l-xylo-3-Hexulose Reductase Is the Missing Link in the Oxidoreductive Pathway for d-Galactose Catabolism in Filamentous Fungi

In addition to the well established Leloir pathway for the catabolism of d-galactose in fungi, the oxidoreductive pathway has been recently identified. In this oxidoreductive pathway, d-galactose is converted via a series of NADPH-dependent reductions and NAD+-dependent oxidations into d-fructose. The pathway intermediates include galactitol, l-xylo-3-hexulose, and d-sorbitol. This study identified the missing link in the pathway, the l-xylo-3-hexulose reductase that catalyzes the conversion of l-xylo-3-hexulose to d-sorbitol. In Trichoderma reesei (Hypocrea jecorina) and Aspergillus niger, we identified the genes lxr4 and xhrA, respectively, that encode the l-xylo-3-hexulose reductases. The deletion of these genes resulted in no growth on galactitol and in reduced growth on d-galactose. The LXR4 was heterologously expressed, and the purified protein showed high specificity for l-xylo-3-hexulose with a Km = 2.0 ± 0.5 mm and a Vmax = 5.5 ± 1.0 units/mg. We also confirmed that the product of the LXR4 reaction is d-sorbitol.