citric acid cycle

main role of the citric acid cycle

oxidizes acetyl coa → co2

• makes NADH and FADH2

key functions of the citric acid cycle

1. harvest high-energy electrons from carbon fuels

2. hub for metabolic pathways

overall output of citric acid cycle

• 2 CO₂

• 1 ATP

• NADH + FADH₂

first stage of citric acid cycle

makes 2 CO2 molecules by oxidative decarboxylation

citrate synthase

• Acetyl-CoA + oxaloacetate → citrate

• catalyzes the condensation

Aconitase

catalyzes citrate → isocitrate

Isocitrate dehydrogenase

• catalyzes the oxidative decarboxylation of isocitrate

• makes CO2 and NADH

α-Ketoglutarate dehydrogenase

• makes NADH and succinyl coa

• enzyme and reactions are structurally and functionally similar to pyruvate dehydrogenase complex

Succinyl CoA synthetase

cleaves thioester linkage and forms atp

stage 2 of citric acid cycle

regenerates oxaloacetate

and harvests energy-rich electrons

Substrate-level phosphorylation

makes atp

Substrate-level phosphorylation

ATP made directly (no ETC)

Succinate → oxaloacetate

makes FADH2 and nadh

• when oxaloacetate regenerates, the cycle continues

The reaction mechanism of succinyl CoA synthetase

1. Phosphate replaces CoA to create a high-energy molecule called succinyl phosphate.

2. The enzyme's histidine grabs that phosphate, releasing succinateand leaving the enzyme phosphorylated (charged with energy)

3. the phosphohistidine residue then swings over to a bound nucleosidediphosphate.

4. The phosphoryl group is transferred to form the nucleoside triphosphate.

what enzymes catalyze reactions to regenerate oxaloacetate

• succinate dehydrogenase

• fumarase

• malate dehydrogenase

Oxaloacetate is regenerated by the oxidation of Succinate

• FADH2 and NADH are generated.

• Oxaloacetate can condense with another acetyl CoA to initiate another cycle.

atp yield from electrons in the citric acid cycle

• NADH → 2.5 ATP

• FADH₂ → 1.5 ATP

regulation of the citric acid cycle

controlled by ATP and NADH

key enzymes serving as control points in the citric acid cycle

• Isocitrate dehydrogenase

• α-ketoglutarate dehydrogenase

Pyruvate carboxylase replenishes the citric acid cycle

• The rate of the citric acid cycle increases during exercise, oxaloacetate and acetyl CoA needs to be replenished.

• Oxaloacetate is replenished by forming pyruvate.

• Acetyl CoA can be produced from the metabolism of both pyruvate and fatty acids.