Xylose reductases catalyse the reduction of xylose to xylitol, the initial reaction in the fungal D-xylose degradation pathway. Most of the enzymes exhibit a strict requirement for NADPH (cf. EC 18.104.22.1681
, D-xylose reductase (NADPH)). Some D-xylose reductases have dual coenzyme specificity, though they still prefer NADPH to NADH (cf. EC 22.214.171.1247
, D-xylose reductase [NAD(P)H]). The enzyme from Candida parapsilosis is a rare example of a xylose reductase that significantly prefers NADH, with Km and Vmax values for NADH being 10-fold lower and 10-fold higher, respectively, than for NADPH.