Structural and mechanistic aspects of a new family of dehydrogenases, the beta-hydroxyacid dehydrogenases.

Abstract

The catabolism of valine, unlike that of other branched-chain amino acids, occurs with the formation of a free branched-chain acid, (S)-β-hydroxyisobutyrate or HIBA, whereas other branched-chain amino acids are metabolized solely as coenzyme A thioesters. Because it exists as a free acid, HIBA can be released into the blood stream by specific tissues and is cleared by the liver where it can serve as a substrate for gluconeogenesis (Letto et al., 1986). During the past decade there has been significant interest in the metabolism and interorgan trafficking of the R- and S- enantiomers of HIBA. HIBA is oxidized in mitochondria to methylmalonate semialdehyde by a highly specific, NAD+-dependent dehydrogenase (HIBADH or 3-hydroxy-2-methyl-propionate: NAD+ oxidoreductase, EC 1.1.1.31). Previous studies of rat HIBADH tentatively placed the enzyme in the now well-established short-chain alcohol dehydrogenase family (Crabb et al., 1993; Hawes et al., 1995). This assignment was based on amino acid sequence homology, enzymatic properties such as the lack of a metal requirement for catalysis, and effects of tyrosine-specific chemical modification. However, site-directed mutagenesis studies indicated that HIBADH differs in mechanism from the short-chain dehydrogenases studied to date, such as Drosophila alcohol dehydrogenase (Hawes et al., 1995). Furthermore, the short-chain dehydrogenases mostly prefer secondary alcohols as optimal substrates whereas HIBADH is only active with primary alcohol substrates. HIBADH, therefore, is most likely not closely related to the short-chain dehydrogenases. More recent studies showed that HIBADH shares better amino acid sequence homology and enzymatic properties with a separate, previously unrecognized family of enzymes that includes D-phenylserine dehydrogenase from Pseudomonas syringae, 6-phosphogluconate dehydrogenase from numerous species, and numerous hypothetical proteins of microbial origin (Hawes et al., 1996).