• Coenzymes can be classified into two types based on how they

interact with the apoenzyme (Figure 7.1). Coenzymes of one

type—often called cosubstrates— are actually substrates in

enzyme-catalyzed reactions. A cosubstrate is altered in the

course of the reaction and dissociates from the active site.

• The second type of coenzyme is called a prosthetic group. A

prosthetic group remains bound to the enzyme during the

course of the reaction. In some cases, the prosthetic group is

covalently attached to its apoenzyme, while in other cases it is

tightly bound to the active site by many weak interactions.

• The coenzymes flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) are derived from riboflavin, or vitamin B2.

• Thiamine (or vitamin B1) contains a pyrimidine ring and a positively charged thiazolium ring.

• The B6 family of water-soluble vitamins consists of three closely related molecules that differ only in the state of oxidation or amination of the carbon bound to position 4 of the pyridine ring.

• The coenzyme lipoamide is the protein-bound form of lipoic acid. Although lipoic acid is often described as a B vitamin, animals appear to be able to synthesize it.

• Lipoamide is believed to function as a swinging arm that carries acyl groups between active sites in multienzyme complexes.

• The structures of the four lipid vitamins (A, D, E, and K) contain rings and long aliphatic side chains.

• Vitamin A• Vitamin A, or retinol, is a 20-carbon lipid molecule obtained in the

diet either directly or indirectly from β-carotene. Carrots and other yellow vegetables are rich in β-carotene, a 40-carbon plant lipid whose enzymatic oxidative cleavage yields vitamin A (Figure 7.27).

• Vitamin K (phylloquinone) is a lipid vitamin from plants that is required for the synthesis of some of the proteins involved in blood coagulation. It is a coenzyme for a mammalian carboxylase that catalyzes the conversion of specific glutamate residues to γ-carboxyglutamate residues. The reduced (hydroquinone) form of vitamin K participates in the carboxylation as a reducing agent. When calcium binds to the γ-carboxyglutamate residues of the coagulation proteins, the proteins adhere to platelet surfaces where many steps of the coagulation process take place.

• Ubiquinone—also called coenzyme Q and therefore abbreviated Q—is a lipidsoluble coenzyme synthesized by almost all species. Ubiquinone is a benzoquinone with four substituents, one of which is a long hydrophobic chain.

• This chain, which consists of 6 to 10 isoprenoid units, allows ubiquinone to dissolve in lipid membranes. In the membrane, ubiquinone transports electrons between membrane-embedded enzyme complexes.

• Cytochromes are heme-containing protein coenzymes whose Fe(III) atoms undergo reversible one-electron reduction. Cytochromes are classified as a, b, and c on the basis of

• their visible absorption spectra.