Optimization of stereoselective ketone reduction by the white-rot fungus Merulius tremellosus ono991

A recently isolated white-rot fungal strain, Merulius tremellosus ono991, displays high stereoselectivity during the reduction of arylketones. In order to increase the productivity and specific yield of the optically active alcohols, the culture conditions for the reduction of the model ketone compound 1′-acetonaphtone to α-methyl-1-naphtalenemethanol were optimized with respect to oxygen supply, choice of primary substrate and arylketone concentration. Alternative electron acceptors were also used to elucidate the role of reduction equivalents in the reduction process. The optimal yields of α-methyl-1-naphtalenemethanol were obtained in N₂-flushed incubations with glycerol as primary substrate. The specific yield was increased from 57% to 98% compared to incubations under air with glucose. Most of the yield increase was due to N₂-flushing and could be attributed to two factors. First, an increased stability of the product, α-methyl-1-naphtalenemethanol, in anaerobic compared to aerobic atmosphere was demonstrated. Second, fermentative metabolism increased reduced enzyme cofactors available for the reduction. Diverting reducing equivalents away from fermentation with alternative electron acceptors correlated with a decreased yield of α-methyl-1-naphtalenemethanol. Furthermore, the dependency of ketone reductase for common occurring metabolic reducing equivalents, NAD(P)H, was demonstrated by the reduction of 1′-acetonaphtone in cell extracts of M. tremellosus ono991.