A-Ketoacid dehydrogenase

An acyl-transfer and redox coenzyme containing two sulfhydryl groups that form a dithiolane ring in the oxidized (disulfide) form. The redox potential at pH 7 is -0.29 volts. Lipoic acid is attached to the e-amino group of lysyl residues of transacetylases (subunit of a-ketoacid dehydrogenase complexes), thereby permitting acyl... 

Maple syrup disease Branched chain a-ketoacid dehydrogenase... 

Paxton, R. Harris, R.A. Isolation of rabbit liver branched chain a-ketoacid dehydrogenase and regulation by phosphorylation. J. Biol. Chem., 257, 14433-14439 (1982)... 

Shimomura, Y. Kuntz, M.J. Suzuki, M. Ozawa, T. Harris, R.A. Monovalent cations and inorganic phosphate alter branched-chain a-ketoacid dehydrogenase-kinase activity and inhibitor sensitivity. Arch. Biochem. Biophys., 266, 210-218 (1988)... 

Doering, C.B. Coursey, C. Spangler, W. Danner, D.J. Murine branched chain a-ketoacid dehydrogenase kinase cDNA cloning, tissue distribution, and temporal expression during embryonic development. Gene, 212, 213-219 (1998)... 

Correct answer = B. Alkaptonuria is a rare metabolic disease involving a deficiency in homogentisic acid oxidase, and the subsequent accumulation of homogentisic acid in the urine, which turns dark upon standing. The elevation of methylmalonate (due to methylmalonyl CoA mutase deficiency), phenylpyruvate (due to phenylalanine hydroxlyase deficiency), a-ketoisovalerate (due to branched-chain a-ketoacid dehydrogenase deficiency), and homocystine (due to cystathionine synthase deficiency) are inconsistent with a healthy child with darkening of the urine. 

Maple syrup urine disease (MSUD) is a recessive disorder in which there is a partial or complete deficiency in branched-chain a-ketoacid dehydrogenase—an enzyme that decarboxylates leucine, isoleucine, and valine. These amino acids and their corresponding a-keto acids accumulate in the blood, causing a toxic effect that interferes with brain func tion. Symptoms include feeding problems, vomiting, dehydration, severe metabolic acidosis, and a characteristic smell of the urine. If untreated, the disease leads to mental retardation, physical disabilities, and death. Diagnosis is based on a blood sample within 24 hours of birth. Treatment of MSUD involves a synthetic formula that contains limited amounts of leucine, isoleucine, and valine. 

B Zhang, DW Crabb, RA Harris. Nucleotide and deduced amino acid sequence of the El-a subunit of human liver branched-chain a-ketoacid dehydrogenase. Gene... 

I Matsuda, J Asaka, I Akaboshi, F Endo, H Mitsubuchi, Y Nobukuni. Maple syrup urine disease complete primary structure of the El-(3 subunit of human branched chain a-ketoacid dehydrogenase complex deduced from the nucleotide sequence and a gene analysis of patients with this disease. J Clin Invest 86 242-247, 1990. 

MC McKean, KA Winkeler, DJ Danner. Nucleotide sequence of the 5 end including the initiation codon of cDNA for the Ela subunit of the human branched chain a-ketoacid dehydrogenase complex. Biochim Biophys Acta 1171 109-112, 1992. 

P. J. Randle, a-ketoacid dehydrogenase complexes and respiratoty fuel utilisation in diabetes. Diabetohgia, 28, 479-484, 1985. Also P. J. Randle, P. B. Garland, C. N. Hales, and E. A. Newsholme. Ciba Foundation Ctdloqida Endocrin, IS, 1966. 

Methionine is converted into succinyl CoA in nine steps (Figure 23.2.7). The first step is the adenylation of methionine to form methyl donor in the cell (Section 24.2.7). Methyl donation and deadenylation yield homocysteine, which is eventually processed to a-ketobutyrate. The enzyme a-ketoacid dehydrogenase complex oxidatively decarboxylates a-ketobutyrate to propionyl CoA, which is processed to succinyl CoA as described in Section 23.3.3. 

The degradation of the hranched-chain amino acids employs reactions that we have encountered previously in the citric acid cycle and fatty acid oxidation. Leucine is transaminated to the corresponding a-ketoacid, a-ketoisocaproate. This a-ketoacid is oxidatively decarboxylated to isovaleryl CoA by the branched-chain a-ketoacid dehydrogenase complex. 

Whereas redox reactions on metal centres usually only involve electron transfers, many oxidation/reduction reactions in intermediary metabolism, as in the case above, involve not only electron transfer, but hydrogen transfer as well — hence the frequently used denomination dehydrogenase . Note that most of these dehydrogenase reactions are reversible. Redox reactions in biosynthetic pathways usually use NADPH as their source of electrons. In addition to NAD and NADP+, which intervene in redox reactions involving oxygen functions, other cofactors like riboflavin (in the form of flavin mononucleotide, FMN, and flavin adenine dinucleotide, FAD) (Figure 5.3) participate in the conversion of [—CH2—CH2— to —CH=CH—], as well as in electron transfer chains. In addition, a number of other redox factors are found, e.g., lipoate in a-ketoacid dehydrogenases, and ubiquinone and its derivatives, in electron transfer chains. 

Maple syrup urine disease a-Ketoacid dehydrogenase complex Misassembly/misfolding... 

Figure 15-2. The catalytic mechanism shared by the enzymes pyruvate dehydrogenase, a-ketoglutarate dehydrogenase, and branched-chain a-ketoacid dehydrogenase. E is the dehydrogenase complex E is the dihydrohpoyl transacetylase subunit, and Ej is the dihydrolipoyl dehydrogenase component. Ej and E are specific to each enzyme, and Ej is common to all three enzymes.

In these reactions, the C2-atom of ThDP must be deprotonated to allo v this atom to attack the carbonyl carbon of the different substrates. In all ThDP-dependent enzymes this nucleophilic attack of the deprotonated C2-atom of the coenzyme on the substrates results in the formation of a covalent adduct at the C2-atom of the thiazolium ring of the cofactor (Ila and Ilb in Scheme 16.1). This reaction requires protonation of the carbonyl oxygen of the substrate and sterical orientation of the substituents. In the next step during catalysis either CO2, as in the case of decarboxylating enzymes, or an aldo sugar, as in the case of transketo-lase, is eliminated, accompanied by the formation of an a-carbanion/enamine intermediate (Ilia and Illb in Scheme 16.1). Dependent on the enzyme this intermediate reacts either by elimination of an aldehyde, such as in pyruvate decarboxylase, or with a second substrate, such as in transketolase and acetohydroxyacid synthase. In these reaction steps proton transfer reactions are involved. Furthermore, the a-carbanion/enamine intermediate (Ilia in Scheme 16.1) can be oxidized in enzymes containing a second cofactor, such as in the a-ketoacid dehydrogenases and pyruvate oxidases. In principal, this oxidation reaction corresponds to a hydride transfer reaction. 

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