A PH-CONTROLLED FED-BATCH PROCESS CAN OVERCOME INHIBITION BY FORMATE IN NADH-DEPENDENT ENZYMATIC REDUCTIONS USING FORMATE DEHYDROGENASE-CATALYZED COENZYME REGENERATION

The NAD-dependent, formate dehydrogenase-catalyzed oxidation of format e anion into CO2 is known as the method for the regeneration of NADH i n reductive enzymatic syntheses. Inhibition by formate and inactivatio n by alkaline pH-shift that occurs when oxidation of formate is carrie d out at pH approximate to 7.0 may, however, hamper the efficient appl ication of this NADH recycling reaction. Here, we have devised a fed-b atch process using pH-controlled feeding of formic acid that can overc ome enzyme inhibition and inactivation. The reaction pH is thus kept c onstant by addition of acid, and formate dehydrogenase is supplied con tinuously with substrate as required, but the concentration of formate is maintained at a constant, non- or weakly inhibitory level througho ut the enzymatic conversion, thus enabling a particular NADH-dependent dehydrogenase to operate stably and at high reaction rates. For xylit ol production from xylose using yeast xylose reductase (K-i,K-Formate 182 mM), a fed-atch conversion of 0.5M xylose yielded productivities o f 2.8 g (L h)(-1) that are three-fold improved when contrasted to a co nventional batch reaction that employed equal initial concentrations o f xylose and formate. (C) 1998 John Wiley & Sons, Inc.