Nutrient uptake by microorganisms according to kinetic parameters from theory as related to cytoarchitecture

The abilities of organisms to sequester substrate are described by the two kinetic constants specific affinity, a degrees and maximal velocity V-max. Specific affinity is derived from the frequency of substrate-molecule collisions with permease sites on the cell surface at subsaturating concentrations of substrates. V-max is derived from the number of permeases and the effective residence time, tau, of the transported molecule on the permease. The results may be analyzed with affinity plots (nu/S versus nu, where nu is the rate of substrate uptake), which extrapolate to the specific affinity and are usually concave up. A third derived parameter; the affinity constant K-A, is similar to K-M but is compared to the specific affinity rather than V-max and is defined as the concentration of substrate necessary to reduce the specific affinity by half It can be determined in the absence of a maximal velocity measurement and is equal to die Michaelis constant for a system with hyperbolic kinetics. Both are taken as a measure of tau, with departure of K-M from K-A being affected by permease/enzyme ratios. Compilation of kinetic data indicates a 10(8)-fold range in specific affinities and a smaller (10(3)-fold) range in V-max values. Data suggest that both specific affinities and maximal velocities can be underestimated by protocols which interrupt nutrient few prior to kinetic analysis. A previously reported inverse relationship between specific affinity and saturation constants was confirmed. Comparisons of affinities with ambient concentrations of substrates indicated that only the largest a degrees(s) values are compatible with growth in natural systems.