Biochemistry and Molecular Biology - Belyasova N.A. 2002

Structure and Functions of Cellular Components
Cofactors
Acyl Group Carriers

An indispensable cofactor for acyl group transfer in Cells is coenzyme A (CoA), also referred to as coenzyme A. This compound serves as a cofactor for at least 80 known acyl-transferring Enzymes involved in The Tricarboxylic Acid Cycle, fatty acid ß-Oxidation, certain Types of Fermentation, Carbohydrate METABOLISM, and numerous other vital processes.

The coenzyme A molecule contains vitamin B₃ (pantothenic acid) (Fig. 7.8), which is composed of pantoic acid and ß-Alanine. The reactive group of coenzyme A is the sulfhydryl (SH-) group located at the end of a long, relatively flexible chain. Acyl groups are attached to this group via a thioester bond. The resulting derivatives are known as acyl-CoA.

The simplest acyl derivative is acetyl-CoA (Fig. 7.8). This compound is characterized by a high transfer potential for the acetyl group, as evidenced by the large negative value of the Standard Free energy change (∆G^o) for the Hydrolysis of acetyl-CoA:

Class="center">Acetyl-CoA + H₂O ↔ Acetate + CoA + H⁺    ∆G^o = -31.4 kJ/mol

Fig. 7.8. Structure of coenzyme A and its derivatives. The dashed outline highlights the active sulfhydryl group

Coenzyme A is a mobile cofactor that interacts with various enzymes during acyl group transfer. In addition to CoA, cells contain another widespread cofactor that transfers acyl groups: phosphopantetheine (Fig. 7.9). Phosphopantetheine is covalently linked (via a Serine residue) to the acyl carrier protein involved in FATTY ACID Biosynthesis. Thus, this cofactor acts as a prosthetic group of Proteins. Its Functions are analogous to those of CoA.

Bound phosphopantetheine has also been found in the citrate-Cleavage enzyme and in enzymes involved in the synthesis of peptide Antibiotics.

Fig. 7.9. Structure of phosphopantetheine