title: Transport of branched-chain amino acids in membrane vesicles of Streptococcus cremoris.
abstract: The kinetics, specificity, and mechanism of branched-chain amino acid transport in Streptococcus cremoris were studied in a membrane system of S. cremoris in which beef heart mitochondrial cytochrome c oxidase was incorporated as a proton motive force (delta p)-generating system. Influx of L-leucine, L-isoleucine, and L-valine can occur via a common transport system which is highly selective for the L-isomers of branched chain amino acids and analogs. The pH dependency of the kinetic constants of delta p-driven L-leucine transport and exchange (counterflow) was determined. The maximal rate of delta p-driven transport of L-leucine (Vmax) increased with increasing internal pH, whereas the affinity constant increased with increasing external pH. The affinity constant for exchange (counterflow) varied in a similar fashion with pH, whereas Vmax was pH independent. Further analysis of the pH dependency of various modes of facilitated diffusion, i.e., efflux, exchange, influx, and counterflow, suggests that H+ and L-leucine binding and release to and from the carrier proceed by an ordered mechanism. A kinetic scheme of the translocation cycle of H+-L-leucine cotransport is suggested.
authors.affiliation: Department of Microbiology, University of Groningen, Haren, The Netherlands.
authors.forename: A J
authors.initials: AJ
authors.lastname: Driessen
authors.forename: S
authors.initials: S
authors.lastname: de Jong
authors.forename: W N
authors.initials: WN
authors.lastname: Konings
mesh_terms: D000597:Amino Acids, Branched-Chain / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002417:Cattle; D002462:Cell Membrane / Q000378:metabolism / Q000648:ultrastructure; D003576:Electron Transport Complex IV / Q000378:metabolism; D007700:Kinetics; D007930:Leucine / Q000378:metabolism; D008929:Mitochondria, Heart / Q000201:enzymology; D008954:Models, Biological; D013291:Streptococcus / Q000378:metabolism*
chemical_list: D000597:Amino Acids, Branched-Chain; D003576:Electron Transport Complex IV; D007930:Leucine