The catabolism of valine occurs with the formation of a free branched-chain acid, (S)-β-hydroxyisobutyrate or HIBA which is reversibly oxidized to methylmalonate semialdehyde by a highly specific dehydrogenase (HIBADH or 3-hydroxy-2methylpropanoate: NADoxidoreductase, EC 188.8.131.52). Previous studies placed this enzyme in the now well established short-chain dehydrogenase gene family based on sequence homology and enzymatic properties such as the lack of a metal requirement for catalysis. However, site-directed mutagenesis studies showed that HIBADH differs mechanistically from the short-chain dehydrogenases. We now show that HIBADH shares more significant sequence homology with a separate group of enzymes including 6-phosphogluconate dehydrogenase from numerous species, and several unidentified ORFs of microbial origin. Most of the amino acid residues suggested to be important for substrate-binding and catalysis from X-ray crystal lographic studies of sheep 6-phosphogluconate dehydrogenase are completely conserved in all of these sequences. Furthermore, a conserved lysine residue suggested to be critical to catalysis in 6-phosphogluconate dehydrogenase is now shown by site-directed mutagenesis to be critical to catalysis by HIBADH. This study suggests that 6-phosphogluconate dehydrogenase and HIBADH may share a common evolutionär)' origin and a common enzymatic mechanism. We propose that these homologous proteins represent a distinct family of enzymes most likely consisting of β-hydroxyacid dehydrogenases.
|Original language||English (US)|
|State||Published - Dec 1 1996|
ASJC Scopus subject areas
- Molecular Biology