Following glycolysis, pyruvate moves from the cytosol to the matrix of the mitochondria where it reacts with coenzyme A (CoA) to form acetyl CoA. In the process, one molecule of NADH and one molecule of carbondioxide is produced for each pyruvate made. The process is called pyruvate oxidation of pyruvate decarboxylation

Pyruvate decarboxylation reaction

1 Pyruvate + 1 NAD+ + CoA → 1 Acetyl-CoA + NADH + H+ +CO2

Acetyl CoA then reacts with oxaloacetate in the tricarboxylic acid (TCA) cycle (also known as Krebs and citric acid cycle) to produce citrate. A series of enzymatic reactions then follow which result in the production of the following molecules, for each turn of the cycle:

  • 1 ATP
  • 3 NADH
  • 1 FADH2

NADH and FADH2 are molecules that will be used later in the electron transport chain to produce ATP. Remember that two pyruvates are produced in glycolysis, so multiply the numbers above by 2 to get the total number of ATP, NADH, and FADH2 that are formed in the citric acid cycle. The citric acid cycle is shown below.

Citric Acid Cycle. Image source.

For biochemistry exams, you will need to memorize the cycle. Create a mnemonic to help you remember.

Courtney Simons
Courtney Simons is a food science professor. He holds a BS degree in food science and a Ph.D. in cereal science from North Dakota State University.
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