The TCA Cycle

main goal of TCA cycle

oxidize acetyl groups to release CO2, transfer electrons to NAD+ and FAD, and make ATP

functional phases of TCA cycle

1. entry: acetyl CoA condenses with oxaloacetate to form citrate

2. oxidation: carbon is oxidized, releasing CO2 and generating NADH

3. ATP generation: substrate level phosphorylation to make GTP (later converted to ATP)

4. oxaloacetate regeneration: sustains the cycle

net yield per glucose (2 turns of cycle)

6 NADH, 2 FADH2, 2 ATP, 4 CO2

step 1 and the enzyme that catalyzes it

acetyl CoA (2C) + oxaloacetate (4C) → citrate (6C) + CoA (condensation); citrate synthase

step 3 and the enzyme that catalyzes it

isocitrate + NAD+ → α-ketoglutarate + NADH + CO2 (redox/oxidative decarboxylation); isocitrate dehydrogenase

step 4 and the enzyme that catalyzes it

α-ketoglutarate + NAD+ + CoA → succinyl CoA + NADH + CO2 (redox/oxidative decarboxylation); α-ketoglutarate dehydrogenase

regulation of citrate synthase

inhibited by high energy products (ATP and NADH)

regulation of isocitrate dehydrogenase

inhibited by ATP and NADH (products)

activated by ADP and calcium

regulation of α-ketoglutarate dehydrogenase

inhibited by TCA products (succinyl CoA, ATP/GTP, NADH)

how is the TCA cycle amphibolic

the steps of the cycle, which involve oxidation, are catabolic, but the intermediates act as substrates in other anabolic pathways