A-Keto acid decarboxylases

As the name implies, the odor of urine in maple syrup urine disease (brancbed-chain ketonuria) suggests maple symp or burnt sugar. The biochemical defect involves the a-keto acid decarboxylase complex (reaction 2, Figure 30-19). Plasma and urinary levels of leucine, isoleucine, valine, a-keto acids, and a-hydroxy acids (reduced a-keto acids) are elevated. The mechanism of toxicity is unknown. Early diagnosis, especially prior to 1 week of age, employs enzymatic analysis. Prompt replacement of dietary protein by an amino acid mixture that lacks leucine, isoleucine, and valine averts brain damage and early mortality. 

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... 

This enzyme complex [EC 1.2.4.4], also known as 3-methyl-2-oxobutanoate dehydrogenase (lipoamide) and 2-oxoisovalerate dehydrogenase, catalyzes the reaction of 3-methyl-2-oxobutanoate with lipoamide to produce S-(2-methylpropanoyl)dihydrolipoamide and carbon dioxide. Thiamin pyrophosphate is a required cofactor. The complex also can utilize (5)-3-methyl-2-oxopenta-noate and 4-methyl-2-oxopentanoate as substrates. The complex contains branched-cham a-keto acid decarboxylase, dihydrolipoyl acyltransferase, and dihydrolipoa-mide dehydrogenase [EC 1.8.1.4]. 

Various thiamine diphosphate (ThDP)-dependent a-keto acid decarboxylases have been described as catalyzing C-C bond formation and/or cleavage [48]. Extensive work has already been conducted on transketolase (TK) and pyruvate decarboxylase (PDC) from different sources [49]. Here attention should be drawn to some concepts based on the investigation of reactions catalyzed by the enzymes... 

The a-keto acid decarboxylases such as pyruvate (E.C. 4.1.1.1) and benzoyl formate (E.C. 4.1.1.7) decarboxylases are a thiamine pyrophosphate (TPP)-dependent group of enzymes, which in addition to nonoxidatively decarboxylating their substrates, catalyze a carboligation reaction forming a C-C bond leading to the formation of a-hydroxy ketones.269-270 The hydroxy ketone (R)-phenylacetylcarbinol (55), a precursor to L-ephedrine (56), has been synthesized with pyruvate decarboxylase (Scheme 19.35). BASF scientists have made mutations in the pyruvate decarboxylase from Zymomonas mobilis to make the enzyme more resistant than the wild-type enzyme to inactivation by acetaldehyde for the preparation of chiral phenylacetylcarbinols.271... 

OMP decarboxylase (ODCase) catalyzes the decarboxylation of OMP to UMP, a decarboxylation that must necessarily be mechanistically different from the groups of decarboxylations that occur throughout metabolism [1]. The structure of the substrate does not lend itself to decarboxylation mechanisms involving pyridoxal phosphate (typical of amino acid decarboxylases [2]), thiamine pyrophosphate (typical of a-keto acid decarboxylases [3]), or metal ions (typical of /3-keto acid decarboxylases [4]) although the presence of ions has been detected in some preparations of ODCase [5, 6], the enzyme clearly does not require for catalytic activity [7]. 

The broad synthetic potential ThDP-dependent enzymes for asymmetric C-C bond formation is by far not fully exploited with the acyloin- and benzoin-condensations discussed above. On the one hand, novel branched-chain a-keto-acid decarboxylases favorably extend the limited substrate tolerance of traditirnial enzymes, such as PDC, by accepting sterically hindered a-ketoacids as dcmors [1511], On the other hand, the acceptor range may be significantly widened by using carlxMiyl compounds other than aldehydes Thus, ketones, a-ketoacids and even CO2 lead to novel types of products (Scheme 2.203). 

Figure 14.2. Example of some metabolic pathways adopted by Lactobacillus plantarum during fermentation of vegetable and fruit juices. He, isoleucine Leu, leucine Val, valine His, histidine Glu, glutamic acid BcAT, branched-chain aminotransferase KDC, a-keto acid decarboxylase ADH, alcohol dehydrogenase MLE, malol-actlc enzyme HDC, histidine decarboxylase (Adapted from Filannino etal. 2014)...

Flavor-related activities of LAB depend on the species, with some specific activities found only in a small number of species. The branched-chain a-keto acid decarboxylase activity involved in the formation of malty branched-chain aldehydes from branched-chain AA, for example, has been found only in L lactis and not in the Lactobacillus and Leuconostoc strains tested (Fernandez De Palencia et al. 2006). Most LAB, however, display a great strain-to-strain variability, on the genomic level and/or at the phenotypic level. Table 19.3 gives some examples of intra- and interspecies variability of flavor-related LAB properties, such as proteolytic activities, AA-converting enzymatic activities, ester synthesis, and autolysis. 

A second variant form [90] is characterized by mental retardation (I.Q. 50) and no other evidence of neurological disease. The serum and urine levels of leucine, isoleucine, valine and the corresponding a-keto acids are in the leucinosis range continuously, not merely during infections. The leucocytes contained 15 to 25% of the normal activities of the branched chain a-keto acid decarboxylases. 

Iding H, Siegert P, Mesch K, Pohl M. AppUcation of a-keto acid decarboxylases in biotransformations. Biochim. Bio-phys. Acta 1998 1385 307-322. 

Pyruvate decarboxylase is by far the best characterized enzyme among thiamine pyrophosphate-linked a-keto acid decarboxylases. Properties of this enzyme, its structure, and its catalytic mechanism have been described in a recent review [8]. Two other enzjmies belonging to the class of a-keto acid decarboxylases, benzoylformate decarboxylase and phenylpyruvate decarboxylase, are much less studied. 

The objectives of this chapter are to give a short description of properties of non-oxidative a-keto acid decarboxylases and their applications in stereoselective biotransformations. 

The second representative of the class of thiamine pyrophosphate-dependent nonoxidative a-keto acid decarboxylases is phenylglyoxylate decarboxylase (benzoylformate decarboxylase EC 4.1.1.7). This enzyme participates in the catabolism of aromatic compounds as part of mandelate pathway in different Pseudomonas and Acinetobacter species, normally converting benzoylformate to benzaldehyde [81-83]. This pathway is induced by mandelic acid [84]. 

The least studied among thiamine pyrophosphate-dependent nonoxidative a-keto acid decarboxylases is phenylpyruvate decarboxylase (EC 4.1.1.43), an enzyme found in Achromobacter auridice [95], Acinetobacter calcoaceticus [81] and the denitrifying bacterium Thauera aromatica [96]. This enzyme participates in catabolic pathways of aro-... 

Benzoylformate decarboxylase from Pseudomonas and Acinetobacter species, also an a-keto acid decarboxylase, has higher substrate specificity than pyruvate decarboxylase. Cells of these species grown in media inducing the mandelate pathway enzymes can convert benzoylformate and acetaldehyde to optically active 2-hydroxypropiophenone. Benzaldehyde is produced in this biotransformation reaction, as it is the normal product of benzoylformate decarboxylase. Some benzyl alcohol is also produced, in this case probably by reduction of benzaldehyde by cell oxidoreductases. In the case of P. putida the (S) enantiomer form of 2-hydrox) ropiophenone was produced, with an e.e. of 91-92%. The same product produced by A. calcoaceticus had an e.e. of 98%. An optimal volumetric production of 2-hydroxypropiophenone of 6.95 g per L per h was reported. 

Anodier a-keto acid decarboxylase, phenylpyruvate decarboxylase, has been characterized but there are no reports on condensation reactions by this enzyme. 

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