Simple carrier kinetics in complex membrane transporters.
The four-state simple carrier model (SCM) has been employed to describe facilitative transport of ligands across biological membranes. Two basic mechanisms have been invoked to account for carrier-mediated ligand translocation: (i) binding to a mobile carrier, and (ii) displacement determined by conformational changes of an integral protein. While translatory carriers may be accurately represented by a four-state diagram, 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 with 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 kinetic parameters are subject to the same basic restrictions as in the SCM. The fundamental finding is that there is a general kinetic design embodied with SCM-like properties, that can be shared by many transporters. In particular, the classical SCM is shown here to represent a particular case of the GCM. A main conclusion of this work is therefore that the finding of a macroscopic SCM-like kinetic behavior for a particular process of facilitative transport does not represent a sufficient argument in favor of a particular type of mechanism, like the typical one involving a two-conformational single-site carrier.
Sección Biofisica, Facultad de Ciencias, Universidad de la República, 11400 Montevideo, Uruguay.
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Hernández
D002352:Carrier Proteins / Q000737:chemistry* / Q000378:metabolism*; D007700:Kinetics; D008024:Ligands; D008433:Mathematics; D008954:Models, Biological; D011487:Protein Conformation
D002352:Carrier Proteins; D008024:Ligands