Citric Acid Cycle

Yields reduced coenzymes (NADH and QH2) and some ATP (2).
Preparative step is oxidative decarboxylation involving coenzyme A.
Occurs in eucaryotic mitochondrion and procaryotic cytosol.

How does the pyruvate get into the mitochondrion from the cytosol?
Pyruvate passes through channel proteins called porins (can transport molecules < 10,000 daltons) located in outer mitochondrial membrane.

To get from intermembrane space to matrix involves pyruvate translocase (symporter that also moves H+ into matrix).

CONVERSION OF PYRUVATE TO ACETYL COA

Enzyme is pyruvate dehydrogenase complex, composed of three enzymes:
1) pyruvate dehydrogenase
2) dihydrolipoamide acetyltransferase
3) dihydrolipoamide dehydrogenase

Reaction occurs in 5 steps:
1) E1 uses TPP as a prosthetic group and decarboxylates pyruvate --> forms HETPP intermediate
2) E1 then transfers acetyl group to oxidized lipoamide --> acetyllipoamide
3) E2 transfers acetyl group to coenzyme A to form acetyl CoA; dihydrolipoamide becomes reduced
4) E3 reoxidizes lipoamide portion of E2; prosthetic group of E3 (FAD) oxidizes reduced lipoamide --> FADH2
5) NAD+ is reduced by E3-FADH --> E3-FAD + NADH + H+

E2 acts like a crane by swinging substrate between protein complexes in enzyme.