Metabolic engineering of Escherichia coli: construction of an efficient biocatalyst for D-mannitol formation in a whole-cell biotransformation.

A whole-cell biotransformation system for the conversion of D-fructose to D-mannitol was developed in Escherichia coli by construction of a recombinant oxidation/reduction cycle. First, the mdh gene encoding for the mannitol dehydrogenase of Leuconostoc pseudomesenteroides ATCC 12291 (MDH) was expressed, effecting a strong catalytic activity of a NADH-dependent reduction of D-fructose to D-mannitol in cell extracts of the recombinant E. coli strain but not enabling whole cells of the strain to produce D-mannitol from D-fructose. To provide a source for reduction equivalents needed for D-fructose reduction, the fdh gene from Mycobacterium vaccae N10 (FDH) encoding formate dehydrogenase was functionally co-expressed. FDH generates NADH used for D-fructose reduction by dehydrogenation of formate to carbon dioxide. These recombinant E. coli cells were able to form D-mannitol from D-fructose in a low but significant quantity (15 mM). The introduction of a further gene, encoding for the glucose facilitator protein of Zymomonas mobilis (GLF) enabled the cells to efficiently take up D-fructose into the cells, without simultaneous phosphorylation. Resting cells of this E. coli strain (3 g cell dry weight/l) produced 216 mM D-mannitol in 17 hours. Biotransformations conducted under pH-control by formic acid addition yielded D-mannitol at a concentration of 362 mM within 8 hours. The yield Y(D-mannitol D-fructose) was 84 [mol%]. These results show that the recombinant strain of E. coli can be utilized as an efficient biocatalyst for D-mannitol formation.