Principles of Biochemistry, Volume 1 - A. Lehninger 1985

Biomolecules
Vitamins and trace elements: their role in enzyme function
Pantothenic acid — a component of coenzyme A

Pantothenic acid (Fig. 10-8) was first isolated from Yeast and Liver extracts in 1938 by Roger Williams (brother of Robert Williams, who determined The Structure of thiamine). Pantothenic acid (derived from the Greek prefix panto-, meaning "everywhere") has been found in all studied PLANT AND ANIMAL Tissues, as well as in microorganisms. However, it was not until twelve years after pantothenic acid was obtained in pure form that Fritz Lipmann and Nathan Kaplan demonstrated its function as a coenzyme. Initially, they discovered a heat-stable factor required to accelerate the ATP-dependent enzymatic Acetylation of alcohols and amines. Subsequently, upon purifying and studying The properties of this factor—named coenzyme A (the coenzyme of acetylation)—they established that it contains tightly bound pantothenic acid. Today we know that coenzyme A has a much broader function: it participates in numerous Enzymatic reactions involving not only acetyl groups, but acyl groups of all kinds. Coenzyme A (-CoA or CoA-SH) acts as an intermediate carrier of acyl groups.

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Fig. 10-8. Pantothenic acid and coenzyme A.

The coenzyme A molecule (Fig. 10-8) contains a reactive thiol (—SH) group, to which acyl groups carried by the coenzyme are covalently linked, forming thioesters. Thioesters are esters of thiols, or thioscohols (general formula R—SH). Figure 10-9 illustrates how the thiol

group of coenzyme A Functions as An acyl group carrier. In the first reaction shown in Fig. 10-9, The oxidative decarboxylation of Pyruvate by the pyruvate dehydrogenase complex yields acetyl-CoA. In the second reaction, the acyl group of acetyl-CoA is transferred to oxaloacetate by citrate synthase, converting it into citrate. This reaction initiates The Citric Acid Cycle—the central pathway for the oxidative degradation of CARBOHYDRATES and Fatty acids under aerobic conditions (Chapter 16).