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QUESTION 1: Calcium — Dietary Sources, Biochemical Functions, Normal Serum Levels, Regulation & Disorders

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A. Dietary Sources of Calcium

Daily requirement: 800–1200 mg/day for adults; 1200–1500 mg/day for pregnant and lactating women.

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B. Normal Serum Levels

Note: For every 1 g/dL decrease in albumin below 4 g/dL, total serum calcium decreases by ~0.8 mg/dL. Corrected Ca = Measured Ca + 0.8 × (4 − serum albumin).

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C. Biochemical Functions of Calcium

1. Structural role: Major component of bone (hydroxyapatite — Ca₁₀(PO₄)₆(OH)₂) and teeth; ~99% of body calcium is in bone.
2. Neuromuscular excitability: Ca²⁺ lowers threshold of nerve and muscle excitability; hypocalcemia causes tetany.
3. Muscle contraction: Ca²⁺ binds troponin C → releases inhibition of actin–myosin interaction.
4. Enzyme activation: Activates enzymes like lipase, ATPase, phospholipase A₂; cofactor for coagulation factors (factors II, VII, IX, X — extrinsic and intrinsic pathways).
5. Blood coagulation: Ca²⁺ (Factor IV) is essential for multiple steps of the coagulation cascade.
6. Intracellular second messenger: Released from ER by IP₃; activates calmodulin → downstream signaling.
7. Cell membrane permeability and adhesion: Stabilizes cell membranes; involved in exocytosis (e.g., hormone secretion, neurotransmitter release).
8. Bone remodeling: Regulates osteoblast and osteoclast activity.
9. Cardiac pacemaker activity: Maintains normal cardiac rhythm.

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D. Regulation of Calcium

1. Parathyroid Hormone (PTH)

• Secreted by chief cells of parathyroid glands as an 84-amino acid peptide (pre-proPTH → proPTH → PTH).
• Stimulus: Low serum Ca²⁺ detected by calcium-sensing receptor (CaSR), a G protein-coupled receptor on parathyroid cells.
• Actions:
  • Bone: Stimulates osteoclasts (via osteoblast-mediated RANKL release) → bone resorption → Ca²⁺ release.
  • Kidney: Increases Ca²⁺ reabsorption in distal tubule (upregulates TRPV5, calbindin-D28K); inhibits phosphate reabsorption (internalizes NPT2a/2c in proximal tubule).
  • Kidney: Stimulates 1α-hydroxylase → increases synthesis of 1,25(OH)₂D₃ (active vitamin D); inhibits 24-hydroxylase.
  • Net effect: ↑ serum Ca²⁺, ↓ serum PO₄³⁻

2. Calcitriol (1,25-Dihydroxyvitamin D₃ — Active Vitamin D)

• Synthesis: Skin (UV) → Cholecalciferol (D₃) → Liver (25-hydroxylase) → 25-OH-D₃ → Kidney (1α-hydroxylase, stimulated by PTH) → 1,25(OH)₂D₃.
• Actions:
  • Gut: Major action — increases intestinal absorption of Ca²⁺ and PO₄³⁻ (induces calbindin-D9K synthesis).
  • Bone: Acts with PTH to mobilize calcium.
  • Kidney: Enhances Ca²⁺ reabsorption.
  • Parathyroid: Negative feedback — suppresses PTH synthesis.
  • Net effect: ↑ serum Ca²⁺ and PO₄³⁻

3. Calcitonin

• Secreted by C-cells (parafollicular cells) of the thyroid gland.
• Stimulus: High serum Ca²⁺.
• Actions:
  • Bone: Inhibits osteoclasts → decreases bone resorption.
  • Kidney: Increases Ca²⁺ and PO₄³⁻ excretion.
  • Net effect: ↓ serum Ca²⁺ and PO₄³⁻ (opposes PTH)

4. Calcium-Sensing Receptor (CaSR)

• A GPCR on parathyroid cells, kidney tubular cells, and other tissues.
• High Ca²⁺ activates CaSR → inhibits PTH secretion (negative feedback).
• Low Ca²⁺ → reduced CaSR activity → PTH secretion increases.

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E. Disorders of Calcium

HYPERCALCEMIA (Serum Ca²⁺ > 10.5 mg/dL)

• C – Cancer (most common cause in hospitalized patients; PTHrP-mediated osteolysis, myeloma)
• H – Hyperparathyroidism (primary — most common outpatient cause; usually from parathyroid adenoma; MEN-1, MEN-2 syndromes)
• A – Addison's disease / Acromegaly
• P – Paget's disease, Prolonged immobilization
• R – Renal failure (tertiary hyperparathyroidism)
• I – Iatrogenic (excess Vitamin D, thiazide diuretics, milk-alkali syndrome), Inflammatory (sarcoidosis, TB — granulomas produce 1,25(OH)₂D₃ extrarenally)

• Bones: Osteitis fibrosa cystica, subperiosteal bone resorption, "salt and pepper" skull X-ray
• Stones: Renal calculi (nephrolithiasis), nephrocalcinosis
• Groans: Nausea, vomiting, constipation, pancreatitis, peptic ulcer
• Psychic Moans: Depression, confusion, lethargy, coma
• Cardiovascular: Shortened QT interval on ECG, hypertension
• Polyuria and polydipsia (nephrogenic DI)

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HYPOCALCEMIA (Serum Ca²⁺ < 8.5 mg/dL)

• Hypoparathyroidism (post-thyroidectomy/parathyroidectomy — most common cause; also DiGeorge syndrome — aplasia of 3rd and 4th pharyngeal pouches)
• Vitamin D deficiency / resistance (rickets in children, osteomalacia in adults)
• Pseudohypoparathyroidism (target organ resistance to PTH; Albright's hereditary osteodystrophy)
• Malabsorption syndromes
• Hypomagnesemia (impairs PTH secretion and action)
• Pancreatitis (saponification of calcium in fat necrosis)
• Renal failure (decreased 1α-hydroxylase activity)

• Neuromuscular: Tetany (carpopedal spasm), paresthesias, laryngospasm, seizures
• Chvostek's sign: Tapping facial nerve → ipsilateral facial muscle twitch
• Trousseau's sign: Inflating BP cuff > systolic pressure for 3 min → carpal spasm (more specific)
• Cardiovascular: Prolonged QT interval, heart failure
• Cataracts (chronic hypocalcemia)
• Papilledema, raised intracranial pressure

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QUESTION 2: Beta Oxidation of Palmitic Acid — Steps, Energetics & Disorders

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Overview

• β-Oxidation is the major pathway for catabolism of fatty acids.
• Location: Mitochondrial matrix.
• Activation site: Cytosol (outer mitochondrial membrane).
• Palmitic acid (C16:0, a saturated, even-chain fatty acid) undergoes 7 cycles of β-oxidation to yield 8 acetyl-CoA molecules.

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Preparatory Step: Activation of Palmitic Acid

Palmitoyl-CoA synthetase (acyl-CoA synthetase / fatty acid thiokinase) Palmitate + CoA-SH + ATP → Palmitoyl-CoA + AMP + PPi (PPi is hydrolyzed by pyrophosphatase → irreversible; costs 2 ATP equivalents)

• Carnitine acyltransferase I (CAT-I / CPT-I) on outer mitochondrial membrane: Palmitoyl-CoA + Carnitine → Palmitoylcarnitine + CoA
• Palmitoylcarnitine crosses inner membrane via carnitine-acylcarnitine translocase.
• CAT-II (CPT-II) regenerates Palmitoyl-CoA inside the matrix.
• CPT-I is the rate-limiting, regulated step (inhibited by malonyl-CoA — prevents simultaneous synthesis and oxidation of fatty acids).

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Steps of One Cycle of β-Oxidation (4 Reactions)

Step 1 — Oxidation (Dehydrogenation)

Enzyme: Acyl-CoA dehydrogenase (FAD-dependent; four isoforms: VLCAD, LCAD, MCAD, SCAD)

Palmitoyl-CoA + FAD → Δ²-trans-Enoyl-CoA + FADH₂

• Removes 2H from C2 (α) and C3 (β) carbons.
• Creates a trans double bond between C2 and C3.
• Produces 1 FADH₂ per cycle.

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Step 2 — Hydration

Enzyme: Enoyl-CoA hydratase (Crotonase)

Δ²-trans-Enoyl-CoA + H₂O → L-3-Hydroxyacyl-CoA

• Water added across the double bond.
• Produces the L (S) stereoisomer specifically.

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Step 3 — Oxidation (Dehydrogenation)

Enzyme: L-3-Hydroxyacyl-CoA dehydrogenase (NAD⁺-dependent)

L-3-Hydroxyacyl-CoA + NAD⁺ → 3-Ketoacyl-CoA + NADH + H⁺

• Oxidation of hydroxyl group at C3.
• Produces 1 NADH per cycle.

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Step 4 — Thiolysis (Cleavage)

Enzyme: 3-Ketoacyl-CoA thiolase (β-ketothiolase)

3-Ketoacyl-CoA + CoA-SH → Acetyl-CoA + Acyl-CoA (2 carbons shorter)

• Cleaves the bond between C2 and C3.
• Releases 1 Acetyl-CoA.
• Shortened acyl-CoA re-enters the cycle.

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Summary Table: 7 Cycles for Palmitate (C16)

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Energetics of β-Oxidation of Palmitic Acid

ATP yield per step:

Palmitate is the most energy-dense common fuel: 106 ATP net from a single 16-carbon fatty acid, compared to ~30–32 ATP from glucose (6C).

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Disorders of β-Oxidation

1. Medium-Chain Acyl-CoA Dehydrogenase Deficiency (MCAD Deficiency)

• Most common inherited disorder of fatty acid oxidation.
• Autosomal recessive; mutation in ACADM gene.
• Cannot oxidize medium-chain fatty acids (C6–C10).
• Features: Fasting-induced non-ketotic hypoglycemia, lethargy, vomiting, sudden death in infancy (may mimic SIDS).
• Diagnosis: Neonatal screening; elevated medium-chain acylcarnitines in blood; dicarboxylic aciduria in urine.
• Treatment: Avoid fasting; high-carbohydrate diet; L-carnitine supplementation.

2. Carnitine Deficiency / CPT Deficiency

• Primary carnitine deficiency: Defect in plasma membrane carnitine transporter (OCTN2); can also occur in preterm neonates, hemodialysis patients.
• Features: Hypoglycemia, lipid myopathy (muscle weakness), cardiomyopathy, hyperammonemia.
• CPT-I deficiency: Affects liver → impaired ketogenesis; hypoketotic hypoglycemia.
• CPT-II deficiency: Affects skeletal muscle (and liver in severe forms) → myoglobinuria after exercise, rhabdomyolysis.
• Treatment: L-Carnitine supplementation (oral).

3. Long-Chain 3-Hydroxyacyl-CoA Dehydrogenase Deficiency (LCHAD)

• Deficiency of LCHAD (part of mitochondrial trifunctional protein complex).
• Features: Hypoketotic hypoglycemia, cardiomyopathy, peripheral neuropathy, pigmentary retinopathy.
• Associated with acute fatty liver of pregnancy in heterozygous carrier mothers.

4. Very Long-Chain Acyl-CoA Dehydrogenase Deficiency (VLCAD)

• Cannot oxidize fatty acids >C16.
• Features: Hypertrophic cardiomyopathy, hypoglycemia.

5. Jamaican Vomiting Sickness

• Caused by hypoglycin A (toxin in unripe ackee fruit).
• Inactivates medium- and short-chain acyl-CoA dehydrogenase.
• Severe non-ketotic hypoglycemia, vomiting; can be fatal.

6. Refsum Disease

• Deficiency of phytanoyl-CoA hydroxylase (peroxisomal α-oxidation defect).
• Accumulation of phytanic acid (branched-chain fatty acid from dairy/ruminant fat).
• Features: Cerebellar ataxia, peripheral neuropathy, retinitis pigmentosa, nerve deafness.
• Treatment: Phytanic acid-restricted diet.

7. Zellweger (Cerebrohepatorenal) Syndrome

• Absent or non-functional peroxisomes.
• Failure of peroxisomal β-oxidation of very long-chain fatty acids (C26–C38).
• Features: Severe neurological impairment, hepatomegaly, accumulation of polyenoic acids in brain.
• Most patients die in the first year of life.

8. Dicarboxylic Aciduria

• Lack of medium-chain acyl-CoA dehydrogenase → fatty acids undergo ω-oxidation (microsomal) instead → C6–C10 dicarboxylic acids excreted.
• Features: Non-ketotic hypoglycemia.

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