Comparative energetics of glucose and xylose metabolism in recombinant Zymomonas mobilis

Recombinant Zymomonas mobilis CP4:pZB5 was grown with pH control in batch and continuous modes with either glucose or xylose as the sole carbon and energy source. In batch cultures in which the ratio of the final cell mass concentration to the amount of sugar in the medium was constant (i.e., under conditions that promote "coupled growth"), maximum specific rates of glucose and xylose consumption were 8.5 and 2.1 g/(g of cell.h), respectively; maximum specific rates of ethanol production for glucose and xylose were 4.1 and 1.0 g/(g of cell.h), respectively; and average growth yields from glucose and xylose were 0.055 and 0.034 g of dry cell mass (DCM)/g of sugar, respectively. The corresponding value of YATP for glucose and xylose was 9.9 and 5.1 g of DCM/mol of ATP, respectively. YATP for the wild-type culture CP4 with glucose was 10.4 g of DCM/mol of ATP. For single substrate chemostat cultures in which the growth rate was varied as the dilution rate (D), the maximum or "true" growth yield (max Yx/s) was calculated from Pirt plots as the inverse of the slope of the best-fit linear regression for the specific sugar utilization rate as a function of D, and the "maintenance coefficient" (m) was determined as the y-axis intercept. For xylose, values of max Yx/s and m were 0.0417 g of DCM/g of xylose (YATP = 6.25) and 0.04 g of xylose/(g of cell.h), respectively. However, with glucose there was an observed deviation from linearity, and the data in the Pirt plot was best fit with a second-order polynomial in D. At D > 0.1/h, YATP = 8.71 and m = 2.05 g of glu/(g of cell.h) whereas at D < 0.1/h, YATP = 4.9 g of DCM/mol of ATP and m = 0.04 g of glu/(g of cell.h). This observation provides evidence to question the validity of the unstructured growth model and the assumption that Pirt's maintenance coefficient is a constant that is independent of the growth rate. Collectively, these observations with individual sugars and the values assigned to various growth and fermentation parameters will be useful in the development of models to predict the behavior of rec Zm in mixed substrate fermentations of the type associated with biomass-to-ethanol processes.