coupled oxidation-reduction reactions
In biological systems, a coupled oxidation-reduction reaction is one in which one substance is oxidized (loses electrons) while a second is reduced (gains electrons). An example of one such reaction is the conversion of lactate to pyruvate.
The conversion of lactate to pyruvate is enzymatically catalyzed by lactate dehydrogenase. In this reaction lactate loses two electrons (becomes oxidized) and is converted to pyruvate. NAD+ gains two electrons (is reduced) and is converted to NADH. The coupling of oxidation-reduction reactions is often depicted in the following manner.
Both lactate and NAD+ bind to the active site of the enzyme lactate dehydrogenase and both lactate and NAD+ participate in the catalysis reaction. In fact, catalysis could not occur unless the coenzyme NAD+ bound to the active site.