Nutrient Uptake by Microorganisms according to Kinetic Parameters from Theory as Related to Cytoarchitecture

Abstract

SUMMARY The abilities of organisms to sequester substrate are described by the two kinetic constants specific affinity, a°, 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, τ, of the transported molecule on the permease. The results may be analyzed with affinity plots (v/S versus v, where v 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 the Michaelis constant for a system with hyperbolic kinetics. Both are taken as a measure of τ, 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 flow 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° S values are compatible with growth in natural systems.