bio chem pdh and tca

Why the PDH step is described as the "bridge" and is irreversible

It commits carbon irreversibly from glycolysis to oxidation; pyruvate → acetyl-CoA + CO₂ cannot be reversed, so fatty acids/acetyl-CoA can't make net glucose

Five cofactors of PDH (mnemonic "Tender Loving Care For Nancy")

TPP, Lipoamide, CoA, FAD, NAD⁺

Why the TCA cycle is called catalytic rather than consumptive

OAA is regenerated each turn, so one OAA can oxidise unlimited acetyl-CoA. Intermediates are catalysts, not consumed

Per turn, the TCA cycle yields

3 NADH, 1 FADH₂, 1 GTP, 2 CO₂ (and per glucose, ×2)

The 3 regulated enzymes of the TCA cycle

Citrate synthase, isocitrate dehydrogenase (main), α-ketoglutarate dehydrogenase

Why the two CO₂ released per turn are NOT the carbons that just entered as acetyl-CoA

They derive from the OAA moiety of citrate; the acetyl carbons leave in later turns. A classic exam trap.

The one TCA enzyme embedded in the inner membrane

Succinate dehydrogenase (= Complex II of the ETC) — the physical link between the cycle and the chain