Coenzyme keto acid oxidation

Lipoic acid participates in the coenzyme A (CoA)- and diphosphopyri-diiie nucleotide (DPN)-linkcd oxidative decarboxylation of a-keto acids [Eq. (3)]. There are alternate pathways of a-keto acid oxidation which do... 

Detailed studies of the mechanism of a-keto acid oxidations have revealed that two sulfur-containing coenzymes, lipoic acid and CoASH, are involved. The respective roles of these compounds are illustrated in Fig. 3, which summarizes the results obtained by Korkes et al. (31), Gunsalus et al. (32), and by Reed and DeBusk (33) in their studies on pyruvate oxidation. As shown in the figure, pynivate reacts with oxidized lipoic acid in such a manner as to result in the cleavage of the S—S bond with the formation... 

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. 

Thiamine diphosphate (TDP) is an essential coenzyme in carbohydrate metabolism. TDP-dependent enzymes catalyze carbon-carbon bond-breaking and -forming reactions such as a-keto acid decarboxylations (oxidative and non-oxidative) and condensations, as well as ketol transfers (trans- and phospho-ketolation). Some of these processes are illustrated in Fig. 12. 

The a-keto acids then undergo oxidative decarboxylation to their coenzyme A derivatives catalyzed by branched-chain a-keto acid dehydrogenase. 

TPP-dependent enzymes catalyze either simple decarboxylation of a-keto acids to yield aldehydes (i.e. replacement of C02 with H+), or oxidative decarboxylation to yield acids or thioesters. The latter type of reaction requires a redox coenzyme as well (see below). The best known example of the former non-oxidative type of decarboxylation is the pyruvate decarboxylase-mediated conversion of pyruvate to acetaldehyde and C02. The accepted pathway for this reaction is shown in Scheme 10 (69MI11002, B-70MI11003, B-77MI11001>. 

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). 

Oxidation of a-amino acids to keto acids catalysed by D- and L-amino acid oxidases Oxidation of NADH via the cytochrome system catalyzed by cytochrome reductase Energy production via the TCA or Krebs cycle catalyzed by succinate dehydrogenase Fatty acid oxidation catalyzed by acyl-coenzyme A dehydrogenases Synthesis of fatty acids from acetate (80,81)... 

Branched-chain ketoaciduria (commonly known as Maple Syrup Urine Disease MSUD) is another ailment that may be caused by thiamine deficiency. In MSUD, the oxidative decarboxylation of alpha-keto acids derived from, i.e. valine, isoleucine, and leucine, is blocked due to an inadequate supply of the coenzyme thiamine pyrophosphate (TPP). Clinical symptoms of MSUD include mental and physical retardation. Describe briefly the structure of Riboflavin (Vitamin B-2) and its biochemical role. 

Degradation of all three branched-chain amino acids begins with a transamination followed by an oxidative decarboxylation catalyzed by the branched-chain a-keto acid dehydrogenase complex. This enzyme, like a-ketoglutarate dehydrogenase, requires thiamine pyrophosphate, lipoic acid, coenzyme A, FAD, and NAD+ (Figure 7-11). 

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 is required by the body as the pyrophosphate (TPP) in two general types of reaction, the oxidative decarboxylation of a keto acids catalyzed by dehydrogenase complexes and the formation of a-ketols (ketoses) as catalyzed by transketolase, and as the triphosphate (TTP) within the nervous system. TPP functions as the Mg -coordinated coenzyme for so-called active aldehyde transfers in mul-tienzyme dehydrogenase complexes that affect decarboxyia-tive conversion of a-keto (2 oxo) acids to acyl-coenzyme A (acyl-CoA) derivatives, such as pyruvate dehydrogenase and a-ketoglutarate dehydrogenase. These are often localized in the mitochondria, where efficient use in the Krebs tricarboxylic acid (citric acid) cycle follows. 

Lipoic acid acts as one of the coenzymes in the oxidative decarboxylation of pyruvate and other a-keto acids. It can be separated in an alkaline environment on a strongly basic anion exchanger in the hydroxide form, and can be detected like carbohydrates via pulsed amperometry at a Au working electrode. The corresponding chromatogram of a lipoic acid standard is shown in Fig. 8-88. This method allows to accurately detect 0.1 nmol lipoic acid. 

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). 

Oxidative deamination is stereospecific and is catalyzed by L- or D-amino acid oxidase. The initial step is removal of two hydrogen atoms by the flavin coenzyme, with formation of an unstable a-amino acid intermediate. This intermediate undergoes decomposition by addition of water and forms ammonium ion and the corresponding a-keto acid L-Amino acid oxidase occurs in the liver... 

Glutamate dehydrogenase plays a major role in amino acid metabolism. It is a zinc protein, requires NAD+ or NADP+ as coenzyme, and is present in high concentrations in mitochondria of liver, heart, muscle, and kidney. It catalyzes the (reversible) oxidative deamination of L-glutamate to a-ketoglutarate and NH3. The initial step probably involves formation of a-iminoglutarate by dehydrogenation. This step is followed by hydrolysis of the imino acid to a keto acid and NH3 ... 

The simplest example of such reactions is the decarboxylation of pyruvate. Both model and enzyme studies have shown the intermediacy of covalent complexes formed between the cofactor and the substrate. Kluger and coworkers have studied extensively the chemical and enzymatic behavior of the pyruvate and acetaldehyde complexes of ThDP (2-lactyl or LThDP, and 2-hydroxyethylThDP or HEThDP, respectively) . As Scheme 1 indicates, the coenzyme catalyzes both nonoxidative and oxidative pathways of pyruvate decarboxylation. The latter reactions are of immense consequence in human physiology. While the oxidation is a complex process, requiring an oxidizing agent (lipoic acid in the a-keto acid dehydrogenases , or flavin adenine dinucleotide, FAD or nicotinamide adenine dinucleotide , NAD " in the a-keto acid oxidases and Fe4.S4 in the pyruvate-ferredoxin oxidoreductase ) in addition to ThDP, it is generally accepted that the enamine is the substrate for the oxidation reactions. 

Isoleucine is a good example of branched-chain amino acids for a semi-in-depth examination. Unique aspects of the metabolism of valine and leucine are highlighted. After transamination and oxidative decarboxylation to form the branched-chain fatty-acyl CoA, a double bond is formed between a and b carbons utilizing FAD then water is added to form a b hydroxy derivative (Fig. 18.4). Then a NAD+-dependent dehydrogenase produces a keto derivative of the branched-chain fatty-acyl CoA. The similarity to straight-chain fatty-acid oxidation should be noted. This keto fragment is cleaved with participation of coenzyme A to form acetyl CoA, which ei-... 

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 known occurrences of thioaldehydes in biochemistry are few. One well-studied example is the involvement of a thioaldehyde in the decarboxylation of cysteine in phosphopantothenoyl-cysteine during coenzyme A biosynthesis. In the proposed mechanism for this decarboxylation, a thioaldehyde is generated at the cysteine sulfur by a flavin-dependent oxidation of the thiol. The resulting /3-thioketo acid, acting like a /3-keto acid, facilitates the decarboxylation of the amide-bound cysteine in phosphopantothenoyl-cysteine substrate. Finally, the flavinH2 produced in the thiol oxidation is used to reduce the thioaldehyde back to the thiol. 

Many alcoholics such as Al Martini develop thiamine deficiency because alcohol inhibits the transport of thiamine through the intestinal mucosal cells. In the body, thiamine is converted to thiamine pyrophosphate (TPP). TPP acts as a coenzyme in the decarboxylation of a-keto acids such as pyruvate and a-ketoglutarate (see Fig. 8.11) and in the utilization of pentose phosphates in the pentose phosphate pathway. As a result of thiamine deficiency, the oxidation of a-keto acids is impaired. Dysfunction occurs in the central and peripheral nervous system, the cardiovascular system, and other organs. 

Photosynthetic carbon dioxide fixation into oi-keto acids has recently been found to be the major pathway in some organisms. The process appears to be essentially a reversal of the mitochondrial oxidative decarboxylation processs. The photoreduction is mediated through a ferredoxin system similar to the photosynthetic nicotinamide coenzyme reductase. The involvement of lipoic add has not yet been shown, but it would be expected and could provide the long-sought role of lipoate in photosynthesis. 

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