Pyruvate Oxidation & The Citric Acid Cycle
NOTE: Glycolysis produces two pyruvate molecules per glucose molecule, so pyruvate oxidation and the citric acid cycle are actually gone through twice for each glucose molecule.
Pyruvate oxidation, which follows glycolysis, is essentially the conversion of pyruvate molecules to carbon dioxide, acetyl coenzyme A, and NADH. The last step of glycolysis produces two pyruvate molecules in the cytosol, which are then brought to the mitochondrial matrix via active transport. There, a multi-enzyme complex cleaves off a –COO- group, which is removed as a molecule of carbon dioxide. The enzyme complex then oxidizes what is left of the pyruvate molecule (a two carbon fragment), turning it into an acetyl group. In these reactions, two electrons and two protons are released; the electrons and one proton are accepted by NAD+, which make it into NADH, while the other proton is simply released as an H+ ion. The acetyl group is attached to coenzyme A, a carrier formed from nucleotides, and the resulting molecule, Acetyl-CoA, is transported to the citric acid cycle, which also occurs in the mitochondrial matrix.
The overall reaction for pyruvate oxidation is:
Pyruvate + CoA + NAD+ → Acetyl-CoA + NADH + H+ + CO2
CITRIC ACID CYCLE (KREBS CYCLE, TRICARBOXYLIC CYCLE)
There are eight steps to the citric acid cycle.
1) The acetyl group from Acetyl-CoA is transferred to the acceptor molecule, oxaloacetate. Coenzyme A can then return to pyruvate oxidation. The resulting molecule is citrate.
2) The citrate is then rearranged into its isomer, isocitrate.
3) Then, isocitrate is oxidized to alpha-ketoglutarate; one of the –COO- groups is removed and released as CO2. The resulting electron reduces NAD+ to NADH, while the resulting proton is released as an H+ ion.
4) Then, the alpha-ketoglutarate is oxidized again; another –COO- group is removed as CO2, and NADH and H+ are also released. The molecule is simultaneously joined to coenzyme A to form Succinyl-CoA.
5) Then, coenzyme A is released, forming succinate. The energy from this release converts GDP to GTP, which then converts ADP to ATP via substrate-level phosphorylation.
6) Succinate is then oxidized to fumarate; the two electrons and two protons produced are transferred to FAD, which becomes FADH2.
7) Fumarate is then converted to malate through the addition of H2O.
8) Malate can then be oxidized to form oxaloacetate; this action, in addition to reducing NAD+ to NADH + H+, completes the cycle.
The overall reaction for the citric acid cycle is:
1 Acetyl-CoA + 3 NAD+ + 1 FAD + 1 ADP + 1 Pi + 2H2O →
2 CO2 + 3 NADH + 1 FADH2 + 1 ATP + 3 H+ + 1 CoA
While the citric acid cycle does produce ATP, its primary function is to produce high energy electrons, carried by 3NADH and FADH2, for the mitochondrial electron transport chain. Also, like glycolysis, the citric acid cycle is regulated to match its rate to the cell’s ATP needs. For instance, the enzyme that catalyzes the synthesis of citrate is inhibited by high levels of ATP (an example of feedback inhibition). In slowing or stopping the cycle when there is not a need for ATP, the cell can conserve its resources.