title: Active transport of Ca2+ by an artificial photosynthetic membrane.
abstract: Transport of calcium ions across membranes and against a thermodynamic gradient is essential to many biological processes, including muscle contraction, the citric acid cycle, glycogen metabolism, release of neurotransmitters, vision, biological signal transduction and immune response. Synthetic systems that transport metal ions across lipid or liquid membranes are well known, and in some cases light has been used to facilitate transport. Typically, a carrier molecule located in a symmetric membrane binds the ion from aqueous solution on one side and releases it on the other. The thermodynamic driving force is provided by an ion concentration difference between the two aqueous solutions, coupling to such a gradient in an auxiliary species, or photomodulation of the carrier by an asymmetric photon flux. Here we report a different approach, in which active transport is driven not by concentration gradients, but by light-induced electron transfer in a photoactive molecule that is asymmetrically disposed across a lipid bilayer. The system comprises a synthetic, light-driven transmembrane Ca2+ pump based on a redox-sensitive, lipophilic Ca2+-binding shuttle molecule whose function is powered by an intramembrane artificial photosynthetic reaction centre. The resulting structure transports calcium ions across the bilayer of a liposome to develop both a calcium ion concentration gradient and a membrane potential, expanding Mitchell's concept of a redox loop mechanism for protons to include divalent cations. Although the quantum yield is relatively low (approximately 1 per cent), the Ca2+ electrochemical potential developed is significant.
authors.affiliation: Department of Chemistry and Biochemistry and Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe, Arizona 85287, USA.
authors.forename: Ira M
authors.initials: IM
authors.lastname: Bennett
authors.forename: Hebe M Vanegas
authors.initials: HM
authors.lastname: Farfano
authors.forename: Federica
authors.initials: F
authors.lastname: Bogani
authors.forename: Alex
authors.initials: A
authors.lastname: Primak
authors.forename: Paul A
authors.initials: PA
authors.lastname: Liddell
authors.forename: Luis
authors.initials: L
authors.lastname: Otero
authors.forename: Leonides
authors.initials: L
authors.lastname: Sereno
authors.forename: Juana J
authors.initials: JJ
authors.lastname: Silber
authors.forename: Ana L
authors.initials: AL
authors.lastname: Moore
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Moore
authors.forename: Devens
authors.initials: D
authors.lastname: Gust
mesh_terms: D001693:Biological Transport, Active / Q000528:radiation effects; D002118:Calcium / Q000378:metabolism*; D004579:Electron Transport / Q000528:radiation effects; D008027:Light; D008051:Lipid Bilayers / Q000378:metabolism / Q000528:radiation effects; D008081:Liposomes / Q000378:metabolism / Q000528:radiation effects; D008564:Membrane Potentials / Q000528:radiation effects; D008567:Membranes, Artificial; D010084:Oxidation-Reduction / Q000528:radiation effects; D010788:Photosynthesis / Q000528:radiation effects
chemical_list: D008051:Lipid Bilayers; D008081:Liposomes; D008567:Membranes, Artificial; D002118:Calcium