Kinetic and nuclear magnetic resonance studies of xylose metabolism by recombinant Zymomonas mobilis ZM4(pZB5)

The specific rates of growth, substrate utilization, and ethanol production as well as yields of biomass and ethanol production on xylose for the reco mbinant Zymomonas mobilis ZM4(pZB5) were shown to be much less than those o n glucose or glucose-xylose mixtures, Typical fermentations with ZM4(pZB5) growing on glucose-xylose mixtures followed two-phase growth kinetics with the initial uptakes of glucose and xylose being follow ed by slower growth and metabolic uncoupling on xylose after glucose depletion. The reductions in rates and yields from xylose metabolism were considered in the present i nvestigation and may be due to a number of factors, including the following : (i) the increased metabolic burden from maintenance of plasmid-related fu nctions, (ii) the production of by-products identified as xylitol, acetate, lactate, acetoin, and dihydroxyacetone by C-13-nuclear magnetic resonance (NR-IR) spectroscopy and high-performance liquid chromatography, (iii) grow th inhibition due to xylitol by the putative inhibitory compound xylitol ph osphate, and (iv) the less energized state of ZM4(pZB5). In vivo P-31-NMR s tudies have established that the levels of NTP and UDP sugars on xylose wer e less than those on glucose, and this energy limitation is likely to restr ict the growth of the recombinant strain on xylose media.