Biochemistry Glossary: Citric Acid Cycle

The CITRIC ACID CYCLE (aka Krebs cycle and tricarboxylic acid cycle) occurs under aerobic conditions (has many exits and entry points) and occurs in the mitochondrial matrix (pyruvate transported from cytosol). The first intermediate is Acetyl CoA (two carbon molecule). There are 8 more intermediates to complete the cycle, often remembered by the mnemonic: Oh, Can I Keep Studying Science For Med-school?

ACETYL CoA

Acetyl CoA is a 2-carbon molecule. This molecule can come from carbohydrates (via pyruvate), fatty acids and amino acids. It is produced through the Pyruvate dehydrogenase complex: Pyruvate + NAD + --> Acetyl CoA + CO2 + NADH.

Reactions and Intermediates

CITRATE AND ISOCITRATE

These are 6-carbon molecules. The cycle progresses when Oxaloacetate (4 carbons) + Acetyl CoA (2 carbons) = Citrate. This Citrate is then isomerized to form isocitrate.

ALPHA-KETOGLUTARATE AND SUCCINYL CoA

Alpha-ketoglutarate is a 5-carbon molecule where 1 carbon dioxide and 1 NADH released in its production. Similarly, Succinyl CoA is a 4-carbon molecule where 1 carbon dioxide and 1 NADH released in its production.

SUCCINATE, FUMARATE, AND MALATE

Succinate is a 4-carbon molecule that Lost CoA and 1 ATP produced via substrate level phosphorylation. Next, Fumarate is a 4-carbon molecule where FADH2 released in its production (F for FADH2 and Fumarate). Malate to oxaloacetate is the last reaction in the cycle. Both are 4-carbon molecules, and the last NADH released in the cycle.

TOTAL OUTPUT

• Pyruvate decarboxylation: 1 NADH and 1 carbon dioxide (x2 per glucose).
• Per turn: 2 carbon dioxide molecules, 3 NADH and 2 FADH2 (2 turns per glucose).
• Electron Transport: NADH and FADH2 deliver electrons to electron transport chain on inner mitochondrial membrane.