File:BIOL 111 Figure 9.10.png

Junction Reaction

Conversion of pyruvate to acetyl CoA, the junction between glycolysis and the citric acid cycle

Pyruvate is a charged molecule, so in eukaryotic cells it must enter the mitochondrion via active transport, with the help of a transport protein. Next a complex of several enzymes (the pyruvate dehydrogenase complex) catalyzes the three numbered steps, which are described below. The acetyl group of acetyl CoA will enter the citric acid cycle. The CO₂ molecule will diffuse out of the cell.

1. Pyruvate's carboxyl group (—COO^–), which is already fully oxidized and thus has little chemical energy, is removed and given off as a molecule of CO₂. (This is the first step in which CO₂ is released during respiration.)
2. The remaining two-carbon fragment is oxidized, forming a compound named acetate (the ionized form of acetic acid). An enzyme transfers the extracted electrons to NAD⁺, storing energy in the form of NADH.
3. Finally, coenzyme A (CoA), a sulfur-containing compound derived from a B vitamin, is attached to the acetate by an unstable bond (the wavy line) that makes the acetyl group (the attached acetate) very reactive.