic: Oxidizes acetyl CoA → CO₂ + energy
• Anabolic: Provides precursors:
  • OAA → Aspartate, Gluconeogenesis
  • α-KG → Glutamate
  • Succinyl CoA → Heme synthesis
  • Citrate → Fatty acid synthesis (exported to cytosol)

• Pyruvate → OAA (pyruvate carboxylase; requires Biotin, ATP)
• PEP → OAA (PEP carboxykinase)
• Glutamate → α-KG
• Propionyl CoA → Succinyl CoA

Applied

• Thiamine (Vit B1) deficiency (Beriberi/Wernicke's encephalopathy): PDH and α-KG dehydrogenase require TPP. Deficiency → pyruvate and lactate accumulate → lactic acidosis. Peripheral neuropathy (dry beriberi), cardiac failure (wet beriberi), Wernicke's (confusion, ophthalmoplegia, ataxia).
• Fluoroacetate (rat poison): Metabolized to fluorocitrate → inhibits aconitase → TCA blocked → cell death.
• Malonate inhibits succinate dehydrogenase (competitive inhibitor) → classic example of competitive inhibition in biochemistry.

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8. HMP SHUNT AND ITS SIGNIFICANCE

Phase 1: Oxidative (Irreversible) - generates NADPH and Ribulose-5-P

Phase 2: Non-oxidative (Reversible) - interconverts sugar phosphates

• Ribulose-5-P → Ribose-5-P (ribose phosphate isomerase)
• Ribulose-5-P → Xylulose-5-P (epimerase)
• Transketolase and transaldolase reactions interconvert C3, C4, C5, C6, C7 sugar phosphates
• Transketolase requires TPP (Thiamine) as cofactor

Significance (Key for exams)

1. NADPH production - required for:
   • Reductive biosynthesis: Fatty acid synthesis, cholesterol synthesis, steroid hormone synthesis
   • Regeneration of glutathione (GSH): NADPH reduces oxidized glutathione (GSSG) via glutathione reductase
   • Respiratory burst in phagocytes: NADPH oxidase uses NADPH to generate superoxide (O₂•⁻) → kills bacteria
   • Cytochrome P450 reactions (drug detoxification)
2. Ribose-5-phosphate - required for nucleotide synthesis (purine and pyrimidine rings) → DNA/RNA synthesis → important in rapidly dividing cells
3. Erythrocyte protection from oxidative stress: GSH neutralizes H₂O₂ (via glutathione peroxidase)

• G6PD deficiency (most common enzyme deficiency worldwide; X-linked): G6PD → NADPH → GSH. Without G6PD, RBCs cannot regenerate GSH → oxidative stress (from infections, drugs like primaquine, dapsone, oxidative foods like fava beans) → Heinz bodies (precipitated hemoglobin) form → hemolytic anemia.
• Drugs precipitating G6PD hemolysis: primaquine, dapsone, rasburicase, nitrofurantoin.
• Diagnosis: G6PD enzyme assay (during non-hemolytic period).

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9. DIABETES MELLITUS

Classification

Biochemical Changes in Diabetes

1. Hyperglycemia: Glucose uptake by muscle/fat requires GLUT-4 (insulin-dependent). Without insulin, GLUT-4 not translocated → glucose stays in blood.
2. Glycosuria: When blood glucose exceeds renal threshold (~180 mg/dL) → glucose spills into urine → osmotic diuresis → polyuria → polydipsia.
3. Protein catabolism: Amino acids released → gluconeogenesis → more hyperglycemia.
4. Fat mobilism: Hormone-sensitive lipase (insulin normally inhibits it) becomes active → free fatty acids released → liver converts to ketone bodies.

HbA1c

• Glycosylated hemoglobin formed by non-enzymatic glycation of Hb.
• Reflects average blood glucose over past 2-3 months (life span of RBC).
• Normal: <5.7%; Pre-DM: 5.7-6.4%; DM: ≥6.5%

Diagnosis (WHO Criteria)

• Fasting blood glucose ≥126 mg/dL
• Random blood glucose ≥200 mg/dL with symptoms
• 2-hour OGTT ≥200 mg/dL
• HbA1c ≥6.5%

Acute Complications

• Diabetic Ketoacidosis (DKA): Type 1 DM. Insulin deficiency → glucagon excess → lipolysis → FFA → ketogenesis → ketonemia → metabolic acidosis. Features: Kussmaul breathing, fruity breath (acetone), anion gap metabolic acidosis.
• HHNK (Hyperosmolar Hyperglycemic Non-Ketotic state): Type 2 DM. Extreme hyperglycemia without ketosis. Serum osmolality >320 mOsm/kg.
• Hypoglycemia: Due to excess insulin.

Chronic Complications (biochemical basis)

1. Polyol pathway: Excess glucose → sorbitol (aldose reductase, NADPH-dependent) → sorbitol accumulates in lens (cataract), nerves (neuropathy), glomeruli (nephropathy). Sorbitol doesn't exit cells easily.
2. Glycation of proteins: Non-enzymatic → AGEs (Advanced Glycation End products) → cross-linking of collagen → basement membrane thickening → microangiopathy.
3. PKC activation: Diacylglycerol increases → activates Protein Kinase C → vascular changes.
4. Oxidative stress: Increased ROS → endothelial damage.

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10. GLYCOGEN METABOLISM

Glycogen Synthesis (Glycogenesis)

• Occurs in fed state, stimulated by insulin

1. Glucose → Glucose-6-P (hexokinase/glucokinase)
2. Glucose-6-P → Glucose-1-P (phosphoglucomutase)
3. Glucose-1-P + UTP → UDP-Glucose (UDP-glucose pyrophosphorylase) - activated glucose
4. UDP-Glucose → Glycogen (Glycogen synthase adds to non-reducing end, α-1,4 bond)
5. Branching enzyme: creates α-1,6 bonds (moves 6-7 glucose units to create branch)

• Glycogen synthase is the key enzyme; requires a primer (glycogenin)

Glycogen Breakdown (Glycogenolysis)

• Occurs in fasting/exercise, stimulated by glucagon (liver), epinephrine (liver+muscle)

1. Glycogen → Glucose-1-P: Glycogen phosphorylase (cleaves α-1,4 bonds; requires PLP/Vit B6)
2. Debranching enzyme: removes α-1,6 branches (transfers branch to main chain by α-1,4 bond; releases free glucose from α-1,6 bond)
3. Glucose-1-P → Glucose-6-P (phosphoglucomutase)
4. Glucose-6-P → Glucose (glucose-6-phosphatase) - ONLY in liver (muscle lacks this enzyme)

Regulation

• Activated: Epinephrine/glucagon → cAMP → PKA → phosphorylates and activates phosphorylase kinase → activates glycogen phosphorylase b → phosphorylase a (active)
• AMP also allosterically activates (in muscle)
• Inhibited by glucose (in liver), ATP, G6P

• Active (dephosphorylated) form
• Inhibited by PKA phosphorylation (reciprocal regulation with phosphorylase)
• Activated by glucose-6-P

Glycogen Storage Diseases (GSD)

• In McArdle disease, ischemic forearm exercise test: normally, muscle glycogenolysis produces lactate. In McArdle's, no phosphorylase activity → no lactate rise, but ammonia (from AMP deamination) still rises.

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11. HOMEOSTASIS OF BLOOD GLUCOSE

Hormonal Regulation

Mechanisms to Maintain Blood Glucose

• Insulin released → GLUT-4 translocation to muscle/fat → glucose uptake
• Liver: Glycogen synthesis, glycolysis activated
• Adipose: Glucose → triglycerides (lipogenesis)
• Glucagon suppressed

• Glucagon released → glycogenolysis (liver) → blood glucose maintained
• After 8-12 hours, liver glycogen depleted → gluconeogenesis begins
• Substrates for gluconeogenesis: Lactate, alanine, glycerol, glutamine

• Occurs mainly in liver (90%), kidney cortex (10%)
• Substrates: Lactate (Cori cycle), Alanine (glucose-alanine cycle), Glycerol (from lipolysis), Propionate
• Key enzymes (bypass glycolysis irreversible steps):
  1. Pyruvate carboxylase (pyruvate → OAA; requires Biotin, ATP)
  2. PEPCK (OAA → PEP; requires GTP)
  3. Fructose-1,6-bisphosphatase (F-1,6-BP → F-6-P)
  4. Glucose-6-phosphatase (G-6-P → Glucose)

• Metformin (Type 2 DM drug): Inhibits hepatic gluconeogenesis (inhibits Complex I of ETC → reduces ATP → activates AMPK → inhibits gluconeogenesis). Also increases peripheral insulin sensitivity.
• Cori cycle: Lactate from exercising muscle goes to liver → gluconeogenesis → glucose back to muscle. Transfers metabolic burden from muscle to liver.
• Alcohol-induced hypoglycemia: Ethanol metabolism generates large amounts of NADH → NADH:NAD+ ratio increases → OAA reduced to malate → less OAA for gluconeogenesis → hypoglycemia (especially in fasting alcoholics).

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SECTION B: LIPIDS

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12. DEFINITION AND CLASSIFICATION OF LIPIDS

Classification

• Fats/Oils: Esters of fatty acids + glycerol (Triacylglycerols / Triglycerides)
• Waxes: Esters of fatty acids + long-chain alcohol (e.g., beeswax = palmitate + myricyl alcohol)

• Phospholipids: Glycerophospholipids (phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, plasmalogenes) and Sphingomyelin
• Glycolipids: Cerebrosides, Gangliosides, Globosides
• Lipoproteins: Lipid + protein complex

• Fatty acids, Glycerol, Sterols (cholesterol, ergosterol), Fat-soluble vitamins (A, D, E, K), Ketone bodies

Fatty Acids

• Saturated: No double bonds (palmitic C16:0, stearic C18:0)
• Unsaturated: One (oleic C18:1) or more (linoleic C18:2, linolenic C18:3, arachidonic C20:4) double bonds
• Essential fatty acids: Linoleic acid (ω-6) and α-Linolenic acid (ω-3) - CANNOT be synthesized by humans
• Arachidonic acid is synthesized from linoleic acid; precursor of prostaglandins, thromboxanes, leukotrienes

Biomedical Importance

• Energy storage: 1g fat provides 9 kcal (vs 4 kcal from carbs/proteins)
• Membrane components: Phospholipids, cholesterol
• Signaling: Prostaglandins, steroid hormones, diacylglycerol (DAG), Inositol trisphosphate (IP3)
• Insulation: Myelin sheath, adipose tissue
• Absorption of fat-soluble vitamins (A, D, E, K)

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13. BIOMEDICAL IMPORTANCE OF LIPIDS

1. Energy source: Triacylglycerols (triglycerides) are the most concentrated energy store. Adipose tissue provides fuel during fasting.
2. Membrane structure: Phospholipids and cholesterol form the lipid bilayer; determines membrane fluidity and permeability.
3. Cholesterol precursor: Bile acids (digestion), steroid hormones (cortisol, aldosterone, sex hormones), Vitamin D.
4. Myelin: High in lipids (sphingomyelin, cerebrosides) → electrical insulation of neurons.
5. Surfactant: Dipalmitoylphosphatidylcholine (DPPC) in lungs → reduces surface tension → prevents alveolar collapse.
6. Prostaglandins and eicosanoids: From arachidonic acid → inflammation, fever, pain, platelet aggregation, vasodilation.
7. Fat-soluble vitamin carriers: A (vision, antioxidant), D (calcium homeostasis), E (antioxidant), K (clotting factors).
8. Second messengers: DAG, IP3 (from phosphatidylinositol), platelet-activating factor (PAF).
9. Protein modification: Lipid anchors (myristoylation, palmitoylation, GPI anchors) attach proteins to membranes.

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14. LIPOPROTEINS

Classification (by density - ultracentrifugation)

Apolipoproteins and their functions

• Apo B-48: Chylomicron synthesis (intestine)
• Apo B-100: Liver (VLDL/LDL); LDL receptor ligand
• Apo C-II: Activates lipoprotein lipase (LPL)
• Apo E: Ligand for LDL receptor and remnant receptor; Apo E4 allele → risk of Alzheimer's disease
• Apo A-I: Activates LCAT (Lecithin-Cholesterol Acyl Transferase)

LDL Receptor Pathway (Goldstein and Brown - Nobel Prize)

1. LDL binds Apo B-100 to LDL receptor (liver, other cells)
2. Endocytosis → LDL degraded in lysosome → cholesterol released
3. Cholesterol feedback: inhibits HMG-CoA reductase (stops cholesterol synthesis), activates ACAT (stores as cholesterol ester), down-regulates LDL receptor synthesis

• Familial Hypercholesterolemia (FH): Autosomal dominant. Mutation in LDL receptor → LDL not cleared → very high plasma LDL → premature atherosclerosis, xanthomas, corneal arcus.
• Statins (HMG-CoA reductase inhibitors): Reduce cholesterol synthesis → upregulate LDL receptors → more LDL cleared from blood. First-line treatment for dyslipidemia.
• Apo C-II deficiency/LPL deficiency: Cannot hydrolyze chylomicron TG → massive hypertriglyceridemia → pancreatitis.
• Lipoprotein (a) [Lp(a)]: Independent risk factor for CVD.

HDL and Reverse Cholesterol Transport

1. Nascent HDL (disc-shaped) released from liver/intestine
2. ABCA1 transporter in peripheral cells exports cholesterol to HDL
3. LCAT (activated by Apo A-I) esterifies cholesterol → CE stored in HDL core → HDL becomes spherical
4. CE transferred to VLDL/LDL via CETP (Cholesterol ester transfer protein)
5. HDL returns to liver → SR-BI receptor → CE taken up → bile acids excreted

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15. PHOSPHOLIPIDS

Glycerophospholipids

Sphingomyelin (Sphingophospholipid)

• Backbone: Sphingosine (not glycerol)
• Components: Sphingosine + Fatty acid = Ceramide + Phosphocholine = Sphingomyelin
• Found in myelin sheath and RBC membrane
• Niemann-Pick disease: Sphingomyelinase deficiency → sphingomyelin accumulation → hepatosplenomegaly, foam cells in marrow, cherry-red spot in eye.

Glycolipids (no phosphate, but contain sugar)

• Cerebrosides: Ceramide + single sugar (glucose or galactose) → brain white matter
• Gangliosides: Ceramide + oligosaccharide with sialic acid → nerve cell membranes, signal transduction
  • Tay-Sachs disease: Hex A (β-hexosaminidase A) deficiency → GM2 ganglioside accumulates → neurodegeneration, cherry-red spot, blindness, death in early childhood.
• Gaucher disease (most common lysosomal storage disorder): Glucocerebrosidase deficiency → glucocerebroside accumulates → Gaucher cells (crinkled tissue paper cytoplasm) in liver, spleen, bone marrow.

Lung Surfactant

• DPPC (dipalmitoylphosphatidylcholine) = main component
• Reduces surface tension in alveoli → prevents collapse during expiration
• Respiratory Distress Syndrome (RDS/HMD) in premature infants: Insufficient surfactant (type II pneumocytes not fully matured before 35 weeks) → lungs collapse → cyanosis, tachypnea.
• Lecithin:Sphingomyelin (L:S) ratio in amniotic fluid: ≥2 = lung maturity; <1.5 = high risk RDS.

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16. β-OXIDATION OF FATTY ACIDS

• Short and medium chain fatty acids: Enter freely
• Long chain fatty acids (>12C): Need carnitine shuttle
  • Fatty Acyl CoA + Carnitine → Acyl-Carnitine (CPT-I, outer membrane)
  • Translocase carries across inner mitochondrial membrane
  • CPT-II (inner membrane) regenerates Acyl CoA
  • Malonyl CoA inhibits CPT-I (prevents β-oxidation when fatty acid synthesis is active)

4 Steps of β-Oxidation Spiral (per cycle)

Energy yield for Palmitic acid (C16:0)

• 7 cycles of β-oxidation → 8 Acetyl CoA + 7 NADH + 7 FADH₂
• 8 Acetyl CoA × 10 ATP = 80 ATP
• 7 NADH × 2.5 = 17.5 ATP
• 7 FADH₂ × 1.5 = 10.5 ATP
• Total = 108 ATP - 2 ATP (activation) = 106 ATP

Special Cases

• Odd-chain fatty acids: Final product is Propionyl CoA (3C) → Methylmalonyl CoA → Succinyl CoA (enters TCA). Requires Vit B12 (methylcobalamin) for methylmalonyl CoA mutase.
• Unsaturated fatty acids: Need additional enzymes: isomerase (monounsaturated) and reductase (polyunsaturated). Yield slightly less ATP (lose 1 FADH₂ per double bond already present).
• Peroxisomal β-oxidation: For very long chain fatty acids (>C22). First step produces H₂O₂ (not FADH₂) → catalase destroys H₂O₂. Product is medium-chain acyl CoA exported to mitochondria.
  • Zellweger syndrome: Peroxisome biogenesis disorder → cannot oxidize VLCFA → accumulation of VLCFA → brain/liver damage, hypotonia.

• Carnitine deficiency: Cannot transport long-chain FA into mitochondria → hypoglycemia (can't use fat for fuel), muscle weakness, cardiomyopathy. Treatment: carnitine supplementation.
• MCAD deficiency (Medium-chain acyl-CoA dehydrogenase): Cannot oxidize medium chain FA → hypoketotic hypoglycemia on fasting. Common cause of SIDS (sudden infant death) in Northern Europeans.

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17. KETONE BODIES METABOLISM AND KETOSIS

Synthesis (Ketogenesis) - in liver mitochondria

1. 2 Acetyl CoA → Acetoacetyl CoA (thiolase)
2. Acetoacetyl CoA + Acetyl CoA → HMG-CoA (HMG-CoA synthase - RATE LIMITING)
3. HMG-CoA → Acetoacetate + Acetyl CoA (HMG-CoA lyase)
4. Acetoacetate → β-Hydroxybutyrate (β-hydroxybutyrate dehydrogenase; NADH required)
5. Acetoacetate → Acetone + CO₂ (spontaneous decarboxylation; minor pathway)

Utilization (Ketolysis) - in peripheral tissues

1. β-Hydroxybutyrate → Acetoacetate (reverse reaction)
2. Acetoacetate + Succinyl CoA → Acetoacetyl CoA + Succinate (succinyl CoA transferase/thiophorase - ABSENT in liver)
3. Acetoacetyl CoA → 2 Acetyl CoA → TCA cycle

Physiological Ketosis vs Pathological Ketosis (DKA)

DKA (Diabetic Ketoacidosis)

• Absent insulin → unrestrained lipolysis → excess acetyl CoA → OAA depleted (used for gluconeogenesis) → acetyl CoA cannot enter TCA → ketogenesis
• Clinical features: Nausea, vomiting, abdominal pain, Kussmaul breathing (deep, labored - compensatory respiratory alkalosis), fruity odor (acetone), dehydration.
• Lab: High glucose, high anion gap metabolic acidosis, ketonemia, ketonuria.

• Breath acetone in DKA: Acetone (volatile) exhaled → fruity odor. Also measurable by breathalyzer (can falsely elevate ethanol reading).
• In starvation, brain adapts to use ketone bodies (reduces glucose requirement - spares protein from gluconeogenesis).

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18. FATTY LIVER

Causes and Mechanisms

Alcohol-induced fatty liver

1. Ethanol → Acetaldehyde (alcohol dehydrogenase) → Acetate (ALDH). Generates large NADH.
2. High NADH: ↓ OAA (needed for TCA) → ↓ β-oxidation, ↓ gluconeogenesis
3. Acetyl CoA diverted to fatty acid synthesis and ketogenesis
4. Excess FFA → VLDL formation but also direct triglyceride accumulation
5. Acetaldehyde inhibits microtubule function → impairs VLDL secretion → fat accumulates

Stages of Alcoholic Liver Disease:

Non-Alcoholic Fatty Liver Disease (NAFLD):

• Spectrum: NAFL (simple steatosis) → NASH (non-alcoholic steatohepatitis) → cirrhosis
• Associated with insulin resistance, obesity, type 2 DM, metabolic syndrome

Applied:

• Kwashiorkor: Protein deficiency → cannot synthesize apolipoproteins (Apo B-100) → VLDL cannot be assembled → triglycerides cannot be exported → fatty liver (+ edema from low albumin).
• Carbon tetrachloride (CCl₄) toxicity: Damages endoplasmic reticulum → impairs VLDL synthesis/secretion → fatty liver.
• Choline deficiency: Phosphatidylcholine required for VLDL synthesis; deficiency → fatty liver.

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19. CHOLESTEROL

Synthesis (HMG-CoA Reductase Pathway)

1. 2 Acetyl CoA → Acetoacetyl CoA → HMG-CoA
2. HMG-CoA → Mevalonate (HMG-CoA reductase - RATE-LIMITING; requires NADPH)
3. Mevalonate → Isopentenyl pyrophosphate (IPP)
4. 6 IPP → Squalene (C30)
5. Squalene → Lanosterol → Cholesterol (C27)

• Inhibited by: Cholesterol, bile acids, statins
• Activated by: Insulin
• Inhibited by: Glucagon, phosphorylation (AMP kinase)

Functions of Cholesterol

1. Membrane component: Regulates fluidity; cholesterol stiffens fluid membranes, fluidizes rigid membranes (cholesterol ↔ sphingomyelin interaction)
2. Bile acids synthesis: Chenodeoxycholic acid and cholic acid (primary bile acids) in liver. Helps in fat digestion and absorption.
3. Steroid hormone precursor: Progesterone → Cortisol, Aldosterone, Testosterone, Estrogen
4. Vitamin D synthesis: Skin (7-dehydrocholesterol + UV light → cholecalciferol/Vit D3)
5. Bile salts: Emulsify dietary fats in small intestine

Bile Acids and Enterohepatic Circulation

• Primary bile acids: Cholic acid, Chenodeoxycholic acid (made in liver)
• Secondary bile acids: Deoxycholic acid, Lithocholic acid (made by gut bacteria)
• Bile salts = bile acids conjugated with glycine or taurine
• Enterohepatic circulation: 90-95% of bile acids reabsorbed from ileum → portal blood → liver → recycled. Only 5-10% lost in feces.
• Disruption (cholestyramine, ileal disease) → more cholesterol used for bile acid synthesis → lower plasma cholesterol.

• Atherosclerosis: LDL carries cholesterol to vessel walls; oxidized LDL taken up by macrophages → foam cells → plaques → atherosclerosis.
• Statins: Competitively inhibit HMG-CoA reductase → less cholesterol synthesis → upregulate LDL receptor → more LDL cleared → lower LDL.
• Cholestyramine (bile acid sequestrant): Binds bile acids in gut → prevents reabsorption → liver uses more cholesterol to make bile acids → lower plasma cholesterol.

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SECTION C: MINERALS

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20. CALCIUM

Forms in blood:

• Protein-bound (mainly albumin): ~40%
• Ionized (physiologically active): ~50%
• Complexed (citrate, phosphate): ~10%

Functions

1. Bone and teeth mineralization (99% of body calcium in bone as hydroxyapatite)
2. Nerve conduction and neuromuscular excitability
3. Muscle contraction (Ca²+ binds troponin C → conformational change → actin-myosin interaction)
4. Blood clotting (factor IV; needed in coagulation cascade)
5. Enzyme activation (amylase, lipase, ATPase)
6. Second messenger (calmodulin-Ca²+ complex)
7. Cardiac rhythm

Regulation

• PTH (Parathyroid hormone): ↑ Blood Ca²+ → ↑ bone resorption (osteoclasts), ↑ renal reabsorption of Ca²+, ↑ renal production of 1,25-(OH)₂D
• Calcitriol (1,25-(OH)₂D₃ = Active Vit D): ↑ intestinal Ca²+ absorption, ↑ renal reabsorption
• Calcitonin (Thyroid C-cells): ↓ Blood Ca²+ → inhibits osteoclasts

Applied

• Hypocalcemia: Tetany (Chvostek's sign - facial muscle twitch; Trousseau's sign - carpal spasm), prolonged QT interval, convulsions. Causes: hypoparathyroidism, Vit D deficiency, hypomagnesemia (prevents PTH secretion).
• Hypercalcemia (Stones, Bones, Groans, Psychic moans): Renal stones, osteitis fibrosa cystica, abdominal pain, depression. Causes: Hyperparathyroidism (1°), malignancy (PTHrP), Vit D toxicity, sarcoidosis.
• Rickets: Vit D deficiency in children → softening of bones (osteomalacia in adults). Bow-legs, craniotabes, rachitic rosary, widened growth plates.
• Milk-alkali syndrome: Excess calcium + antacids → hypercalcemia, alkalosis, renal failure.

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21. IRON

• Hemoglobin: 70%
• Storage (ferritin, hemosiderin): 25%
• Myoglobin, cytochromes, enzymes: 5%

Absorption

• Site: Duodenum and upper jejunum
• Ferric Fe³+ → reduced to Fe²+ by gastric acid and ferrireductase (duodenal cytochrome b, Dcytb)
• Fe²+ enters enterocyte via DMT-1 (Divalent Metal Transporter-1)
• Inside cell: stored as ferritin OR exported via Ferroportin
• Ferroportin controlled by Hepcidin (liver peptide hormone - master regulator of iron)
• In blood: Fe²+ → Fe³+ (ceruloplasmin oxidizes) → binds transferrin → transported to tissues
• Transferrin → binds transferrin receptor (TfR) on target cells → endocytosis → Fe released

Regulation

• Hepcidin: Key regulator. High iron/inflammation → high hepcidin → internalizes and degrades ferroportin → blocks iron export → less iron in blood.
• Ferritin: Storage form; best indicator of iron stores.
• TIBC (Total Iron Binding Capacity): Measures transferrin. High in iron deficiency; low in iron overload.

Functions

• Hemoglobin O₂ transport
• Myoglobin O₂ storage
• Cytochromes (electron transport)
• Enzymes: ribonucleotide reductase (DNA synthesis), catalase, peroxidase

Applied

• Iron Deficiency Anemia (IDA): Most common nutritional deficiency worldwide. Stages: depleted stores (↓ferritin) → ↓serum iron, ↑TIBC → microcytic hypochromic anemia. Symptoms: fatigue, pallor, koilonychia (spoon nails), angular stomatitis, pica.
• Iron overload / Hemochromatosis: Autosomal recessive (HFE gene mutation). Excess iron deposits in liver (cirrhosis), pancreas (bronze diabetes), heart (cardiomyopathy), joints, skin (bronze pigmentation). Treatment: phlebotomy.
• Anemia of chronic disease: High hepcidin (due to inflammation/IL-6) → iron trapped in macrophages → not available for erythropoiesis → normocytic/microcytic anemia with LOW serum iron, LOW TIBC, NORMAL/HIGH ferritin.
• Vitamin C enhances iron absorption (reduces Fe³+ to Fe²+). Phytates (cereals), tannins (tea), calcium inhibit iron absorption.

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22. COPPER

Functions

1. Component of metalloenzymes:
   • Ceruloplasmin (ferroxidase): iron metabolism
   • Cytochrome c oxidase (Complex IV): electron transport chain
   • Superoxide dismutase (Cu-Zn SOD): antioxidant defense
   • Dopamine β-hydroxylase: converts dopamine → norepinephrine
   • Lysyl oxidase: cross-linking collagen and elastin (bone and connective tissue)
   • Tyrosinase: melanin synthesis

Applied

• Wilson's disease (hepatolenticular degeneration): Autosomal recessive. Mutation in ATP7B gene (copper-transporting ATPase) → copper cannot be incorporated into ceruloplasmin or excreted in bile → copper accumulates in liver, brain, kidney, cornea. Features: liver disease (cirrhosis), neuropsychiatric symptoms, Kayser-Fleischer rings (copper in Descemet's membrane of cornea), Fanconi syndrome (renal tubular damage). Low serum ceruloplasmin; high urinary copper. Treatment: D-penicillamine (chelation), zinc acetate.
• Menkes disease (Kinky hair disease): X-linked recessive. Mutation in ATP7A gene → impaired copper absorption and transport → copper deficiency in brain and connective tissue. Features: kinky/brittle hair (deficient lysyl oxidase → poor cross-linking), hypotonia, seizures, arterial aneurysms (deficient lysyl oxidase → weak elastin/collagen). Low serum copper and ceruloplasmin. Fatal in early childhood.

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23. IODINE

Thyroid Hormone Synthesis

1. Iodide uptake: Thyroid follicular cells actively transport I⁻ from blood via Na+/I⁻ symporter (NIS) (driven by Na+ gradient).
2. Oxidation: I⁻ → I⁰ (by thyroid peroxidase (TPO) using H₂O₂) - "organification"
3. Iodination: I⁰ + Tyrosine residues on thyroglobulin → MIT (monoiodotyrosine) and DIT (diiodotyrosine)
4. Coupling: MIT + DIT → T3 (triiodothyronine); DIT + DIT → T4 (thyroxine). By TPO.
5. Storage: As thyroglobulin colloid in follicular lumen.
6. Release: TSH → endocytosis of colloid → lysosomal proteolysis → T3 and T4 released. MIT and DIT deiodinated (iodine recycled).

Functions of Thyroid Hormones

• Control basal metabolic rate
• Essential for brain development (critical in fetal/neonatal period)
• Promote growth (permissive for GH action)
• Increase O₂ consumption and heat production
• Cardiac effects: increase heart rate and contractility
• Stimulate catecholamine sensitivity
• Promote protein synthesis, glucose absorption, lipid mobilism

Applied

• Iodine deficiency: Most common preventable cause of brain damage worldwide.
  • In adults: Goiter (thyroid hypertrophy due to excess TSH stimulation); hypothyroidism.
  • In pregnancy: Maternal hypothyroidism → cretinism in offspring (irreversible mental retardation, growth failure, deaf-mutism, myxedema). Critical period: first trimester.
  • Prevention: Iodized salt.
• Iodine excess (Wolff-Chaikoff effect): Acute excess iodine temporarily inhibits thyroid hormone synthesis (increased organification inhibits itself). Normally escapes; in diseased thyroid (Hashimoto's), may cause persistent hypothyroidism.
• Drugs blocking thyroid: Propylthiouracil (PTU) and Methimazole: inhibit TPO → block organification and coupling. PTU also inhibits peripheral T4→T3 conversion.
• Radioactive iodine (¹³¹I): Used to ablate thyroid in Graves' disease or thyroid cancer.

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24. FLUORINE

Functions

1. Dental caries prevention:
   • Fluoride replaces hydroxyl groups in hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) → fluorapatite (Ca₁₀(PO₄)₆F₂)
   • Fluorapatite is harder, less soluble, more resistant to acid attack by Streptococcus mutans
   • Also inhibits bacterial glucosyltransferase (reduces plaque)
2. Bone strength: Fluoride incorporated into bone → harder bone
3. Enzyme inhibitor: At high doses: inhibits enolase (glycolysis), ATPase, adenylyl cyclase

Applied

• Optimal fluoride concentration in water: 0.7-1.0 ppm (mg/L)
• Dental fluorosis (mottled enamel): Excess fluoride (>2 ppm) during tooth development → white spots, brown staining, pitting of enamel. Aesthetic problem.
• Skeletal fluorosis: Chronic excess (>10 ppm) → osteosclerosis, bone deformity, calcification of ligaments, neurological compression.
• Acute fluoride toxicity: Soluble fluoride (e.g., fluorosilicic acid) in excess → nausea, vomiting, tetany (binds Ca²+), cardiac arrhythmias.
• Sodium fluoride (NaF) is used in blood tubes for glucose estimation → inhibits enolase → prevents glycolysis ex vivo.
• Fluoride prophylaxis in children: Supplements where water fluoride is low; fluoride varnish applied topically.

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SECTION D: CLINICAL BIOCHEMISTRY

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25. LIVER FUNCTION TESTS (LFT)

Tests and Normal Values

• Viral hepatitis: ALT > AST (ratio >1)
• Alcoholic hepatitis: AST > ALT (ratio >2, because alcohol depletes pyridoxal phosphate needed for ALT)

• Gilbert syndrome: Mild unconjugated hyperbilirubinemia due to reduced UDP-glucuronyl transferase activity. Benign. Jaundice precipitated by fasting, stress, illness. No treatment needed.
• Crigler-Najjar syndrome type I: Complete absence of UDP-glucuronyl transferase → severe unconjugated hyperbilirubinemia → kernicterus (bilirubin enters brain). Treatment: phototherapy + liver transplant.
• Neonatal jaundice (physiological): Transient due to immature conjugating system + increased hemolysis. Treated with phototherapy (converts bilirubin to water-soluble isomers).

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26. KIDNEY FUNCTION TESTS (KFT/RFT)

Tests

• Inulin clearance = Gold standard (not clinical routine)
• Creatinine clearance = GFR estimation: Ccr = (UCr × V) / PCr; Normal 90-120 mL/min
• eGFR (estimated, using CKD-EPI or MDRD formula)
• GFR <60 mL/min/1.73m² for >3 months = CKD

• Urea is the main end product of protein/amino acid catabolism. Made in liver via urea cycle (Krebs-Henseleit cycle).
• Urea cycle requires: Carbamoyl phosphate synthetase I (CPS-I), ornithine transcarbamylase (OTC), argininosuccinate synthetase, argininosuccinate lyase, arginase.
• OTC deficiency = most common urea cycle disorder. X-linked. Hyperammonemia (ammonia toxic to brain).

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27. MECHANISMS FOR ACID-BASE BALANCE

Three Main Buffer Systems

• H+ + HCO₃⁻ ⇌ H₂CO₃ ⇌ CO₂ + H₂O (carbonic anhydrase)
• pKa = 6.1; most effective because HCO₃⁻ and CO₂ are regulated independently
• CO₂ controlled by lungs (fast); HCO₃⁻ controlled by kidneys (slow)

• H₂PO₄⁻ ⇌ HPO₄²⁻ + H+; pKa = 6.8
• Good buffer for urine pH regulation

• Histidine residues (imidazole group, pKa ≈ 6.0) on proteins (Hb especially)
• Haldane effect: Deoxyhemoglobin binds CO₂ and H+ better than oxyhemoglobin
• Bohr effect: ↑CO₂/↑H+ → Hb releases O₂ more easily in tissues

Compensatory Mechanisms

• Acidosis → hyperventilation → blows off CO₂ → pH rises
• Alkalosis → hypoventilation → retains CO₂ → pH falls

• Acidosis: kidneys excrete H+ (as NH₄+ and titratable acidity), reabsorb HCO₃⁻, generate new HCO₃⁻
• Alkalosis: kidneys excrete HCO₃⁻, reabsorb less HCO₃⁻

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28. METABOLIC ACIDOSIS

Two Types

• Acid added, HCO₃⁻ consumed, gap increases
• MUDPILES mnemonic:
  • M - Methanol
  • U - Uremia
  • D - DKA (Diabetic Ketoacidosis)
  • P - Paraldehyde
  • I - Isoniazid/Iron
  • L - Lactic acidosis
  • E - Ethylene glycol
  • S - Salicylates

• HCO₃⁻ lost, replaced by Cl⁻ → gap normal
• Causes: Diarrhea (HCO₃⁻ lost in stool), RTA (renal tubular acidosis), TPN, carbonic anhydrase inhibitors (acetazolamide)

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29. DEHYDRATION

Types Based on Tonicity

Compartments

• ICF (Intracellular fluid): 2/3 of TBW; mainly K+, phosphate
• ECF (Extracellular fluid): 1/3 of TBW = Plasma (1/4 ECF) + Interstitial (3/4 ECF); mainly Na+, Cl⁻, HCO₃⁻

Signs and Symptoms

• Mild (3-5%): Thirst, dry mouth, decreased urine output
• Moderate (5-8%): Tachycardia, decreased skin turgor, sunken eyes, oliguria
• Severe (>10%): Hypotension, confusion, coma, death

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30. REGULATION OF WATER BALANCE

Antidiuretic Hormone (ADH/Vasopressin)

• Made in supraoptic and paraventricular nuclei of hypothalamus; stored/released from posterior pituitary.
• Stimuli for ADH release: ↑plasma osmolality (most potent), ↓blood volume, pain, stress, nausea, some drugs.
• Action: V2 receptors on collecting duct principal cells → cAMP → PKA → Aquaporin-2 (AQP2) insertion into luminal membrane → water reabsorption.
• Also: V1 receptors → vasoconstriction.

• Secreted by adrenal cortex zona glomerulosa in response to: ↓blood volume → RAAS (Angiotensin II → aldosterone); ↑K+; ACTH.
• Acts on principal cells of collecting duct: ↑Na+ reabsorption (via ENaC channels), ↑K+ and H+ secretion.
• Water follows Na+ (osmotically) → volume expansion.

RAAS (Renin-Angiotensin-Aldosterone System)

• SIADH: Inappropriate ADH secretion → excessive water reabsorption → dilutional hyponatremia, concentrated urine, low plasma osmolality. Causes: CNS disorders, lung cancer (ectopic ADH), drugs (carbamazepine, SSRIs). Treatment: fluid restriction, demeclocycline, tolvaptan.
• Diabetes Insipidus (DI): Failure of ADH (central DI) or renal resistance to ADH (nephrogenic DI) → large volumes of dilute urine, hypernatremia. Distinguish by desmopressin (ADH analog) challenge - central DI responds (urine concentrates); nephrogenic does not.

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31. DETOXIFICATION

Mechanisms of Detoxification

• Cytochrome P450 (CYP) enzymes (microsomal): Most important. Monooxygenases. Require O₂ and NADPH.
• Reactions: Hydroxylation, deamination, dealkylation, sulfoxidation, epoxidation.
• CYP inducers (↑detox + drug interactions): Rifampicin, phenobarbitone, phenytoin, carbamazepine, alcohol (chronic).
• CYP inhibitors (slow detox + drug interactions): Cimetidine, erythromycin, ketoconazole, grapefruit juice.

• P-glycoprotein (MDR1) and MRP transporters export conjugated products into bile or urine.

Paracetamol (Acetaminophen) Toxicity - Classic Example

• Normal: 90% metabolized by glucuronidation/sulfation; 10% by CYP2E1 → NAPQI (toxic) → immediately neutralized by glutathione (GSH) → mercapturic acid → excreted.
• Overdose: GSH depleted → NAPQI accumulates → covalent binding to hepatocyte proteins → acute hepatic necrosis (centrilobular).
• Treatment: N-Acetylcysteine (NAC) - replenishes GSH.

• Slow vs Fast acetylators: INH (isoniazid) metabolized by N-acetyltransferase (NAT2). Slow acetylators (genetic): higher INH blood levels → peripheral neuropathy (competes with pyridoxal phosphate). Fast acetylators: more hepatotoxic metabolites.
• First pass effect: Oral drugs absorbed via portal vein → liver → hepatic CYP metabolism before reaching systemic circulation → reduced bioavailability. Drugs with high first-pass: morphine, propranolol, lidocaine.

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32. cAMP (CYCLIC AMP)

Mechanism of Action (Second Messenger)

1. Hormone (first messenger) binds Gs-coupled receptor
2. Gs protein activated → α-subunit binds GTP → activates Adenylyl cyclase
3. ATP → cAMP (↑)
4. cAMP activates Protein Kinase A (PKA)
5. PKA phosphorylates target proteins → biological effects
6. PDE degrades cAMP → terminates signal

Effects of cAMP/PKA

• Glycogenolysis: PKA → Phosphorylase kinase → Glycogen phosphorylase (active)
• Lipolysis: PKA → Hormone-sensitive lipase (HSL) → breaks TG → FFA
• Inhibition of glycogen synthesis: PKA → Inactivates glycogen synthase
• Gluconeogenesis: PKA → CREB → PEPCK gene expression
• Cardiac stimulation: PKA → ↑heart rate, ↑contractility (β1)

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33. MECHANISM OF ACTION OF GROUP I AND GROUP II HORMONES

Group I Hormones (Lipophilic - Intracellular Receptors)

1. Lipid-soluble → diffuse across plasma membrane
2. Bind to intracellular receptors (cytoplasmic or nuclear) - nuclear receptor superfamily
3. Hormone-receptor complex enters nucleus (or is already nuclear)
4. Binds to Hormone Response Elements (HRE) on DNA (specific DNA sequences)
5. Acts as transcription factor → stimulates/inhibits gene transcription
6. → New mRNA → New proteins → Biological effect

• Slow onset (hours-days)
• Long duration
• Effects mediated through gene expression (new protein synthesis)
• Receptor is in cytoplasm (e.g., glucocorticoid receptor) or nucleus (e.g., thyroid hormone receptor)

• Cortisol → binds glucocorticoid receptor (GR) → GR-cortisol complex enters nucleus → induces anti-inflammatory genes, gluconeogenic enzymes → anti-inflammatory, gluconeogenic effects
• Estrogen → estrogen receptor (ER) → HRE → induces genes for cell proliferation

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Group II Hormones (Water-soluble - Cell Surface Receptors)

1. Hormone → Gq receptor → Phospholipase C (PLC) activated
2. PLC cleaves PIP2 → IP3 (Inositol trisphosphate) + DAG (Diacylglycerol)
3. IP3: → ER/SR → releases Ca²+ → Ca²+-calmodulin → calmodulin kinase → biological effects
4. DAG: → activates Protein Kinase C (PKC) → phosphorylation → biological effects

1. Hormone binds → dimerization of receptor → autophosphorylation of tyrosine residues
2. Phosphotyrosines → recruit adaptor proteins (IRS-1, Grb2, SOS) → Ras-MAP kinase pathway (cell growth/differentiation) OR PI3K-Akt pathway (GLUT-4 translocation, anti-apoptosis)
3. Insulin-specific: PI3K → Akt → GLUT-4 translocation to membrane → glucose uptake

1. Hormone → cytokine receptor (no intrinsic kinase) → receptor dimerization → activates associated JAK (Janus Kinase)
2. JAK phosphorylates STAT proteins
3. p-STAT dimerizes → enters nucleus → activates transcription

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34. METABOLISM IN FED AND FASTING CONDITIONS

FED STATE (Post-absorptive, 0-2 hours after meal)

• Dietary glucose → portal vein → liver → glycogen + glycolysis + de novo lipogenesis
• Excess glucose → Acetyl CoA → fatty acids → TG (de novo lipogenesis; RQ = 0.85)

• Amino acids → protein synthesis → excess to gluconeogenesis or fat

• Dietary fat → chylomicrons → LPL hydrolyzes TG → FFA taken up by adipose/muscle
• Liver: fatty acid synthesis from glucose

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FASTING STATE (Hours to Days)

• Insulin ↓, Glucagon ↑
• Glycogenolysis (liver) → maintains blood glucose
• Muscle protein catabolism → alanine → liver → gluconeogenesis
• Adipose: hormone-sensitive lipase active → FFA released → liver → β-oxidation → ATP and ketone bodies

• Liver glycogen depleted
• Gluconeogenesis dominant (substrates: lactate, alanine, glycerol, glutamine)
• Ketogenesis increases significantly
• Brain adapts: uses ketone bodies (reduces glucose need, spares protein)
• Protein breakdown for gluconeogenesis continues but slowed

• Adipose fat stores = primary fuel for muscle and other organs
• Ketone bodies = brain fuel (up to 75% of brain's energy)
• Muscle protein conserved (less gluconeogenesis from protein)
• RQ drops to 0.7 (pure fat oxidation)
• Basal metabolic rate decreases (conservation)

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35. TUMOUR MARKERS

Classification and Important Markers

• AFP and CEA are classic oncofetal antigens.

• PTHrP (malignancy-associated hypercalcemia), ACTH (ectopic in small cell lung cancer), ADH (SIADH in lung cancer)

Applied reasoning

• AFP monitoring: After treatment of HCC or yolk sac tumor, serial AFP should fall. Rising AFP = recurrence.
• CEA in colorectal cancer: Pre-op CEA baseline → post-op should fall → rising CEA in follow-up → metastasis.
• β-hCG in choriocarcinoma: Highly chemosensitive tumour; β-hCG is an excellent monitoring tool. Falls with treatment; rises with recurrence.
• CA-125 in ovarian cancer: Also elevated in endometriosis, PID, peritonitis → not useful for diagnosis but good for monitoring post-treatment.
• PSA velocity (rate of PSA rise over time) more important than single value.

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QUICK REVISION MNEMONICS

• Wilson = Waist up (liver, brain) + Kayser-Fleischer rings + Low ceruloplasmin + High urine copper + ATP7B mutation
• Menkes = Male (X-linked) + Mental retardation + Minky hair + Low copper/ceruloplasmin + ATP7A mutation

• Pre-hepatic: ↑indirect only
• Hepatic: ↑both
• Post-hepatic: ↑direct only + dark urine + pale stools

• Liver: AFP
• Colon: CEA
• Prostate: PSA
• Ovary: CA-125
• Pancreas: CA 19-9
• Breast: CA 15-3
• Choriocarcinoma/testicular: β-hCG
• Medullary thyroid: Calcitonin
• Carcinoid: 5-HIAA
• Myeloma: Bence-Jones proteins

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References: Basic Medical Biochemistry: A Clinical Approach, 6e (Lieberman & Peet) | Harper's Illustrated Biochemistry, 32e | Lippincott Illustrated Reviews: Biochemistry, 8e | Content aligned with Vasudevan & Sreekumari's Textbook of Biochemistry and Satyanarayan's Biochemistry (Indian curriculum standard)