KINETICS OF TRANSPORT-SYSTEMS DEPENDENT ON PERIPLASMIC BINDING-PROTEINS

Rate equations are derived for a transport model involving a water-soluble binding protein outside the plasma membrane. On addition of the substrate, the conformation of the binding protein changes; the complex then combines with the membrane carrier, transferring the substrate to the carrier site. The free binding protein leaves and the carrier shifts inward, releasing the substrate inside the cell. Exit follows the reverse path. The predicted behaviour is as follows. (i) Uptake does not necessarily conform to Michaelis-Menten kinetics. (ii) In both the energized and de-energized states, the maximum rate of exit is far lower than that of entry; the asymmetry is determined by the conformational change in the binding protein, which is independent of the energy state of the system. (iii) Exchange transport is inhibited by external substrate and is extremely slow; consequently counter-transport is not expected. (iv) The half-saturation constant in uptake can differ from the dissociation constant of the binding protein. (v) The maximum rate of uptake depends on the intrinsic substrate affinity of the membrane carrier relative to that of the binding protein. (vi) The maximum rate of uptake and the substrate half-saturation constant depend on' the concentration of the binding protein.