Malic acid is a four-carbon dicarboxylic acid with one hydroxyl group. Its deprotonated form, malate, is an intermediate of the citric acid cycle.
Oxidation of malate to oxaloacetate couples a change in the carbon skeleton's functional groups to reduction of NAD+. The reaction illustrates why protonation state and oxidation state are separate aspects of a metabolite's identity.[ref]
The citric acid cycle by Hans Adolf Krebs. Table 1, malic and oxaloacetic acids
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