Biochemical Basis for Whole-Cell Uptake Kinetics: Specific Affinity, Oligotrophic Capacity, and the Meaning of the Michaelis Constant

Abstract

Formulations are presented that describe the concentration dependency of nutrient-limited transport and growth in molecular terms. They relate the rate of transport at steady state through a two-sequence process, transport and metabolism, to ambient concentrations according to the amounts and kinetic characteristics of the two rate-limiting proteins in these sequences. Sequences are separated by a metabolic pool. A novel feature of these formulations is the translation coefficient, which relates the transport rate attained at given ambient nutrient concentrations and membrane transporter characteristics to the nutrient concentrations sustained in the metabolic pools. The formulations, termed janusian kinetics, show that hyperbolic kinetics are retained during independent changes in transporter and enzyme contents or characteristics. Specific affinity ( a ° A ) depends strongly on the amount and kinetic characteristics of the transporters; it is also mildly affected by the amount and characteristics of the rate-limiting enzyme. This kinetic constant best describes the ability to accumulate substrate from limiting concentrations. Maximal velocity ( V max ) describes uptake from concentrated solutions and can depend strongly on either limiting enzyme content or the associated content of transporters. The whole-cell Michaelis constant ( K T ), which depends on the ratio of rate-limiting enzyme to transporter, can be relatively independent of change in a ° A and is best used to describe the concentration at which saturation begins to occur. Theory specifies that good oligotrophs have a large a ° A for nutrient collection and a small V max for economy of enzyme, giving a small K T . The product of the two constants is universally rather constant so that oligotrophy is scaled on a plot of a ° A versus K T , with better oligotrophs toward one end. This idea is borne out by experimental data, and therefore typical small difficult-to-culture aquatic bacteria may be classified as oligobacteria.