Chapter 7 — Metabolism of Lipids and Proteins
Key Concepts Quick Recap
| Pathway | Location | Key Products |
|---|
| Beta-oxidation (fatty acid oxidation) | Mitochondria | Acetyl-CoA + NADH + FADH₂ |
| Fatty acid synthesis (de novo) | Cytoplasm | Palmitate (C16) from acetyl-CoA |
| Ketogenesis | Liver mitochondria | Acetoacetate, β-hydroxybutyrate, acetone |
| Transamination | Mitochondria + cytoplasm | Transfer of amino group; PLP cofactor |
| Deamination | Liver | Removes NH₃; converts amino acids to keto acids |
| Urea cycle | Liver (mitochondria + cytoplasm) | Urea (excretion of NH₃) |
I. Choose the Best Answer (MCQs)
1. Beta-oxidation of fatty acids occurs in the: (a) Cytoplasm (b) Mitochondria (c) Smooth ER (d) Peroxisomes only
Answer(b) Mitochondria Beta-oxidation: occurs in mitochondrial matrix (after fatty acids are activated to acyl-CoA in cytoplasm and transported into mitochondria via carnitine carrier). Very long chain fatty acids are partially oxidised in peroxisomes first.
2. The product of each round of beta-oxidation includes: (a) 1 acetyl-CoA + 1 NADH + 1 FADH₂ (b) 2 acetyl-CoA (c) 1 acetyl-CoA only (d) 1 glucose
Answer(a) 1 acetyl-CoA + 1 NADH + 1 FADH₂ Each cycle of beta-oxidation: fatty acyl-CoA (Cn) → fatty acyl-CoA (Cn-2) + acetyl-CoA + NADH + FADH₂.
3. Ketone bodies are produced in the: (a) Adipose tissue (b) Liver (c) Muscle (d) Brain
Answer(b) Liver Ketogenesis: excess acetyl-CoA (from beta-oxidation) → ketone bodies: acetoacetate, β-hydroxybutyrate, acetone. Produced in liver. Used as fuel by brain, heart, muscle during fasting or diabetes. Diabetic ketoacidosis: uncontrolled production → blood pH falls.
4. Transamination involves transfer of: (a) Phosphate group (b) Amino group (c) Carboxyl group (d) Methyl group
Answer(b) Amino group Transamination: amino group (–NH₂) transferred from amino acid to a keto acid. Catalysed by aminotransferases (transaminases). Coenzyme: Pyridoxal phosphate (PLP, from Vitamin B₆). Example: Alanine + α-ketoglutarate → Pyruvate + Glutamate (ALT enzyme).
5. Urea is synthesised in the: (a) Kidney (b) Brain (c) Liver (d) Muscle
Answer(c) Liver Urea cycle occurs exclusively in the liver. NH₃ (toxic) is converted to urea (less toxic, water-soluble) → released into blood → kidneys → urine. Liver disease → hyperammonaemia → coma.
6. The enzyme involved in oxidative deamination of glutamate is: (a) Glutamate dehydrogenase (b) Glutaminase (c) Aspartate aminotransferase (d) Alanine aminotransferase
Answer(a) Glutamate dehydrogenase Glutamate dehydrogenase (GDH): in liver mitochondria. Glutamate + NAD⁺ (or NADP⁺) → α-ketoglutarate + NH₃. Major route for NH₃ release from amino acids. The NH₃ enters the urea cycle.
7. Fatty acid synthesis (de novo lipogenesis) occurs in the: (a) Mitochondria (b) Cytoplasm (c) Nucleus (d) Peroxisomes
Answer(b) Cytoplasm De novo fatty acid synthesis: occurs in cytoplasm using acetyl-CoA units (transported from mitochondria as citrate). Key enzyme: Acetyl-CoA carboxylase (ACC, rate-limiting: acetyl-CoA → malonyl-CoA) and fatty acid synthase (FAS). Products: primarily palmitate (C16:0).
II. Short Answer Questions
Q1. Calculate the ATP yield from complete oxidation of palmitic acid (C16:0).
Answer
Activation of palmitate: –2 ATP equivalents (forms palmitoyl-CoA).
Beta-oxidation of C16: 7 cycles.
Each cycle: 1 FADH₂ + 1 NADH + 1 acetyl-CoA.
Total from 7 cycles: 7 FADH₂ + 7 NADH + 8 acetyl-CoA.
ATP calculation (modern values):
- 7 FADH₂ × 1.5 = 10.5 ATP
- 7 NADH × 2.5 = 17.5 ATP
- 8 Acetyl-CoA × 10 ATP (Krebs + ETC) = 80 ATP
- Total = 108 − 2 (activation) = 106 ATP net (approximately 129 by older calculation).
Q2. Describe the urea cycle briefly.
Answer
Location: liver (mitochondria + cytoplasm).
1. Carbamoyl phosphate synthesis (mitochondria): NH₃ + CO₂ + 2ATP → carbamoyl phosphate (CPS-I enzyme).
2. Citrulline formation (mitochondria): Carbamoyl phosphate + Ornithine → Citrulline.
3. Citrulline exits mitochondria → cytoplasm.
4. Argininosuccinate formation (cytoplasm): Citrulline + Aspartate → Argininosuccinate.
5. Arginine formation: Argininosuccinate → Arginine + Fumarate.
6. Urea release: Arginine + H₂O → Urea + Ornithine (arginase).
7. Ornithine returns to mitochondria → cycle repeats.
Net: 2 NH₃ + CO₂ + 3ATP → Urea + 2ADP + AMP + 3Pi + H₂O.
Exam Checklist
- Beta-oxidation: mitochondria; each cycle: 1 acetyl-CoA + 1 NADH + 1 FADH₂.
- Palmitate (C16): 7 cycles → 8 acetyl-CoA + 7 NADH + 7 FADH₂.
- Ketone bodies: liver → acetoacetate, β-hydroxybutyrate, acetone; fuel during fasting.
- Transamination: amino group transfer; PLP coenzyme; ALT and AST enzymes.
- Urea cycle: liver; NH₃ → urea; ornithine catalyst.
- Fatty acid synthesis: cytoplasm; acetyl-CoA → palmitate (ACC is rate-limiting).
