Abstract
Substitution of active-site Tyr-51 by Ala (Y51A) disrupted the activity of Candida tenuis xylose reductase by six orders of magnitude. External bromide brought about unidirectional rate enhancement (≈2 × 10 3-fold at 300 mM) for NAD +-dependent xylitol oxidation by Y51A. Activity of the wild-type reductase was dependent on a single ionizable protein group exhibiting a p K of 9.2 ± 0.1 and 7.3 ± 0.3 in the holo-enzyme bound with NADH and NAD +, respectively. This group which had to be protonated for xylose reduction and unprotonated for xylitol oxidation was eliminated in Y51A, consistent with a catalytic acid–base function of Tyr-51. Bromide may complement the xylitol dehydrogenase activity of Y51A by partly restoring the original hydrogen bond between the reactive alcohol and the phenolate of Tyr-51.
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