TCA / Krebs cycle
Eight steps around, two carbons in, two CO₂ out.
Per turn: 3 NADH · 1 FADH₂ · 1 GTP · 2 CO₂
This mnemonic, unpacked word by word →
Runs twice per glucose. Pyruvate oxidation (step 0) is the one-way bridge in from glycolysis. The last three steps (oxidize, hydrate, oxidize) are the same chemistry as β-oxidation, so learning one pattern gets you both.
The one-way door between carbohydrate and everything downstream: irreversible, which is why acetyl-CoA can never become glucose. Five cofactors from four B vitamins, Tender Loving Care For Nancy: TPP, Lipoic acid, CoA, FAD, NAD⁺. Switched off by phosphorylation (PDH kinase, activated by ATP, NADH, acetyl-CoA) and on by PDH phosphatase (Ca²⁺, insulin). Arsenite poisons the lipoic acid arm, and hits α-ketoglutarate dehydrogenase, its twin at step 4, at the same time.
Inhibited by ATP, NADH, succinyl-CoA and its own product citrate.
Moves the hydroxyl so it can be oxidized. Blocked by fluoroacetate.
Rate-limiting. Activated by ADP and Ca²⁺, inhibited by ATP and NADH, so the cycle runs when the cell is spending.
Mechanistically a twin of pyruvate dehydrogenase, with the same five cofactors and the same regulation logic.
The only substrate-level phosphorylation in the cycle. Odd-chain fatty acids enter here, arriving as propionyl-CoA converted to succinyl-CoA (a B₁₂-dependent step).
Complex II of the electron transport chain. It is the only TCA enzyme embedded in the inner membrane, and the only FADH₂ of the cycle.
Hydration across the double bond.
Regenerates oxaloacetate. Strongly unfavorable on its own; pulled forward by citrate synthase consuming the product.