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 K(m) = 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.
VTT Technical Research Centre of Finland, Espoo, 02044 VTT, Finland.
Dominik
D
Mojzita
Silvia
S
Herold
Benjamin
B
Metz
Bernhard
B
Seiboth
Peter
P
Richard
D001234:Aspergillus niger / Q000201:enzymology* / Q000254:growth & development; D005656:Fungal Proteins / Q000235:genetics; D005690:Galactose / Q000378:metabolism*; D017353:Gene Deletion; D015966:Gene Expression Regulation, Fungal; D006601:Hexoses / Q000737:chemistry / Q000378:metabolism; D007661:Ketoses / Q000737:chemistry / Q000378:metabolism; D007700:Kinetics; D053858:Metabolic Networks and Pathways; D009249:NADP / Q000737:chemistry; D010084:Oxidation-Reduction; D013012:Sorbitol / Q000378:metabolism; D013379:Substrate Specificity; D013401:Sugar Alcohol Dehydrogenases / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D014158:Transcription, Genetic; D014242:Trichoderma / Q000201:enzymology* / Q000254:growth & development
D005656:Fungal Proteins; D006601:Hexoses; D007661:Ketoses; C441468:xylo-3-hexulose; D013012:Sorbitol; D009249:NADP; D013401:Sugar Alcohol Dehydrogenases; D005690:Galactose