SIMPLE CARRIER KINETICS IN COMPLEX MEMBRANE TRANSPORTERS

The four-state simple carrier model (SCM) has been employed to describ e facilitative transport of ligands across biological membranes. Two b asic mechanisms have been invoked to account for carrier-mediated liga nd translocation: (i) binding to a mobile carrier, and (ii) displaceme nt determined by conformational changes of an integral protein. While translatory carriers may be accurately represented by a four-state dia gram, it is unlikely that the transport process mediated by a complex membrane protein can be strictly described by the elementary SCM. The purpose of this article is to test whether facilitative transporters w ith a more complex kinetic design than the SCM can exhibit macroscopic kinetic properties indistinguishable from it. For this, I studied a ' 'general carrier model'' (GCM), and evaluated whether the relevant kin etic parameters are subject to the same basic restrictions as in the S CM. The fundamental finding is that there is a general kinetic design embodied with SCM-like properties, that can be shared by many transpor ters. In particular, the classical SCM is shown here to represent a pa rticular case of the GCM. A main conclusion of this work is therefore that the finding of a macroscopic SCM-like kinetic behavior for a part icular process of facilitative transport does not represent a sufficie nt argument in favor of a particular type of mechanism, like the typic al one involving a two-conformational single-site carrier.