4.1

Metabolites and Related Products

The production of a range of byproducts from Z. mobilis is reviewed compre­hensively by Johns et al. [6] and Panesar et al. [8] with the former authors identifying potential commercial opportunities for the following products: fructose (using sucrose and a fructokinase negative mutant), sorbitol and glu­conic acid, levan (a fructose polymer), fructo-oligosaccharides and various enzymes. As pointed out in a review by Scopes [91], Z. mobilis is a rich source (on an enzyme content per g cell basis) of many of the enzymes currently used in diagnostic analysis and research. Interestingly, in other studies by Park et al. [92], it was established that the activities of some of the key ED enzymes (e. g. glucokinase, G-6-phosphate dehydrogenase) were unaffected by the relatively high ethanol concentrations produced during fermentation, while the activity of an enzyme such as transketolase decreased appreciably above ethanol concentrations of 60 gL-1. It has been estimated that for ac­tively growing cells, as much as 30-50% of the cellular protein is comprised of ED enzymes [93]. However, the greatest difficulty for the commercial pro­duction of such enzymes is the low cell yield of Z. mobilis which is typically 0.02-0.03 gg-1 substrate sugar, compared to cell yields close to 0.5 gg-1 for many aerobically grown microorganisms.

4.2