IMPROVED OPERATIONAL STABILITY OF CELL-FREE GLUCOSE-FRUCTOSE OXIDOREDUCTASE FROM ZYMOMONAS-MOBILIS FOR THE EFFICIENT SYNTHESIS OF SORBITOL AND GLUCONIC ACID IN A CONTINUOUS ULTRAFILTRATION MEMBRANE REACTOR

For the continuous, enzymatic synthesis of sorbitol and gluconic acid by cell-free glucose-fructose oxidoreductase (GFOR) from Zymomonas mob ilis, the principal determinants of productivity have been identified. Most important, the rapid inactivation of the soluble enzyme during s ubstrate conversion can be avoided almost completely when weak bases s uch as tris(hydroxymethyl) aminomethan or imidazol are used for the ti tration of the produced gluconic acid and when 5-10 mM dithiothreitol are added to prevent thiol oxidations. With regard to a long-term oper ational stability of the enzyme for continuous syntheses, thermal deac tivation becomes significant at reaction temperatures above 30 degrees C. Without any additional purification being required, the crude cell extract of Z. mobilis can be employed in a continuous ultrafiltration membrane reactor over a time period of more than 250 h without signif icant decrease in substrate conversion or enzyme activity. The use of soluble GFOR thus appears to be an interesting alternative to employin g permeabilized cells of Zymomonas for the production of sorbitol and gluconic acid and may be superior with regard to reactor productivitie s, at comparable operational stabilities. (C) 1997 John Wiley & Sons, Inc.