Generalized kinetic analysis of ion-driven cotransport systems: a unified interpretation of selective ionic effects on Michaelis parameters.

摘要: A major obstacle to the understanding of gradient-driven transport systems has been their apparently wide kinetic diversity, which seemed require a variety ad hoc mechanisms. Ordinary analysis, however, hampered by one mathematically powerful but physically dubious assumption: that rate limitation occurs in transmembrane transit, so ligand-binding reactions are at equilibrium. Simple models lacking assumption turn out be highly flexible and able describe most observed diversity co- counter-transport systems. Our “minimal” model cotransport consists single loop linking six discrete states carrier-type molecule. The state transitions include charge-transport step, step each for binding substrate cosubstrate (driver ion) side membrane. properties this developed sequential use realisticexperimental simplifications generalized numerical computations, focussed create known effects substrate, driver ion, membrane potential upon apparent Michaelis parameters (J max,K m ) isotopic influx. Specific behavior minimal depends arrangement magnitudes individual reaction constants among whole set (12) loop. Well defined arrangements have found permit either increasing or external driverion selectively reduce substrateK , elevateJ max, jointly raise bothK andJ max, lowerK while raisingJ max. Other allow rising internal ion act like competitive noncompetitive inhibitor entry, shut down (“transinhibit”) influx despite large inward driving forces. These findings obviate postulates special mechanisms cotransport: e.g., stoichiometry changes, wells, carrier-mediated leakage, gating —at least as explanations existing data. They also provide simple interpretation certain kinds homeostatic regulation, lead speculation kinetics reflects control-related selection constants, rather than fundamental differences mechanism.