Molecular aspects of functional differences between alcohol and sorbitol dehydrogenases

Abstract

The amino acid sequence of sheep liver sorbitol dehydrogenase has been fitted to the high-resolution model of the homologous horse liver alcohol dehydrogenase by computer graphics. This has allowed construction of a model of sorbitol dehydrogenase that provides explanations why sorbitol is not a substrate for alcohol dehydrogenase, why ethanol is not a substrate for sorbitol dehydrogenase, and what determines its specificity for polyols. An important feature of the model is that one of the ligands to the active site zinc atom is a glutamic acid residue instead of a cysteine residue, which is the corresponding ligand in the homologous alcohol dehydrogenases. This is one component of the structural change that can be related to the different substrate specificities, showing how altered enzymic activity might be brought about by structural changes of the kind that it is now possible to introduce by site-directed mutagenesis and recombinant DNA techniques.

Methylglyoxal and the polyol pathway

“…This is consistent with a nonessential role for the 3-hydroxyl, and lends strength to the concept that a hydroxyl group in this position interacts unfavourably with the nicotinamide ring of the coenzyme [26,29].…”

“…The presence of including (2R,3R)-2,3-butanediol, have been reported a primary hydroxyl group adjacent to the oxidation site to serve as substrates for the enzyme from human liver is considered essential for positioning the reactive 2- [27]. The present findings show that the presence of the hydroxyl at the catalytic zinc atom of SDH, probably C,H(OH) moiety per se is not essential, and the subby hydrogen bond formation between this functional strate specificity of sorbitol dehydrogenase can thus be group and the Glu-154 residue [26]. That l-deoxysorextended to include di-and trifunctional alcohols with bitol or monofunctional secondary alcohols like proa three-carbon chain.…”

“…[NADH] = 100 ,uM. chain compounds, of a 1,2-dihydroxy compound [25,26]. The presence of including (2R,3R)-2,3-butanediol, have been reported a primary hydroxyl group adjacent to the oxidation site to serve as substrates for the enzyme from human liver is considered essential for positioning the reactive 2- [27].…”

Rat liver alcohol dehydrogenase of class III

“…It appears possible that longer and more hydrophobic substrate chains may be ncecessary for the class I11 enzymes to displace the enlarged water content thereby allowing for correct binding at the substrate-binding site. The explanation for the substrate specificity differences between class I11 and the enzyme of other classes may then, at least in part, derive from the effects of water molecules, in a manner similar to the effects implied in explaining the differentiation between ethanol and sorbitol at the active site of sorbitol dehydrogenase [28], or between methanol and larger substrates at the active site of horse liver alcohol dehydrogenase [38].…”

Variability within mammalian sorbitol dehydrogenases

“…Like alcohol dehydrogenase and apparently all known niembers of the family except l-crystallin (2), sorbitol dehydrogenase has zinc at the active site [4]. However, based on fitting the primary structure of sheep liver sorbitol dehydrogenase to the three-dimensional model of horse liver alcohol dehydrogenase, it has been proposed that the coordination of the catalytic zinc is different in the two enzymes; of the two cysteine ligands in alcohol dehydrogenase, one could be glutamic acid in sorbitol dehydrogenase, the third ligand in both cases being a histidine residue [5]. In relation to substrates, sorbitol dehydrogenase does not oxidize primary alcohols such as ethanol, while alcohol dehydrogenase essentially does not oxidize sorbitol [6 -81.…”