A coenzyme is defined as an organic molecule that binds to the active sites of certain enzymes to assist in the catalysis of a reaction. More specifically, coenzymes can function as intermediate carriers of electrons during these reactions or be transferred between enzymes as functional groups.
For example, during the conversion of pyruvate to acetyl coenzyme A (CoA), several coenzymes including free CoA, thiamine pyrophosphate (TPP), lipoic acid (LA), flavin adenine dinucleotide (FAD), two cellular redox enzymes including oxidized nicotinamide adenine dinucleotide (NAD) and reduced nicotinamide adenine dinucleotide (NADH) are required.
Coenzymes, which are often vitamins or derivatives of vitamins, therefore play a crucial role in the regulation of most enzyme activities. In addition to some of the aforementioned coenzymes that are involved in the generation of the energy molecule adenosine triphosphate (ATP), several other coenzymes are considered to be fundamental to the existence of all living cells.
These include two additional redox coenzymes of oxidized nicotinamide adenine dinucleotide phosphate (NADP+) and its reduced counterpart of NADPH, as well as other energy coenzymes like adenosine diphosphate (ADP) and adenosine monophosphate (AMP).
Several coenzymes also function as antioxidants to eliminate reactive oxygen species (ROS), some of which include oxidized glutathione (GSSG) and reduced glutathione (GSH).
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