ycle, in which an acetyl unit (the breakdown product of carbohydrates) is introduced into the cycle to be converted eventually into carbon dioxide, water, and chemical energy. Coenzyme A is derived from adenine, ribose, and pantothenic acid (a vitamin of the B complex).
The two flavin coenzymes, riboflavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), occur universally in living organisms and play important roles in biochemical oxidations and reductions. They are usually found tightly bound to certain enzymes (flavoproteins) and are derived from riboflavin (vitamin B2).
Glutathione, a tripeptide consisting of residues of glutamic acid, cysteine, and glycine, is known to act as a coenzyme in a few enzymatic reactions, but its importance may lie in its role as a nonspecific reducing agent within the cell. It is hypothesized that glutathione serves to maintain the biological activity of certain proteins by keeping selected cysteine sidechains in the reduced thiol form, thereby not allowing these residues to oxidize and cross-link with one another to form cystine residues. (Unnecessary cross-links often result in distortions of protein structure.)
Heme, a complicated molecule containing iron in the ferrous state, serves as a coenzyme in a variety of biochemical processes. It forms an essential part of the structure of hemoglobin and participates intimately in the uptake and release of oxygen by this protein. (In this case the use of the word coenzyme may be inappropriate in that often hemoglobin is not considered to be an enzyme, since it does not catalyze a chemical reaction.) Heme is an important part of the cytochromes, enzymes that catalyze the biochemical oxidations and reductions involved in the production of chemical energy in the form of ATP; heme is also associated with the various enzymes that catalyze the cleavage of peroxides.
Lipoic acid seems to be involved in the removal of carboxyl groups from α-keto acids and in the transfer of the remaining acyl groups to various acceptors. Lipoic acid in fact transfers the acetyl group of pyruvic acid to coenzyme A. Like biotin, lipoic acid is commonly found attached to lysine residues within certain enzymes. It was first reported to have been purified and isolated in crystalline form in 1953.
The nicotinamide nucleotides were the first coenzymes to be detected (1904) in extracts of a living organism. Nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) are derived from adenine, ribose, and nicotinic acid or niacin (a vitamin of the B complex) and are important intermediates in the biochemical oxidations and reductions that provide chemical energy within the cell. Both NAD and NADP can be reduced by accepting a hydride ion (H - , a proton with two electrons) from an appropriate donor; the resulting NADH and NADPH can then be oxidized back to their original states by transferring their hydride ions to various acceptors. In this fashion electron pairs (and protons) are shuttled about in the cell from high-energy donors to lower-energy acceptors. As a general rule, NADPH donates its hydride ions to biosynthetic processes, such as the fixing of carbon dioxide to make carbohydrates during the dark reaction of photosynthesis. NADH, on the other hand, donates its hydride ions to systems such as the cytochromes, which eventually donate them to oxygen to make (with the addition of a proton) water, producing chemical energy in the form of ATP as a byproduct; the process is not yet completely understood.
Pyridoxal phosphate is a coenzyme that is essential for many enzymatic reactions, almost all of which are associated with amino acid metabolism. It is, for example, involved in the synthesis of tryptophan, a derivative of pyridoxine (another vitamin of the B complex).
The coenzyme tetrahydrofolic acid is derived in humans from the B-complex vitamin folic acid. This coenzyme and its close relatives participate in the transfer of various carbon fragments from one molecule to another; they are, for instance, involved in the synthes