FIG. 6.
Effect of enzyme regulation on the relationship between specific substrate consumption rate and steady-state substrate concentration (note the link between panels a and d, panels b and e, and panels c and f). (a) Steady-state concentrations of glucose and galactose in chemostat cultures of E. coli growing under carbon-controlled conditions at a constant dilution rate (D = 0.3 h−1) with different glucose-galactose mixtures in the feed. The total sugar concentration in the inflowing medium was always 10 mg liter−1, and the composition of glucose-galactose is given in weight proportions. Data from reference 148. (b) Regulated catabolic enzyme level. Concentrations of 3-PPA during growth of E. coli in carbon-controlled chemostat cultures with different mixtures of 3-PPA and glucose at constant dilution rate are shown (D = 0.3 h−1). The shaded area indicates the range from 0 mg of 3-PPA liter−1 up to the apparent threshold concentration below which the 3-PPA was not utilized (i.e., the same residual concentrations [□] as those in medium feed were measured). Once induced, 3-PPA was utilized down to concentrations (■) that were lower than those required to trigger induction. Data from reference 134. (c) Steady-state methanol concentration (■) and specific activity of alcohol oxidase (□) in the yeast Kloeckera sp. strain 2201 during simultaneous utilization of glucose-methanol mixtures in carbon-controlled chemostat culture at a constant dilution rate (D = 0.14 h−1). The specific activity of alcohol oxidase is given in micromoles per milligram of protein per minute. Adapted from references 63 and 64. (d to f) Different enzyme expression patterns (the details of the meaning of the numbers are discussed in the text). It is assumed that the consumption kinetics of a microbial culture for a substrate can be described by a Monod-type relationship. qmax(ind) and qmax(rep) are the maximum specific substrate consumption rates under fully induced and repressed conditions, respectively.