Kinetic analysis of a family of cotransport models.

The kinetic properties of a family of cotransport models are studied. The most general model allows the substrate and activator to bind in a random fashion to the transporter (iso random bi-bi mechanism). The other models require an ordered binding sequence (iso ordered bi-bi mechanism) and differ according to the order and symmetry of the binding events at the two membranes faces. In all cases it has been assumed that the translocation of the carrier is the rate-limiting step in the transport process. It is demonstrated that under zero trans, equilibrium exchange and infinite trans experimental conditions the usual kinetic parameters Km and V can be expressed as simple functions of the activator concentration and a minimal set of model dependent constants with well defined kinetic interpretations. Kinetic criteria for distinguishing between the various models are established. The incorporation of the effects of membrane potential into the flux equations is also treated with the aid of certain simplifying assumptions. The usefullness of the concept of 'effective charge' for non mobile carrier mechanisms is emphasized.