A-Ketoglutarate decarboxylase

The biosynthesis of menaquinones in E. coli (Fig. 5) starts with the conversion of isochorismic acid and a-ketoglutaric acid in the presence of thiamine pyrophosphate [105-107] to 2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylic acid (SHCHC) catalyzed by SHCHC synthase, and a-ketoglutarate decarboxylase, both encoded by the menD gene [104,108,109]. SHCHC is converted to o-succinylbenzoic acid by dehydration, catalyzed by a protein encoded by menC (Table 1) [110]. Palaniappan et al., [111] showed for the first time the biosynthesis of menaquinones via o-succinyl benzoic acid in B. subtilis including the activity of the enzymes. 

Figure 11.2 Reaction sequences catalyzed by 2-oxoacid dehydrogenase complex Pyruvate dehydrogenase complex (PDC) and a-ketoglutarate dehydrogenase complex (aKGDC) catalyze the oxidative decarboxylation of pyruvate (R = CH3) and a-ketoglutarate (R = CH2CH2COOH) to Acetyl-CoA and succinyl CoA respectively. Three component enzymes 2-oxoacid (pyruvate/a-ketoglutarate) decarboxylase, lipoate acetyltransferase/succinyltransferase, dihydrolipoate dehydrogenase as well as five cofactors, namely (1) thiamine pyrophosphate (TPP) and its acylated form, (2) lipoamide (LipS2), reduced form and acylated form, (3) flavin adenine dinucleotide (FAD) and its reduced form, (4) nicotinamide adenine dinucleotide (NAD ) and its reduced form, and (5) coenzyme A (CoASH) and its acylated product are involved.

Isocitric Dehydrogenase a-Ketoglutarate Decarboxylase Succinic Dehydrogenase Fumarase... 

The changes in a-ketoglutarate decarboxylase and pyruvic acid decarboxylase activities are not the only biochemical alterations observed in thiamine deficiency. The activities of some other enzymes were found to be altered, for example, erythrocyte transke-tolase and adenosine-5 -phosphatase activities are decreased in chicken brain. 

An explanation for the pathogenesis of the lesions observed in thiamine deficiency would seem to follow logically from these biochemical observations, for in the thiamine-deficient animal, at least two enzymes involved in the Krebs cycle are blocked. The block of pyruvic decarboxylase prevents the entry of the products of glycolysis into the Krebs cycle. The block of a-ketoglutarate decarboxylase restricts the oxidation of both carbohydrates and fatty acids. A severe metabolic distortion follows, and one of the main manifestations of the distortion is a reduction of the amount of chemical energy available in the form of ATP. Clearly, those organs that suffer the most from such alterations are those that are metabolically most active, and the heart and the peripheral nervous system surely qualify as such. 

Mutation of the dihydrolipoate reductase component impairs decarboxylation of branched-chain a-keto acids, of pyruvate, and of a-ketoglutarate. In intermittent branched-chain ketonuria, the a-keto acid decarboxylase retains some activity, and symptoms occur later in life. The impaired enzyme in isovaleric acidemia is isovaleryl-CoA dehydrogenase (reaction 3, Figure 30-19). Vomiting, acidosis, and coma follow ingestion of excess protein. Accumulated... 

I. 1.1.42], also known as oxalosuccinate decarboxylase, catalyzes the reaction of isocitrate with NADP+ to produce a-ketoglutarate, carbon dioxide, and NADPH. The enzyme is reported to be able to decarboxylate added oxalosuccinate. 

TPP involved in reactions catalysed by pyruvate decarboxylase (alcoholic fermentation), pyruvate dehydrogenase a-ketoglutarate dehydrogenase (TCA cycle), transketolase (photosynthesis Calvin cycle) acetolactate synthetase (Val, Leu biosynthesis)... 

Figure 6.3. GABA shunt as an alternative to a-ketoglutarate dehydrogenase in the citric acid cycle. 2-Oxoglutarate dehydrogenase, EC 1.2.4.2 glutamate decarboxylase, EC 4.1.1.15 GABA aminotransferase, EC 2.6.1.19 and succinic semialdehyde dehydrogenase, ECl.2.1.16.

In humans, TPP is a coenzyme for transketolation, an important reaction in the pentose-phosphate pathway, and for the oxidative decarboxylations catalyzed by pyruvate dehydrogenase, branched-chain a-ketoacid decarboxylase, and a-ketoglutarate dehydrogenase complexes. In lower organisms, TPP is also a cofactor for nonoxida-tive decarboxylations such as the conversion of pyruvate to acetaldehyde that occurs in yeast. 

Vitamin Bi is an essential co-factor for several enzymes of carbohydrate metabolism such as transketolase, pyruvate dehydrogenase (PDH), pyruvate decarboxylase and a-ketoglutarate dehydrogenase. To become the active co-factor thiamin pyrophosphate (TPP), thiamin has to be salvaged by thiamin pyrophosphokinase or synthesized de novo. In Escherichia coli and Saccharomyces cerevisiae thiamin biosynthesis proceeds via two branches that have to be combined. In the pyrimidine branch, 4-amino-5-hydroxymethy-2-methylpyrimidine (PIMP) is phosphorylated to 4-amino-2-methyl-5-hydroxymethyl pyrimidine diphosphate (PIMP-PP) by the enzyme HMP/HMP-P kinase (ThiD) however, the step can also be catalyzed by pyridoxine kinase (PdxK), an enzyme also responsible for the activation of vitamin B6 (see below). The second precursor of thiamin biosynthesis, 5-(2-hydroxyethyl)-4-methylthiazole (THZ), is activated by THZ kinase (ThiM) to 4-methyl-5-(2-phosphoethyl)-thiazole (THZ-P), and then the thia-zole and pyrimidine moieties, HMP-PP and THZ-P, are combined to form thiamin phosphate (ThiP) by thiamin phosphate synthase (ThiE). The final step, pyrophosphorylation, yields TPP and is carried out by thiamin pyrophosphorylase (TPK). 

The most useful, and thus far successful, examples have involved irreversible reactions of nucleophilic functions of an enzyme s reactive site with an enzymatically activated Kcat inhibitor of a Michael-type addition reaction. The activation invariably requires participation of the enzyme s prosthetic group (e.g., flavin of monoamine oxidase) or coenzymes such as pyridoxal (vitamin B) as its phosphate, which is associated with several enzymes (e.g., threonine dehydrase, ornithine decarboxylase, a-ketoglutarate transaminase). 

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