CoASH coenzyme

Gly-3-PDH glycerol-3-phosphate dehydrogenase DHAP dihydroxyacetone phosphate CoASH Coenzyme A... 

As noted earlier, coenzymes are frequently altered structurally in the course of an enzymatic reaction. However, they are usually reconverted to their original structure in a subsequent reaction, as opposed to being further metabolized. One turn of the citric acid cycle converts NAD+ into NADH, FAD into FADH2, and acetyl-SCoA into CoASH. Coenzyme A is consumed in the metabolism of pyruvate (see below) but regenerated in the citric acid cycle. Both NADH and FADH2 are reconverted into NAD+ and FAD by the electron transport chain. 

CoASH Coenzyme A FAD Oxidized flavin adenine dinucleotide... 

Figure 19-1. Pathways for the metabolic disposal of phenylalanine. There are two competitive pathways for the disposal of phenylalanine. One pathway involves a transaminase enzyme phenylpyruvate, while the first step in the second pathway requires phenylalanine to be initially converted to tyrosine. Continued metabolism of the phenylpyruvate produced by the first pathway leads to products that cannot be further metabolized, while tyrosine can be converted into citric acid cycle intermediates. Glu, glutamate aKG CoASH, coenzyme A BH4, tetrahydrobiopterin TPP, thiamine pyrophosphate.

TPP thiamine pyrophosphate LipS2 and Lip(SH)2 lipoyl moiety and its reduced form CoASH coenzyme A... 

Coenzyme A (CoASH) and glutathione (GSH) have anionic charges in addition to the thiol group and are readily bound on to a cationic micelle or a cationic polysoap. It was discovered that the nucleophilicity of these coenzymes towards PNPA is markedly enhanced in the presence of cationic... 

Examples of coenzymes vitamin-derived nucleotides for example adenosine phosphates ATP, ADP, AMP nicotinamide derivatives NAD+, NADH, NADP+, NADPH flavin derivatives FAD, FADH2 coenzyme A (abbreviated to CoA, CoASH or CoA-SH). 

Finally, we come to the last of the vitamins that appear on the contents list of my multivitamin pill—pantothenic acid. This water-soluble vitamin serves a single purpose in physiology and biochemistry it is a precursor to a far more complex molecule known as coenzyme A or, simply, CoASH. 

Note that this overall reaction requires three coenzymes that we encountered as metabolites of vitamins in chapter 15 NAD+, derived from lucotiiuc acid or nicotinamide FAD, derived from riboflavin and coenzyme A(CoASH), derived from pantothenic acid. In the overall process, acetyl-SCoA is oxidized to two molecules of carbon dioxide with the release of CoASH. Both NAD+ and FAD are reduced to, respectively, NADH and FADH2. Note that one molecule of guanosine triphosphate, GTP, functionally equivalent to ATP, is generated in the process. 

In this reaction, pyruvic acid is oxidized to carbon dioxide with formation of acetyl-SCoA and NAD+ is reduced to NADH. As noted in chapter 15, this reaction requires the participation of thiamine pyrophosphate as coenzyme. Here too the NADH formed is converted back to NAD+ by the electron transport chain. As noted above, the acetyl-SCoA is consumed by the citric acid cycle and CoASH is regenerated. 

RGURE 13-6 Hydrolysis of acetyl-coenzyme A Acetyl-CoA is a thioester with a large, negative, standard free energy of hydrolysis Thioesters contain a sulfur atom in the position occupied by an oxygen atom in oxygen esters. The complete structure of coenzyme A (CoA, or CoASH) is shown in Rgure 8-41. 

CH3CSCoA + HOCH2CH2N(CH3)3 acetylase> CH3COCH2CH2N(CH3)3 + CoASH Acetyl coenzyme A Choline Acetylcholine Coenzyme A... 

Considerable confusion is possible because of the way in which biochemists use abbreviated names and formulas for the acyl derivatives of coenzyme A. To emphasize the vital —SH group, coenzyme A is usually written as CoASH. However, the acyl derivatives most often are called acetyl CoA and the like, not acetyl SCoA, and you could well get the erroneous impression that the sulfur has somehow disappeared in forming the acyl derivative. We will include the sulfur in formulas such as CH3COSCoA, but use the customary names such as acetyl CoA without including the sulfur. To make clear that CoA does not contain cobalt, CoA is printed in this text in boldface type. 

Several of the B vitamins function as coenzymes or as precursors of coenzymes some of these have been mentioned previously. Nicotinamide adenine dinucleotide (NAD) which, in conjunction with the enzyme alcohol dehydrogenase, oxidizes ethanol to ethanal (Section 15-6C), also is the oxidant in the citric acid cycle (Section 20-10B). The precursor to NAD is the B vitamin, niacin or nicotinic acid (Section 23-2). Riboflavin (vitamin B2) is a precursor of flavin adenine nucleotide FAD, a coenzyme in redox processes rather like NAD (Section 15-6C). Another example of a coenzyme is pyri-doxal (vitamin B6), mentioned in connection with the deamination and decarboxylation of amino acids (Section 25-5C). Yet another is coenzyme A (CoASH), which is essential for metabolism and biosynthesis (Sections 18-8F, 20-10B, and 30-5A). 

Ethanoic acid is activated for biosynthesis by combination with the thiol, coenzyme A (CoASH, Figure 18-7) to give the thioester, ethanoyl (acetyl) coenzyme A (CH3COSC0A). You may recall that the metabolic degradation of fats also involves this coenzyme (Section 18-8F) and it is tempting to assume that fatty acid biosynthesis is simply the reverse of fatty acid metabolism to CH3COSCoA. However, this is not quite the case. In fact, it is a general observation in biochemistry that primary metabolites are synthesized by different routes from those by which they are metabolized (for example, compare the pathways of carbon in photosynthesis and metabolism of carbohydrates, Sections 20-9,10). 

Endogenous pantothenic acid occurs in food primarily in the bound form as a component of coenzyme A (CoA or CoASH), acyl-coenzyme A, and acyl carrier protein (ACP) (185,186). These are the principal vitamers in foods free pantothenic acid (Fig. 9) is much less common. Only the D( + ) or (R) enantiomer of pantothenic acid occurs naturally. 

Structures of the vitamin pantothenic acid (in red) and coenzyme A. The terminal —SH (in blue) is the reactive group in coenzyme A (CoASH). 

The conversion of pyruvate to acetyl-CoA. The reactions are catalyzed by the enzymes of the pyruvate dehydrogenase complex. This complex has three enzymes pyruvate decarboxylase, dihydrolipoyl transacetylase, and dihydrolipoyl dehydrogenase. In addition, five coenzymes are required thiamine pyrophosphate, lipoic acid, CoASH, FAD, and NAD+. Lipoic acid is covalently attached to... 

The following coenzymes contain the AMP moiety NAD+, NADH, NADP+, NADPH, FAD, FADH2, and CoASH. 

Figure 7-2. Reactions of the pyruvate dehydrogenase (PDU) multienzyme complex (PDC). Pyruvate is decarboxylated by the PDH subunit (I ,) in the presence of thiamine pyrophosphate (TPP). The resulting hydroxyethyl-TPP complex reacts with oxidized lipoamide (LipS3), the prosthetic group of dehydrolipoamide transacetylase (Ii2), to form acetyl lipoamide. In turn, this intermediate reacts with coenzyme A (CoASH) to yield acetyl-CoA and reduced lipoamide [Lip(SH)2]. The cycle of reaction is completed when reduced lipoamide is reoxidized by the flavoprotein, dehydrolipoamide dehydrogenase (E3). Finally, the reduced flavoprotein is oxidized by NAD+ and transfers reducing equivalents to the respiratory chain via reduced NADH. PDC is regulated in part by reversible phosphorylation, in which the phosphorylated enzyme is inactive. Increases in the in-tramitochondrial ratios of NADH/NAD+ and acetyl-CoA/CoASH also stimulate kinase-mediated phosphorylation of PDC. The drug dichloroacetate (DCA) inhibits the kinase responsible for phosphorylating PDC, thus locking the enzyme in its unphosphory-lated, catalytically active state. Reprinted with permission from Stacpoole et al. (2003).

Step 4 is the production of succinyl-CoA from 2-oxoglutarate and coenzyme A (CoASH), catalyzed by the 2-oxoglutarate dehydrogenase complex (which is often called a-ketoglutarate... 

In general, the entry of a fatty acid into a metabolic pathway is preceded by its conversion to its coenzyme A (CoASH) derivative this acyl derivative is called an alkanoyl- or alkenoyl-CoA, and in this form the fatty acid is said to have been activated. 

By now you will realize that most of this molecule is there to allow interaction with the various enzymes that catalyse the reactions of coenzyme A. We will abbreviate it from now on as CoASH where the SH is the vital thiol functional group, and all the reactions we will be interested in are those of esters of CoASH. These are thiol esters, as opposed to normal alcohol esters , and the difference is worth a few comments. 

Citrate is a key intermediate of the tricarboxylic acid (TCA) cycle, also known as the Krebs cycle, in the central metabolism of cells. (The set of reactions of the TCA cycle will be considered in some detail in Chapter 6.) One reaction in the cycle is the combination of oxaloacetate (OAA) and the acetyl group from acetyl coenzyme A (ACCOA), in the presence of H2O, to form citrate (CIT), thiol coenzyme A (COASH), and hydrogen ion (H+). The chemical reference reaction for this aldol condensation-hydrolysis reaction catalyzed by citrate synthase is ... 

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