Kreb cycle
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Here is a thorough breakdown of the Krebs cycle, sourced primarily from Harper's Illustrated Biochemistry (32nd Ed).
The Krebs Cycle (Citric Acid Cycle / TCA Cycle)
Overview
The Krebs cycle is a cyclic sequence of 8 enzyme-mediated reactions occurring in the mitochondrial matrix. It is the central hub of aerobic metabolism, oxidizing acetyl-CoA (derived from carbohydrates, fats, and amino acids) to CO2 while generating reducing equivalents (NADH, FADH2) that drive ATP synthesis via the respiratory chain.
Entry into the Cycle: Acetyl-CoA
Before entering the cycle, pyruvate (from glycolysis) is converted to acetyl-CoA by the pyruvate dehydrogenase complex (PDC) in the mitochondrial matrix. This reaction requires:
- Thiamin diphosphate (B1), lipoate, NAD+, FAD, CoA
The 8 Steps
Here is the detailed diagram from Harper's showing all enzymes, intermediates, and inhibitors:
| Step | Substrate → Product | Enzyme | Key Output |
|---|---|---|---|
| 1 | Oxaloacetate (C4) + Acetyl-CoA (C2) → Citrate (C6) | Citrate synthase | Exothermic; CoA released |
| 2 | Citrate → Cis-aconitate → Isocitrate | Aconitase (requires Fe2+) | Dehydration then rehydration |
| 3 | Isocitrate → Oxalosuccinate → α-Ketoglutarate (C5) + CO2 | Isocitrate dehydrogenase (NAD+, Mn2+) | 1st NADH, 1st CO2 |
| 4 | α-Ketoglutarate → Succinyl-CoA (C4) + CO2 | α-Ketoglutarate dehydrogenase complex | 2nd NADH, 2nd CO2 |
| 5 | Succinyl-CoA → Succinate | Succinate thiokinase (succinyl-CoA synthetase, Mg2+) | 1 ATP or GTP (substrate-level phosphorylation) |
| 6 | Succinate → Fumarate | Succinate dehydrogenase (FAD, Fe-S protein) | 1 FADH2 |
| 7 | Fumarate + H2O → L-Malate | Fumarase (fumarate hydratase) | - |
| 8 | L-Malate → Oxaloacetate | Malate dehydrogenase (NAD+) | 3rd NADH |
Products per Turn
| Product | Number | ATP equivalents |
|---|---|---|
| NADH | 3 | ~2.5 each = 7.5 ATP |
| FADH2 | 1 | ~1.5 ATP |
| GTP/ATP (substrate level) | 1 | 1 ATP |
| CO2 | 2 | - |
| Total | ~10 ATP |
Regulation (3 Key Points)
- Citrate synthase - inhibited by ATP, NADH, succinyl-CoA, citrate; activated by ADP
- Isocitrate dehydrogenase - inhibited by ATP and NADH; activated by ADP and Ca2+
- α-Ketoglutarate dehydrogenase - inhibited by succinyl-CoA, NADH; activated by Ca2+
High [NADH] and [ATP] signal energy sufficiency and slow the cycle. High [ADP] and [Ca2+] (during muscle contraction) accelerate it.
Cofactors and B Vitamins
The cycle depends on 4 B vitamins:
- Thiamin (B1) - thiamin diphosphate, cofactor for α-ketoglutarate dehydrogenase
- Riboflavin (B2) - as FAD, cofactor for succinate dehydrogenase
- Niacin (B3) - as NAD+, electron acceptor for isocitrate DH, α-ketoglutarate DH, and malate DH
- Pantothenic acid (B5) - as CoA, esterified to form acetyl-CoA and succinyl-CoA
Key Inhibitors
| Inhibitor | Target | Effect |
|---|---|---|
| Fluoroacetate | Aconitase (via fluorocitrate) | Citrate accumulates |
| Malonate | Succinate dehydrogenase | Competitive inhibitor (structural analogue of succinate) |
| Arsenite | α-Ketoglutarate dehydrogenase | α-Ketoglutarate accumulates |
Amphibolic Role
The cycle is described as amphibolic - it serves both catabolic and anabolic roles:
- Gluconeogenesis: oxaloacetate exits as phosphoenolpyruvate
- Amino acid synthesis: intermediates undergo transamination (e.g., α-ketoglutarate → glutamate)
- Fatty acid synthesis: citrate is exported to the cytosol as an acetyl-CoA source
- Heme synthesis: succinyl-CoA is a precursor for porphyrin rings
Anaplerosis (carbon entering the cycle, e.g., pyruvate → oxaloacetate via pyruvate carboxylase) must equal cataplerosis (carbon leaving, e.g., oxaloacetate → PEP) to sustain the cycle.
Location
All enzymes are in the mitochondrial matrix except:
- Succinate dehydrogenase and aconitase - bound to the inner mitochondrial membrane
Sources: Harper's Illustrated Biochemistry, 32nd Ed., p. 168-171; Guyton and Hall Textbook of Medical Physiology
Formation and utilization of ketone bodies.
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I now have comprehensive information from the textbooks. Here is the complete answer.
Formation and Utilization of Ketone Bodies
The Three Ketone Bodies
| Compound | Nature | Notes |
|---|---|---|
| Acetoacetate | Primary ketone body | Main product released into blood |
| D-β-Hydroxybutyrate | Reduced form | Most abundant in blood (ratio ~3:1 to acetoacetate); not technically a ketone (has -OH group, not C=O) |
| Acetone | Volatile byproduct | Formed by spontaneous decarboxylation of acetoacetate; expired via lungs |
PART 1: Formation (Ketogenesis) - Occurs ONLY in Liver
Location
Mitochondrial matrix of hepatocytes only. All enzymes are present at sufficient levels only in liver.
Precursor
Acetyl-CoA, generated from:
- β-oxidation of fatty acids (major source)
- Catabolism of ketogenic amino acids (leucine, isoleucine, lysine, tryptophan, phenylalanine, tyrosine)
- Minor contribution from pyruvate oxidation
Why does ketogenesis occur?
When acetyl-CoA production overwhelms the TCA cycle capacity (due to low oxaloacetate - which is being diverted to gluconeogenesis during fasting), excess acetyl-CoA is channeled into ketone body synthesis.
Steps of Ketogenesis (HMG-CoA Pathway)
| Step | Reaction | Enzyme |
|---|---|---|
| 1 | 2 Acetyl-CoA → Acetoacetyl-CoA + CoA | Thiolase (reversible; favors acetyl-CoA normally, reverses when acetyl-CoA is high) |
| 2 | Acetoacetyl-CoA + Acetyl-CoA → HMG-CoA + CoA | HMG-CoA synthase (rate-limiting step; mitochondrial isoform - distinct from cytosolic HMG-CoA synthase used in cholesterol synthesis) |
| 3 | HMG-CoA → Acetoacetate + Acetyl-CoA | HMG-CoA lyase |
| 4a | Acetoacetate + NADH → D-β-Hydroxybutyrate + NAD+ | D-β-Hydroxybutyrate dehydrogenase (reversible; ratio determined by mitochondrial NADH/NAD+) |
| 4b | Acetoacetate → Acetone + CO2 | Spontaneous (non-enzymatic decarboxylation) |
The ratio of β-hydroxybutyrate to acetoacetate in blood (~3:1) is governed by the mitochondrial NADH/NAD+ ratio. During severe fasting/DKA, NADH is high, so β-hydroxybutyrate predominates.
PART 2: Utilization (Ketolysis) - Occurs in Extrahepatic Tissues
Tissues that USE ketone bodies
- Skeletal muscle, cardiac muscle, brain (after 2-3 days of starvation), kidney cortex, intestinal mucosa
Why the LIVER cannot use its own ketone bodies
The liver lacks succinyl-CoA:acetoacetate CoA transferase (thiotransferase) in sufficient quantities - the key enzyme needed to reactivate acetoacetate. This ensures ketone bodies are exported to peripheral tissues.
Steps of Ketolysis
| Step | Reaction | Enzyme |
|---|---|---|
| 1 | D-β-Hydroxybutyrate + NAD+ → Acetoacetate + NADH | D-β-Hydroxybutyrate dehydrogenase |
| 2 | Acetoacetate + Succinyl-CoA → Acetoacetyl-CoA + Succinate | Succinyl-CoA:acetoacetate CoA transferase (thiotransferase) - KEY STEP; costs 1 GTP equivalent |
| 3 | Acetoacetyl-CoA + CoA → 2 Acetyl-CoA | Thiolase |
| 4 | Acetyl-CoA → TCA cycle → ATP |
Energy Yield
| Ketone Body | Net ATP |
|---|---|
| Acetoacetate | ~20 ATP (2 × acetyl-CoA via TCA) minus 1 ATP equivalent for activation = ~19 ATP |
| β-Hydroxybutyrate | ~21.5 ATP (1 extra NADH from step 1) |
Regulation of Ketogenesis
The rate-limiting enzyme is HMG-CoA synthase (mitochondrial). Key regulators:
| Factor | Effect on Ketogenesis | Mechanism |
|---|---|---|
| Insulin (fed state) | Inhibits ↓ | Suppresses adipose lipolysis → less fatty acid to liver; promotes malonyl-CoA production → inhibits carnitine palmitoyltransferase-I (CPT-I) → blocks fatty acid entry into mitochondria |
| Glucagon/Fasting | Stimulates ↑ | Activates lipolysis; decreases malonyl-CoA → CPT-I active |
| High fatty acid supply | Stimulates ↑ | More acetyl-CoA delivered |
| Low oxaloacetate | Stimulates ↑ | OAA diverted to gluconeogenesis → TCA cycle slows → acetyl-CoA diverted to ketogenesis |
| High NADH/NAD+ | Shifts toward β-hydroxybutyrate | Favors reduction of acetoacetate |
Kinetics During Fasting
- 3-4 hours post-meal: fatty acids begin rising
- Day 2-3 of fasting: ketone bodies reach levels sufficient to cross blood-brain barrier
- Prolonged starvation: ketone bodies can supply up to 2/3 of brain's energy - sparing muscle protein from gluconeogenesis
Ketosis vs. Diabetic Ketoacidosis (DKA)
| State | Cause | Blood pH | Ketone Level |
|---|---|---|---|
| Physiologic ketosis (fasting) | Starvation; insulin present but low | Normal | Mildly elevated |
| Diabetic ketoacidosis | Absent insulin (Type 1 DM); unrestrained lipolysis + ketogenesis | Acidotic (<7.3) | Severely elevated |
In DKA: absence of insulin → maximal lipolysis + maximal ketogenesis → ketonemia → ketonuria → metabolic acidosis. β-Hydroxybutyrate predominates (may be missed by dipstick tests which detect only acetoacetate/acetone via nitroprusside reaction).
Alternative Fates of Ketone Bodies
- Cytosolic acetyl-CoA source: Acetoacetate can be activated in the cytosol → acetoacetyl-CoA → acetyl-CoA for acetylcholine synthesis (neuronal cells) and cholesterol synthesis
- Ketogenic diet: High fat:carbohydrate (3:1 ratio) diet induces therapeutic ketosis - used in refractory epilepsy in children and pyruvate dehydrogenase deficiency
- Ketogenic amino acids: Leucine, isoleucine, lysine, tryptophan, phenylalanine, tyrosine are catabolized to acetyl-CoA/acetoacetyl-CoA, providing an additional route to ketone bodies
Sources: Basic Medical Biochemistry: A Clinical Approach, 6th Ed., p. 1097-1101; Schwartz's Principles of Surgery, 11th Ed., p. 92
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