Transport of sugars and amino acids in bacteria. XVI. Theory and evaluation of a model for the membrane transport reaction mediated by a single carrier with three binding sites for substrate.

A novel model is presented for bacterial active transport reactions which show a curvilinear Eadie-Hofstee plot and negative homotropic cooperativity in the kinetics of substrate uptake. Various models of a single carrier with multi-binding sites for substrate were constructed and examined theoretically. The fit of these models with experimental data on the kinetics of branched chain amino acid transport reactions were tested by iterative computation using the non-linear least square method. The transport model which fitted the experimental data best consisted of a single carrier with three binding sites for substrate in which one of the substrate-carrier complexes, CSS, is not active in translocating substrate across the cytoplasmic membrane. The mechanism of homeostatic regulation of the intracellular concentration of amino acids by active transport systems is discussed on the basis of this transport model.