Acetyl coenzyme 3 keto acid

Step 4 of Figure 29.12 Oxidative Decarboxylation The transformation of cr-ketoglutarate to succinyl CoA in step 4 is a multistep process just like the transformation of pyruvate to acetyl CoA that we saw in Figure 29.11. In both cases, an -keto acid loses C02 and is oxidized to a thioester in a series of steps catalyzed by a multienzynie dehydrogenase complex. As in the conversion of pyruvate to acetyl CoA, the reaction involves an initial nucleophilic addition reaction to a-ketoglutarate by thiamin diphosphate vlide, followed by decarboxylation, reaction with lipoamide, elimination of TPP vlide, and finally a transesterification of the dihydrolipoamide thioester with coenzyme A. 

Another observation on oxalate formation is that other a-keto acids, such as oxalosuccinic acid (74) and a-ketoglutaric acid (106) do not seem to yield oxalate directly but indirectly (123). This appears to be due to the fact that only oxaloacetic acid can function as an acetate donor. In this connection the intervention of Coenzyme A may be considered, since it is reported to function in the acetylation of sulfanilamide and choline (73) and recently was shown to take part in the enzymatic synthesis of citric acid. This concept may be illustrated as follows ... 

Oxidative decarboxylations of a-keto acids are mediated by either enzymes having more than one cofactor or complex multienzyme systems utilizing a number of cofactors. For example, pyruvate oxidase uses TPP and FAD as coenzymes, the function of the latter being to oxidize the intermediate (41). Conversion of pyruvate to acetyl-CoA requires a multienzyme complex with the involvement of no less than five coenzymes, TPP, CoA, dihydrolipoate, FAD and NAD+ (74ACR40). 

The final step in the removal of two carbons from the fatty acid is the thiolytic cleavage to release acetyl-CoA. The term thiolytic refers to the use of Coenzyme A to bond with the carbonyl carbon of the P-keto acid. 

D. Valine, isoleucine, and leucine (the branched-chain amino acids) are transaminated and then oxidized by an a-keto acid dehydrogenase that requires lipoic acid as well as thiamine pyrophosphate, coenzyme A, FAD, and NAD+. Four of the carbons of valine and isoleucine are converted to succinyl CoA. Isoleucine also produces acetyl CoA Leucine is converted to HMG CoA, which is cleaved to acetoacetate and acetyl CoA... 

Thiamine pyrophosphate has two important coenzyme roles, both of which focus mostly on carbohydrate metabolism (Figs. 8.26 and 8.27). The active portion of the coen- rae is the thiazole ring. The first step in the oxidative decarboxylation of a-keto acids requires TPP. The two most common examples are pyruvate and a-ketoglutarate, oxidatively decarboxyatedto acetyl CoA and succinyl CoA, respectively. The same reaction is found in the metabolism of the branched-chain amino acids valine, isoleucine, leucine, and methionine. In all cases, TPP is a coenzyme in a mitochondrial multienzyme complex, consisting of TPP, lipoic acid, coenzyme A, FAD, and NAD. Note the number of vitamins required for the oxidative decarboxylation of a-keto acids thiamine (TPP), pantothenic acid (coenzyme A), riboflavin (FAD),and niacin (NAD). 

The physiological role of lipoic acid was first determined to be its involvement in the acetyl-coenzyme A (acetyl-CoA) and NAD-dependent oxidative decarboxylation of pyruvate and a-keto acids. Lipoic acid was shown to be essential for the oxidation of pyruvate, a-ketobutyrate, /3-methyl-a-ketobutyrate, and... 

The first step in the reaction sequence that converts pyruvate to carbon dioxide and acetyl-CoA is catalyzed by pyruvate dehydrogenase, as shown in Figure 19.4. This enzyme requires thiamine pyrophosphate (TPP a metabolite of vitamin Bj, or thiamine) as a coenzyme. The coenzyme is not covalently bonded to the enzyme they are held together by noncovalent interactions. Mg + is also required. We saw the action of TPP as a coenzyme in the conversion of pyruvate to acetaldehyde, catalyzed by pyruvate decarboxylase (Section 17.4). In the pyruvate dehydrogenase reaction, an a-keto acid, pyruvate, loses carbon dioxide the remaining two-carbon unit becomes covalently bonded to TPP. 

Amide bonds are found in many proteins. One is the acyl carrier protein of Escherichia coli (see 90), which contains the peptide backbone, and a 4 -phosphopantetheine unit (in violet in the illustration) is attached to a serine residue. Note the amine bonds in the pantothenic acid unit and also the 0-P=0 unit, which is a phosphate ester (an ester of phosphoric acid). An acyl carrier protein is involved in fatty acid synthesis, linking acetyl and malonyl groups from acetyl coenzyme A and malonyl coenzyme A to form P-keto acid acyl carrier protein (abbreviated as ACP). The widely utilized acetyl CoA is an ester (91) attached to coenzyme A. Acetyl CoA is a key intermediate in aerobic intermediary metabolism of carbohydrates, lipids, and some amino acids. 

Formation of a poly-j -keto-acyl-CoA [as (5.9)] occurs as for fatty acid biosynthesis by condensation of acetyl-coenzyme A with malonyl-CoA. Malonyl-CoA is generally derived by carboxylation of 3.1) (Scheme 3.3). An alternative path to malonyl-CoA is via oxaloacetate, an intermediate in the citric acid cycle. 

The enzyme which attracted most of our interest became the one which catalyzes the reversible thiolytic cleavage of /8-ketoacyl derivatives of coenzyme A to acetyl-CoA and an acyl-CoA containing two fewer carbon atoms than the original j3-keto acid. We found... 

FIGURE 13.2 The role of BCAAs in energy metabolism. Following removal of their amino group by reversible transamination (a) and the irreversible decarboxylation of the resulting branched-chain a-keto acids to form coenzyme A (CoA) compounds (b), BCAAs act as precursors of acetyl CoA and tricarboxylic acid cycle intermediates. CoA-SH, reduced form of CoA IB-CoA, isobutyryl-CoA ILE, isoleucine IV-CoA, isovaleryl-CoA KIC, a-ketoiso-caproate KIV, a-ketoisovalerate KMV, a-keto-P-methylvalerate LEU, leucine MB-CoA, a-methylbutyryl-CoA NADHj, reduced nicotinamide adenine dinucleotide VAL, valine. 

No information is available about the biochemical process which converts isoleucine and acetic acid (two units) into tenuazonic acid. By analogy with fatty acid and polyketide biosynthesis (cf. Turner, 1971), it is possible that an acetate (via acetyl coenzyme A) and a malonate unit (the monocoenzyme A derivative of malonic acid formed from acetyl-CoA and carbon dioxide) combine with the loss of a free carboxy group to give enzyme-bound aceto-acetic acid. Condensation of this activated keto acid with isoleucine would give rise to tenuazonic acid. This suggested process is analogous to the laboratory synthesis of a-acetyltetramic acids (Lacey, 1954). 

Subsequent metabolism of m,cw-muconic acid by Pseudomonas results in the formation of j3-ketoadipic acid, which is further degraded as its coenzyme A-thioester to acetyl coenzyme A and succinyl coenzyme A (Katagiri and Hayaishi, 1957). Sistrom and Stanier (1954) have identified the two enzymes that together produce ]8-ketoadipic acid. The first, lactonizing, enzyme equilibrates w,w-muconic acid with (-f)-y-carbo-xymethyl-zl -butenolide this product is irreversibly converted to jS-keto-adipic acid by the second, delactonizing, enzyme. 

Further bacterial degradation of m,m-muconic acid in cell-free enzyme extracts occurs via y-carboxymethylene butenolide — )3-keto-adipic acidsuccinic acid -b acetyl coenzyme A (Evans et al., 1951 Dagley et al., 1960 Katagiri and Hayaishi, 1957). 

The cyclized form of this amino-aldehyde is the y-methyl-A -pyrrol-inium salt (6). Condensation of this iminium salt with aceto-acetyl coenzyme A yields the coenzyme A ester of hygrine-1 -carboxylic acid (10). Hydrolysis of this ester and decarboxylation of the resultant 3-keto-acid (9) affords hygrine (8), which has been shown to be a precursor of tropine which is plausibly formed... 

The reverse of the above reaction is a key step in the oxidative degradation of fatty acids. This reverse Claisen condensation (catalyzed by thiolase) involves the cleavage of a carbon-carbon bond of a /3-keto ester of coenzyme A by another molecule of coenzyme A to give a new acyl derivative (RCO—SCoA) and ethanoyl (acetyl) derivative (CH3CO—SCoA) ... 

Like the related fatty acid synthases (FASs), polyketide synthases (PKSs) are multifunctional enzymes that catalyze the decarboxylative (Claisen) condensation of simple carboxylic acids, activated as their coenzyme A (CoA) thioesters. While FASs typically use acetyl-CoA as the starter unit and malonyl-CoA as the extender unit, PKSs often employ acetyl- or propionyl-CoA to initiate biosynthesis, and malonyl-, methylmalonyl-, and occasionally ethylmalonyl-CoA or pro-pylmalonyl-CoA as a source of chain-extension units. After each condensation, FASs catalyze the full reduction of the P-ketothioester to a methylene by way of ketoreduction, dehydration, and enoyl reduction (Fig. 3). In contrast, PKSs shortcut the FAS pathway in one of two ways (Fig. 4). The aromatic PKSs (Fig. 4a) leave the P-keto groups substantially intact to produce aromatic products, while the modular PKSs (Fig. 4b) catalyze a variable extent of reduction to yield the so-called complex polyketides. In the latter case, reduction may not occur, or there may be formation of a P-hydroxy, double-bond, or fully saturated methylene additionally, the outcome may vary between different cycles of chain extension (Fig. 4b). This inherent variability in keto reduction, the greater variety of... 

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