title: A multi-substrate single-file model for ion-coupled transporters.
abstract: Ion-coupled transporters are simulated by a model that differs from contemporary alternating-access schemes. Beginning with concepts derived from multi-ion pores, the model assumes that substrates (both inorganic ions and small organic molecules) hop a) between the solutions and binding sites and b) between binding sites within a single-file pore. No two substrates can simultaneously occupy the same site. Rate constants for hopping can be increased both a) when substrates in two sites attract each other into a vacant site between them and b) when substrates in adjacent sites repel each other. Hopping rate constants for charged substrates are also modified by the membrane field. For a three-site model, simulated annealing yields parameters to fit steady-state measurements of flux coupling, transport-associated currents, and charge movements for the GABA transporter GAT1. The model then accounts for some GAT1 kinetic data as well. The model also yields parameters that describe the available data for the rat 5-HT transporter and for the rabbit Na(+)-glucose transporter. The simulations show that coupled fluxes and other aspects of ion transport can be explained by a model that includes local substrate-substrate interactions but no explicit global conformational changes.
authors.affiliation: Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena 91125, USA.
authors.forename: A
authors.initials: A
authors.lastname: Su
authors.forename: S
authors.initials: S
authors.lastname: Mager
authors.forename: S L
authors.initials: SL
authors.lastname: Mayo
authors.forename: H A
authors.initials: HA
authors.lastname: Lester
mesh_terms: D000818:Animals; D001665:Binding Sites; D055592:Biophysical Phenomena; D001703:Biophysics; D002352:Carrier Proteins / Q000737:chemistry / Q000378:metabolism*; D003198:Computer Simulation; D004563:Electrochemistry; D050485:GABA Plasma Membrane Transport Proteins; D066298:In Vitro Techniques; D017136:Ion Transport / Q000502:physiology*; D008562:Membrane Glycoproteins / Q000737:chemistry / Q000378:metabolism; D008564:Membrane Potentials; D008565:Membrane Proteins / Q000737:chemistry / Q000378:metabolism; D026901:Membrane Transport Proteins; D008954:Models, Biological; D015394:Molecular Structure; D009004:Monosaccharide Transport Proteins / Q000737:chemistry / Q000378:metabolism; D009419:Nerve Tissue Proteins; D027361:Organic Anion Transporters; D012701:Serotonin / Q000378:metabolism; D050486:Serotonin Plasma Membrane Transport Proteins; D012964:Sodium / Q000378:metabolism; D051269:Sodium-Glucose Transporter 1; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
chemical_list: D002352:Carrier Proteins; D050485:GABA Plasma Membrane Transport Proteins; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D009004:Monosaccharide Transport Proteins; D009419:Nerve Tissue Proteins; D027361:Organic Anion Transporters; D050486:Serotonin Plasma Membrane Transport Proteins; C493415:Slc6a1 protein, rat; D051269:Sodium-Glucose Transporter 1; D012701:Serotonin; D005680:gamma-Aminobutyric Acid; D012964:Sodium