DUAL RELATIONSHIPS OF XYLITOL AND ALCOHOL DEHYDROGENASES IN FAMILIES OF 2 PROTEIN TYPES

Xylitol dehydrogenase encoded by gene XYL2 from Pichia stipitis is a m ember of the medium-chain alcohol dehydrogenase family, as evidenced b y the domain organization and a distant homology (24% residue identity with the human class I(gamma1) alcohol dehydrogenase). Much of a loop structure is missing, like in mammalian sorbitol and prokaryotic thre onine dehydrogenases, many additional differences occur, and relations hips are closest with the sorbitol dehydrogenase, the equivalence of w hich in P. stipitis may actually be the xylitol dehydrogenase. A secon d P. stipitis gene, also cloned and corresponding to a xylitol dehydro genase, is highly different from XYL2, but encodes an enzyme with stru ctural properties typical of the short-chain dehydrogenase family, whi ch also contains an alcohol dehydrogenase (from Drosophila). Thus, yea st xylitol dehydrogenases, like alcohol and polyol dehydrogenases from other sources, have dual derivations, combining similar enzyme activi ties in separate protein families. In contrast to the situation with t he other enzymes, both forms of xylitol dehydrogenase are present in o ne organism.