Give me question no 7 viva answer and prepare questions also from this topic
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Topic 7: Metabolism - Viva Answer + Practice Questions
VIVA ANSWER (Topic 7)
1. Definition of Metabolism
Metabolism is the sum total of all chemical reactions occurring in a living organism to maintain life. It includes both anabolism (building up) and catabolism (breaking down).
| Term | Definition | Example |
|---|---|---|
| Anabolism | Biosynthetic reactions that build complex molecules from simpler ones; energy-consuming (endergonic) | Synthesis of proteins, fatty acids, glycogen |
| Catabolism | Degradative reactions that break down complex molecules into simpler ones; energy-releasing (exergonic) | Glycolysis, beta-oxidation, proteolysis |
2. Overview of Integration of Major Metabolic Pathways
All three major fuels - glucose, TAGs (triacylglycerols), and amino acids - converge at acetyl-CoA and the TCA cycle:
- Glucose → (Glycolysis) → Pyruvate → (PDH complex) → Acetyl-CoA → TCA cycle
- TAGs → Glycerol + Fatty acids → (Beta-oxidation) → Acetyl-CoA → TCA cycle
- Amino acids → Transamination/deamination → Carbon skeletons → enter as Pyruvate, Acetyl-CoA, or TCA cycle intermediates (anaplerotic reactions)
The TCA cycle intermediates also serve as precursors for biosynthesis: citrate → fatty acid synthesis; oxaloacetate → gluconeogenesis; alpha-ketoglutarate → GABA (in brain); succinyl-CoA → heme synthesis.
(Basic Medical Biochemistry - A Clinical Approach, 6e)
3. Glycolysis
Glycolysis is the 10-step cytoplasmic pathway that converts 1 molecule of glucose (6C) into 2 molecules of pyruvate (3C).
Net yield:
- 2 ATP (net; 2 used, 4 produced)
- 2 NADH
- 2 Pyruvate
Key irreversible (regulated) steps:
- Glucose → Glucose-6-phosphate (by Hexokinase/Glucokinase) - uses 1 ATP
- Fructose-6-P → Fructose-1,6-bisphosphate (by PFK-1) - rate-limiting step, uses 1 ATP
- PEP → Pyruvate (by Pyruvate Kinase) - produces 1 ATP (x2)
4. Regulation of Glycolysis
Glycolysis is primarily regulated to maintain ATP homeostasis. The three key regulatory enzymes are:
A. Hexokinase (HK) / Glucokinase:
- HK (in most tissues): low Km, inhibited by glucose-6-phosphate (product inhibition)
- Glucokinase (liver): high Km, NOT inhibited by glucose-6-P → allows liver to continue glycolysis for anabolic synthesis (fatty acids, glycogen) even when energy is sufficient
B. Phosphofructokinase-1 (PFK-1) - Rate-Limiting Enzyme:
- Activated by: AMP, ADP, Fructose-2,6-bisphosphate (most potent activator in liver)
- Inhibited by: ATP (high levels = energy sufficiency), Citrate (signals TCA cycle is full)
- Has 6 binding sites: 2 catalytic + 4 allosteric
C. Pyruvate Kinase:
- Activated by: Fructose-1,6-bisphosphate (feed-forward activation)
- Inhibited by: ATP, alanine, acetyl-CoA
- Liver isoenzyme (L-type) is also inhibited by glucagon via phosphorylation
AMP as a key signal:
When ATP is consumed rapidly, the adenylate kinase reaction (2 ADP → AMP + ATP) causes AMP to rise manyfold (up to 300%) even with only 20% drop in ATP. AMP activates glycolysis, glycogenolysis, and fatty acid oxidation to restore ATP.
5. Aerobic vs. Anaerobic Glycolysis
| Feature | Aerobic Glycolysis | Anaerobic Glycolysis |
|---|---|---|
| O₂ requirement | Required | Not required |
| End product | Pyruvate → Acetyl-CoA → CO₂ + H₂O | Pyruvate → Lactate |
| ATP yield per glucose | 30-32 ATP | 2 ATP |
| NADH fate | Oxidized via shuttle → ETC → ATP | Oxidized by reducing pyruvate to lactate |
| Shuttle used | Glycerol-3-P shuttle (1.5 ATP/NADH) or Malate-aspartate shuttle (2.5 ATP/NADH) | None |
| Rate needed | Normal | ~15x faster than aerobic to produce same ATP |
| Pyruvate fate | Enters mitochondria via PDH complex | Converted to lactate by LDH |
| Clinical relevance | Normal aerobic tissues | RBCs (no mitochondria), fast-twitch muscle, hypoxia, cancer (Warburg effect) |
Energy from complete aerobic oxidation:
- Glycolysis: 2 ATP + 2 NADH (→ 3-5 ATP)
- PDH: 2 NADH per glucose (→ 5 ATP)
- TCA cycle: 6 NADH + 2 FADH₂ + 2 GTP per glucose (→ ~20 ATP)
- Total: ~30-32 ATP per glucose
(Basic Medical Biochemistry - A Clinical Approach, 6e, p.876)
PRACTICE VIVA QUESTIONS - Topic 7: Metabolism
Basic/Definition Level
Q1. What is metabolism? Distinguish between anabolism and catabolism with two examples of each.
Q2. What is the significance of ATP in metabolic reactions? How does the ATP/ADP ratio regulate metabolic pathways?
Q3. What is the net gain of ATP in glycolysis? Name the steps where ATP is consumed and where it is produced.
Glycolysis Pathway
Q4. What is glycolysis? Where does it occur in the cell? List all 10 steps briefly.
Q5. Name the three irreversible steps of glycolysis and the enzymes catalyzing them. Why are these steps important?
Q6. What is the role of NAD⁺ in glycolysis? What happens if NAD⁺ is not regenerated?
Q7. What is the bisphosphoglycerate shunt? What is the significance of 2,3-BPG?
Regulation
Q8. What is the rate-limiting enzyme of glycolysis? How is it regulated allosterically?
Q9. What is the difference between hexokinase and glucokinase? What is the clinical/physiological significance of this difference?
Q10. How does fructose-2,6-bisphosphate regulate glycolysis? What enzyme produces it?
Q11. How does AMP act as a metabolic sensor to activate glycolysis? Explain the adenylate kinase reaction.
Q12. What is feed-forward activation? Give an example from glycolysis.
Aerobic vs. Anaerobic
Q13. Compare aerobic and anaerobic glycolysis in terms of ATP yield, oxygen requirement, and end products.
Q14. Why is anaerobic glycolysis needed even when oxygen is available in some tissues (e.g., RBCs)?
Q15. What is the Cori cycle? How does it link glycolysis in muscle with gluconeogenesis in the liver?
Q16. What are the two shuttle systems that transfer cytosolic NADH into the mitochondria? How much ATP does each generate?
Q17. What is the Warburg effect? Name the cancers where it is clinically relevant.
Integration of Pathways
Q18. What is the central molecule connecting glucose, fatty acid, and amino acid metabolism?
Q19. How do amino acids enter the TCA cycle? Give two examples.
Q20. What is an anaplerotic reaction? Give an example with clinical importance.
Q21. What happens to pyruvate under (a) aerobic conditions, (b) anaerobic conditions, (c) when acetyl-CoA is abundant?
Q22. How is the pyruvate dehydrogenase complex (PDH) regulated? What activates and inhibits it?
Clinical Application
Q23. In a patient with thiamine (Vitamin B1) deficiency, which metabolic step is blocked? What are the clinical consequences?
Q24. Why do cancer cells preferentially use glycolysis even in the presence of oxygen? (Warburg Effect)
Q25. What is lactic acidosis? How does it result from anaerobic glycolysis?
Key Tip for Viva: The examiner often starts with "What is glycolysis?" and then moves to regulation. Always mention PFK-1 as the rate-limiting enzyme, ATP/AMP as regulators, and be able to contrast aerobic vs. anaerobic ATP yield (2 vs. 30-32).
You have given long answers give short viva answer so that i can easily memorize
Topic 7: Metabolism - Short Viva Answers 🧠
CORE DEFINITIONS
Q: What is Metabolism?
Sum of all chemical reactions in the body to maintain life. Includes anabolism + catabolism.
Q: What is Anabolism?
Building complex molecules from simple ones. Energy-consuming. Ex: protein synthesis, glycogen synthesis.
Q: What is Catabolism?
Breaking down complex molecules to simple ones. Energy-releasing. Ex: glycolysis, beta-oxidation.
GLYCOLYSIS
Q: What is Glycolysis?
10-step cytoplasmic pathway. Converts 1 glucose → 2 pyruvate. Net yield = 2 ATP + 2 NADH.
Q: Where does glycolysis occur?
Cytoplasm (cytosol).
Q: What are the 3 irreversible steps?
- Glucose → G-6-P → enzyme: Hexokinase
- F-6-P → F-1,6-bisP → enzyme: PFK-1 (rate-limiting)
- PEP → Pyruvate → enzyme: Pyruvate kinase
Q: What is the rate-limiting enzyme of glycolysis?
Phosphofructokinase-1 (PFK-1)
REGULATION OF GLYCOLYSIS
Q: How is PFK-1 regulated?
- Activated by: AMP, ADP, Fructose-2,6-bisphosphate
- Inhibited by: ATP, Citrate
Q: Difference between Hexokinase and Glucokinase?
| Hexokinase | Glucokinase | |
|---|---|---|
| Location | All tissues | Liver only |
| Km | Low (high affinity) | High (low affinity) |
| Inhibited by G-6-P? | Yes | No |
Q: How does AMP regulate glycolysis?
When ATP falls → ADP rises → adenylate kinase reaction (2ADP → AMP + ATP) → AMP rises 300% → activates PFK-1 → speeds up glycolysis.
Q: What is feed-forward activation in glycolysis?
F-1,6-bisP (product of PFK-1) activates Pyruvate kinase downstream.
AEROBIC vs ANAEROBIC GLYCOLYSIS
| Feature | Aerobic | Anaerobic |
|---|---|---|
| O₂ | Required | Not needed |
| End product | CO₂ + H₂O | Lactate |
| ATP yield | 30-32 ATP | 2 ATP |
| NADH fate | → ETC → ATP | Reduces pyruvate → lactate |
| Site | Mitochondria + cytoplasm | Cytoplasm only |
| Example | Normal cells | RBCs, exercising muscle, hypoxia |
Q: Why only 2 ATP in anaerobic?
NADH cannot enter ETC (no O₂), so it's used to convert pyruvate → lactate. No extra ATP generated.
Q: How much ATP from complete glucose oxidation?
- Glycolysis: 2 ATP
- PDH + TCA: ~28 ATP
- Total: 30-32 ATP
INTEGRATION OF PATHWAYS
Q: What is the central connecting molecule?
Acetyl-CoA - junction of glucose, fatty acid, and amino acid metabolism.
Q: How do the 3 fuels connect?
- Glucose → Pyruvate → Acetyl-CoA → TCA
- Fatty acids → Beta-oxidation → Acetyl-CoA → TCA
- Amino acids → carbon skeletons → Pyruvate / Acetyl-CoA / TCA intermediates
Q: What is an anaplerotic reaction?
Reactions that replenish TCA cycle intermediates. Ex: Pyruvate → Oxaloacetate (by pyruvate carboxylase).
Q: What is the Cori cycle?
Lactate from muscle → liver → converted back to glucose (gluconeogenesis) → back to muscle. Costs 6 ATP.
CLINICAL QUICK HITS
Q: Thiamine (B1) deficiency blocks which step?
Pyruvate Dehydrogenase Complex (PDH) - pyruvate cannot enter TCA. Pyruvate and lactate accumulate → lactic acidosis.
Q: What is the Warburg effect?
Cancer cells use glycolysis even in presence of O₂ (aerobic glycolysis). Produces lactate. Basis of PET scan (↑ glucose uptake).
Q: What is lactic acidosis?
Excess lactate accumulation due to anaerobic glycolysis. Seen in hypoxia, shock, sepsis, metformin overdose.
Q: Why do RBCs use only anaerobic glycolysis?
RBCs have no mitochondria → cannot do aerobic respiration → rely entirely on anaerobic glycolysis for ATP.
Memory tip: For regulation, remember "AMP ON, ATP OFF" for PFK-1. For ATP yield, remember "2 anaerobic, 32 aerobic".
You have not given the overview of integration of major metabolism pathways of glucose, fats and amino acids.
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I now have all the content needed. Here is the short, memorizable viva answer:
Overview of Integration of Major Metabolic Pathways
The Central Concept (One Line Answer)
All three fuels - Glucose, Fats (TAGs), and Amino Acids - are broken down to a common molecule: ACETYL-CoA, which enters the TCA cycle for final oxidation to CO₂ + H₂O + ATP.
DIAGRAM (Easy to Remember)
GLUCOSE FATS (TAGs) AMINO ACIDS
↓ ↓ ↓
Glycolysis Glycerol + FA Transamination/
↓ ↓ Deamination
Pyruvate Beta-oxidation ↓
↓ ↓ Carbon skeletons
↓←─────── ACETYL-CoA ──────────────→↓
↓
TCA CYCLE
↓
NADH + FADH₂ → ETC
↓
30-32 ATP
SHORT VIVA ANSWERS
Q: What is the common molecule where all 3 fuels converge?
Acetyl-CoA
Q: How does Glucose enter the common pathway?
Glucose → (Glycolysis) → Pyruvate → (PDH complex) → Acetyl-CoA
Q: How do Fats (TAGs) enter the common pathway?
TAGs → Glycerol + Fatty acids
- Glycerol → DHAP → enters glycolysis
- Fatty acids → (Beta-oxidation in mitochondria) → Acetyl-CoA
Q: How do Amino Acids enter the common pathway?
Amino acids → remove NH₃ (transamination/deamination) → carbon skeleton enters as:
- Pyruvate → Acetyl-CoA (e.g., Alanine, Serine, Cysteine)
- Acetyl-CoA directly (e.g., Leucine, Lysine - ketogenic AAs)
- TCA intermediates directly:
- Oxaloacetate ← Aspartate
- Alpha-ketoglutarate ← Glutamate
- Succinyl-CoA ← Methionine, Valine, Isoleucine
- Fumarate ← Phenylalanine, Tyrosine
Q: What is the final common pathway for all fuels?
TCA cycle (in mitochondrial matrix)
Q: Where is energy finally captured?
TCA → NADH + FADH₂ → Electron Transport Chain (ETC) → ATP (via oxidative phosphorylation)
INTERCONVERSION POINTS (Important for Viva!)
| Conversion | Possible? |
|---|---|
| Glucose → Fatty acids | ✅ Yes (via Acetyl-CoA) |
| Fatty acids → Glucose | ❌ No (Acetyl-CoA cannot make pyruvate) |
| Amino acids → Glucose | ✅ Yes (glucogenic AAs) |
| Glucose → Amino acids | ✅ Yes (via carbon skeletons + NH₃) |
| Amino acids → Fats | ✅ Yes (via Acetyl-CoA) |
Q: Why can't fats convert to glucose?
Because Acetyl-CoA cannot be converted back to pyruvate - the PDH reaction is irreversible.
3 PHASES OF CELLULAR RESPIRATION (Quick Answer)
| Phase | What Happens |
|---|---|
| Phase 1 | Fuels → Acetyl-CoA → TCA → NADH + FADH₂ |
| Phase 2 | NADH/FADH₂ → ETC → proton gradient (Δp) → ATP synthase → ATP |
| Phase 3 | ATP used for muscle contraction, transport, biosynthesis |
MEMORY TRICK
"All Roads Lead to Acetyl-CoA, and Acetyl-CoA leads to TCA"
- Glucose: Glycolysis → Pyruvate → PDH → Acetyl-CoA
- Fats: Beta-oxidation → Acetyl-CoA
- Amino acids: Deamination → Carbon skeleton → Acetyl-CoA or TCA directly
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