Branched-chain a-keto acid

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... 

BRANCHED-CHAIN a-KETO ACID DEHYDROGENASE COMPLEX... 

This enzyme phosphorylates branched-chain a-keto acid dehydrogenase using ATP. 

Thiamin-dependent enzymes, ACETOLACTATE SYNTHASE BENZOYLFORMATE DECARBOXYLASE BRANCHED-CHAIN a-KETO ACID DEHYDROGENASE COMPLEX... 

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

Deficiency in branched-chain a-keto acid dehydrogenase produces high levels of the branched-chain amino acids and their a-keto acids in the blood, causing neurotoxic effects and potential brain damage. 

Reed, L.J. Damuni, Z. Merryfield, M.L. Regulation of mammalian pyruvate and branched-chain a-keto acid dehydrogenase complexes by phosphorylation-dephosphorylation. Curr. Top. Cell. ReguL, 27, 41-49 (1985)... 

Harris, R.A. Kuntz, M.J. Simpson, R. Inhibition of branched-chain a-keto acid dehydrogenase kinase by a-chloroisocaproate. Methods Enzymol., 166, 114-123 (1988)... 

Lee, H.Y. Hall, T.B. Kee, S.M. Tung, H.Y.L. Reed, L.J. Purification and properties of branched-chain a-keto acid dehydrogenase kinase from bovine kidney. BioFactors, 3, 109-112 (1991)... 

Fujii, H. Shimomura, Y. Murakami, T. Nakai, N. Sato, T. Suzuki, M. Harris, R.A. Branched-chain a-keto acid dehydrogenase kinase content in rat skeletal muscle is decreased by endurance training. Biochem. Mol. Biol. Int., 44, 1211-1216 (1998)... 

Maple syrup urine disease (branched-chain ketoaciduria) <0.4 Isoleucine, leucine, and valine degradation Branched-chain a-keto acid dehydrogenase complex Vomiting convulsions mental retardation early death... 

Experiments with rats have shown that the branched-chain a-keto acid dehydrogenase complex is regulated by covalent modification in response to the content of branched-chain amino acids in the diet. With little or no excess dietary intake of branched-chain amino acids, the enzyme complex is phosphorylated and thereby inactivated by a protein kinase. Addition of excess branched-chain amino acids to the diet results in dephosphoiylation and consequent activation of the enzyme. Recall that the pyruvate dehydrogenase complex is subject to similar regulation by phosphorylation and dephosphorylation (p. 621). 

Figure 11 The putative catabolic pathway of L-leucine and its implications for strain improvement. For a promising host strain, the pathway to be blocked is indicated with thick double lines and the pathways to be fortified are indicated with thick arrows. Abbreviations for enzymes participating in the L-leucine catabolism and the acylation of tylosin VDH, valine (branched-chain amino acid) dehydrogenase BCDFI, branched-chain a-keto acid dehydrogenase IVD (AcdH), isovaleryl-CoA dehydrogenase (acyl-CoA dehydrogenase) MCC, 3-methylcrotonyl-CoA carboxylase EH, enoyl-CoA hydratase AcyA, mac-rolide 3-O-acyltransferase AcyBl, macrolide 4"-(9-acyltransferase.

DD Skinner, MR Morgenstern, RW Fedechko, CD Denoya. Cloning and sequencing of a cluster of genes encoding branched chain a-keto acid dehydrogenase from Streptomyces avermitilis and production of a functional El a[f component in Escherichia coli. J Bacteriol 177 183-190, 1995. 

As discussed earlier, the avermectin polyketide backbone is derived from seven acetate and five propionate extender units added to an a branched-chain fatty acid starter, which is either (S( I )-a-mcthylbutyric acid or isobutyric acid. The C25 position of naturally occurring avermectins has two possible substituents a. sec-butyl residue derived from the incorporation of S(+)-a-methy lbutyry 1-CoA ( a avermectins), or an isopropyl residue derived from the incorporation of isobutyiyl-CoA ( b avermectins). These a branched-chain fatty acids, which act as starter units in the biosynthesis of the polyketide ring, are derived from the a branched-chain amino acids isoleucine and valine through a branched-chain amino acid transaminase reaction followed by a branched-chain a-keto acid dehydrogenase (BCDH) reaction (Fig. 5) [23]. 

In applying mutasynthesis to the production of novel avermectins, the elimination of branched-chain a-keto acid dehydrogenase (BCDH) was targeted. This multienzyme complex is responsible for supplying the 2-methylbutyryl- and iso-butyryl-CoA starter units that initiate natural avermectin biosynthesis [21,29],... 

These results confirmed that branched-chain amino acid catabolism via the BCDH reaction provides the fatty acid precursors for natural avermectin biosynthesis in S. avermitilis. In contrast, B. subtilis appears to possess two mechanisms for branched-chain precursor supply. The dual substrate pyruvate/branched-chain a-keto acid dehydrogenase (aceA) and an a-keto acid dehydrogenase (bfmB), which also has some ability to metabolize pyruvate, appears to be primarily involved in supplying the branched-chain initiators of long, branched-chain fatty acid biosynthesis [32,42], Two mutations are therefore required to generate the bkd phenotype in B. subtilis [31,42],... 

CWC Hu, KS Lau, TA Griffin, JL Chuang, CW Fisher, RP Cox, DT Chuang. Isolation and sequencing of a cDNA encoding the decarboxylase (El)-a precursor of bovine branched-chain a-keto acid dehydrogenase complex expression of El-a mRNA and subunit in maple-syrup-urine-disease and 3T3-L1 cells. J Biol Chem... 

CD Denoya, RW Fedechko, EW Hafner, HAI McArthur, MR Morgenstern, DD Skinner, KJ Stutzman-Engwall, RG Wax, WC Wernau. A second branched-chain a-keto acid dehydrogenase gene cluster (bkdFGH) from Streptomyces avermitilis. its relationship to avermectin biosynthesis and the construction of a bkdF mutant suitable for the production of novel anti parasitic avermectins. J Bacteriol 177 3504-3511, 1995. 

Parsons, H., Fung, F., and Snyder, F. F. (1987). Branched-chain a-keto acids for the diagnosis of maple-syrup-urine disease. N- En l- f. Med. 316,951. 

Block, K. R, Aftering, R. P., Mehard, W. B., and Buse, M. G. (1987). Modulation of rat skeletal muscle branched-chain a-keto acid dehydrogenase in vivo.. Clin. Invest. 79, 1349-1358. 

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). 

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