Ketoacids decarboxylases

Enzymes for Carboligation - 2-Ketoacid Decarboxylases and Hydroxynitrile Lyases 327... 

Enantioselective C-C bond formation is gaining more and more importance in bioorganic synthesis. This reaction is efficiently catalyzed by 2-ketoacid decarboxylases (E.C. 4.1.l.X) as well as by hydroxynitrile lyases (E.C. 4.1.2.X). 

The project encompassed the comparative characterization of pyruvate decarboxylase from Z. mohilis (PDC) and benzoylformate decarboxylase from P. putida (BED) as well as their optimization for bioorganic synthesis. Both enzymes require thiamine diphosphate (ThDP) and magnesium ions as cofactors. Apart from the decarboxylation of 2-ketoacids, which is the main physiological reaction of these 2-ketoacid decarboxylases, both enzymes show a carboligase site reaction leading to chiral 2-hydroxy ketones (Scheme 2.2.3.1). A well-known example is... 

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. 

The enzymes described above that convert oxaloacetate to pyruvate and CO2 appear to use metal chelation to stabilize the enolate formed by decarboxylation. Many other /3-ketoacid decarboxylases use a similar mechanism. However, there are a few decarboxylations of /3-keto acids or their functional equivalents in which no metal ion is involved. One is the case of acetoacetate decarboxylase, which functions by means of a Schiif base mechanism. A few additional examples are described below. All these cases involve particularly stable enolates. 

All of the a-ketoacid dehydrogenase complexes are hnge enzyme complexes composed of multiple subunits of three different enzymes, Ej, E2, and E3 (Fig. 20.9). El is an a-ketoacid decarboxylase which contains thiamine pyrophosphate (TPP) it cleaves off the carboxyl group of the a-keto acid. E2 is a transacylase containing lipoate it transfers the acyl portion of the a-keto acid from thiamine to CoASH. E3 is dihydrolipoyl dehydrogenase, which contains... 

The Access Code for the Microbial Production of Branched-Chain Alcohols 2-Ketoacid Decarboxylase and an Alcohol Dehydrogenase... 

THE ACCESS CODE FOR THE MICROBIAL PRODUCTION OF BRANCHED-CHAIN ALCOHOLS 2-KETOACID DECARBOXYLASE AND AN ALCOHOL DEHYDROGENASE... 

FIGURE 12.1 The Ehrlich pathway exemplified for the conversion of the branched-chain amino acids L-valine, L-isoleucine, and L-leucine to the corresponding alcohols isobntanol, 2-methyl-1-bntanol, and 3-methyl-1-butanol. Adh, alcohol dehydrogenase KE)C, 2-ketoacid decarboxylase TA, transaminase. 

FIGURE 12.2 Formation of the higher alcohols 3 -methyl-1 -butanol, isobutanol, 2-methyl-1 -butanol, and 1-butanol derived from intermediates of the branched-chain amino acid pathways. Adh, alcohol dehydrogenase KDC, 2-ketoacid decarboxylase. 

Overview of the Biochemical Properties of Relevant Enzymes with 2-Ketoacid Decarboxylase or Alcohol Dehydrogenase... 

Adh, alcohol dehydrogenase AlsS, acetohydroxyacid synthase KDC, 2-ketoacid decarboxylase YqhD, broad-range alcohol dehydrogenase TPP, thiamine pyrophosphate. 

The 2-ketoglutarate dehydrogenase (2-KGD) complex is composed of three separate enzymes 2-ketoglutarate decarboxylase, or El lipoate succi-nyltransferase, or E2 and lipoamide dehydrogenase, or E3. The complex catalyses the oxidation of 2-ketoglutarate to yield succinyl-CoA and NADH. 2-KGD deficiency together with pyruvate dehydrogenase deficiency and branched chain ketoacid decarboxylase deficiency has been ascribed to E3 deficiency because the three enzyme complexes have the E3 component in common. E3 deficiency will not be discussed. 

Molds, particularly Penicillium roqueforti, utilize P-ketoacyl-CoA deacylase (thiohydrolase) and P-ketoacid decarboxylase to provide the compounds typical for the aroma of semi-soft cheeses e. g., the blue-veined cheese (Roquefort, Stilton, Gorgonzola, cf. Table 10.32). 

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