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 V_max = 5.5 ± 1.0 units/mg. We also confirmed that the product of the LXR4 reaction is D-sorbitol.