Binding-protein Mediated Transport Systems at Maximal Activity Have Increased Effective Substrate Concentrations and Invest Free Energy in Binding-protein Unloading

Many microorganisms rely on binding-protein (BP) assisted, active transport systems for scavenging of scarce nutrients. Several hypotheses have been raised to explain the advantage of such uptake systems; attempting to rationalise the benefits of the relatively high BP concentrations in the periplasm and the coupling of transport to free energy transduction. Despite the appeal of these hypotheses, a systematic, rigorous analysis of binding-protein functionality in terms of underlying kinetics and thermodynamics is lacking. Here we report such an account. We characterise the states of maximal import activity per unit transporter. We find that a BP concentration that exceeds the concentration of the scarce nutrient and of the transporter raises the effective periplasmic nutrient concentration and the encounter rate of transporter and loaded binding proteins. Under these conditions, BPs enhance the steady state nutrient uptake rate per transporter. We also find that free-energy transduction is predominantly used for affinity-enhancement of the BP and liberation of the tightlybound nutrient from the BP to pass it on to the transporter protein. These conditions can be considered experimentally testable predictions of the hypothesis that BPs function to enhance the specific nutrient uptake rate, and indications of its validity already exist in the literature. The kinetic approach that we present is based on basic biochemical and physical principles. It is generally applicable to the functional analysis of small regulatory systems and leads to experimentally testable predictions.