Production of sorbitol and gluconic acid by permeabilized cells of Zymomonas mobilis

Abstract

Zymomonas mobilis is able to convert glucose and fructose to gluconic acid and sorbitol. The enzyme, glucose-fructose oxidoreductase, catalysing the intermolecular oxidation-reduction of glucose and fructose to gluconolactone and sorbitol, was formed in high amounts [1.4 units (U)·mg-1] when Z. mobilis was grown in chemostats with glucose as the only carbon source under non-carbon-limiting conditions. The activity of a gluconolactone-hydrolysing lactonase was constant at 0.2 U·mg-1. Using glucose-grown cells for the conversion of equimolar fructose and glucose mixtures up to 60% (w/v), a maximum product concentration of only 240 g·1-1 of sorbitol was found. The gluconic acid accumulated was further metabolized to ethanol. After permeabilizing the cells using cationic detergents, maximum sorbitol and gluconic acid concentrations of 295 g·1-1 each were reached; no ethanol production occurred. In a continuous process with κ-carrageenan-immobilized and polyethylenimin-hardened, permeabilized cells no significant decrease in the conversion yield was observed after 75 days. The specific production rates for a high yield conversion ( > 98%) in a continuous two-stage process were 0.19 g·g-1·h-1 for sorbitol and 0.21 g·g-1·h-1 for gluconic acid, respectively. For the sugar conversion of cetyltrimethylammonium bromide-treated κ-carrageenan-immobilized cells a V max of 1.7 g·g-1·h-1 for sorbitol production and a K m of 77.2 g·1-1 were determined