The Structure of Apo and Holo Forms of Xylose Reductase, a Dimeric Aldo-Keto Reductase from Candida tenuis

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• 作者：Kathryn L. Kavanagh ; Mario Klimacek ; Bernd Nidetzky ; and David K. Wilson
• 刊名：Biochemistry
• 出版年：2002
• 出版时间：July 16, 2002
• 年：2002
• 卷：41
• 期：28
• 页码：8785 - 8795
• 全文大小：878K
• 年卷期：v.41,no.28(July 16, 2002)
• ISSN：1520-4995

文摘

Xylose reductase is a homodimeric oxidoreductase dependent on NADPH or NADH and belongsto the largely monomeric aldo-keto reductase superfamily of proteins. It catalyzes the first step in theassimilation of xylose, an aldose found to be a major constituent monosaccharide of renewable planthemicellulosic material, into yeast metabolic pathways. It does this by reducing open chain xylose toxylitol, which is reoxidized to xylulose by xylitol dehydrogenase and metabolically integrated via thepentose phosphate pathway. No structure has yet been determined for a xylose reductase, a dimeric aldo-keto reductase or a family 2 aldo-keto reductase. The structures of the Candida tenuis xylose reductaseapo- and holoenzyme, which crystallize in spacegroup C2 with different unit cells, have been determinedto 2.2 Å resolution and an R-factor of 17.9 and 20.8%, respectively. Residues responsible for mediatingthe novel dimeric interface include Asp-178, Arg-181, Lys-202, Phe-206, Trp-313, and Pro-319. Alignmentswith other superfamily members indicate that these interactions are conserved in other dimeric xylosereductases but not throughout the remainder of the oligomeric aldo-keto reductases, predicting alternatemodes of oligomerization for other families. An arrangement of side chains in a catalytic triad shows thatTyr-52 has a conserved function as a general acid. The loop that folds over the NAD(P)H cosubstrate isdisordered in the apo form but becomes ordered upon cosubstrate binding. A slow conformationalisomerization of this loop probably accounts for the observed rate-limiting step involving release ofcosubstrate. Xylose binding (K_m = 87 mM) is mediated by interactions with a binding pocket that ismore polar than a typical aldo-keto reductase. Modeling of xylose into the active site of the holoenzymeusing ordered waters as a guide for sugar hydroxyls suggests a convincing mode of substrate binding.