Mechanistic model of nutrient uptake explains dichotomy between marine oligotrophic and copiotrophic bacteria
Fig 2
Maximal uptake rates, half-saturation concentrations, and specific affinities of PTS and ABC transport systems.
We can approximate cytoplasmic uptake rates using the Michaelis–Menten equation: vc = Vmax[S]p/(KM+[S]p), where Vmax is the maximal uptake rate and KM the half-saturation concentration. While the exact solution of the cytoplasmic uptake rate for our model of PTS is in the form of a Michaelis–Menten equation, the exact solution of the uptake rate for ABC transport is not. Because our simulations suggest that the abundance of binding proteins should exceed the abundance of transport units in the oligotrophic conditions where ABC transport is optimal, we make the approximations that (i) [T:S:BP]+[T:BP]≪[BP]total and (ii) k′1[T]≪k0r (Section B in S1 Appendix) to obtain the above estimates for the effective maximal rate and half-saturation concentration. For PTS, the half-saturation concentration is a constant equal to the dissociation constant KT = k2/k1. For ABC transport, the half-saturation concentration depends on both the transport dissociation constant and the binding protein dissociation constant KD = k′0r/k′0f and is additionally a function of the abundance of binding proteins. Under this approximation, the specific affinity a′ = V′max/K′M of ABC transport is thus proportional to the product of the abundances of transport units and of binding proteins.