Localized proton microcircuits at the biological membrane–water interface

Abstract

Cellular processes such as nerve conduction, energy metabolism, and import of nutrients into cells all depend on transport of ions across biological membranes through specialized membranespanning proteins. Understanding these processes at a molecular level requires mechanistic insights into the interaction between these proteins and the membrane itself. To explore the role of the membrane in ion translocation we used an approach based on fluorescence correlation spectroscopy. Specifically, we investigated exchange of protons between the water phase and the membrane surface, as well as diffusion of protons along membrane surfaces, at a single-molecule level. We show that the lipid head groups collectively act as a proton-collecting antenna, dramatically accelerating proton uptake from water to a membraneanchored proton acceptor. Furthermore, the results show that proton transfer along the surface can be significantly faster than that between the lipid head groups and the surrounding water phase. Thus, ion translocation across membranes and between the different membrane protein components is a complex interplay between the proteins and the membrane itself, where the membrane acts as a proton-conducting link between membranespanning proton transporters.diffusion ͉ fluorescence ͉ membrane protein ͉ pH ͉ proton transfer E nergy metabolism in living organisms involves translocation of protons across biological membranes through specific membrane-spanning proton transporters, thereby generating a proton-motive force (⌬p)where ⌬ is the electrical potential, R is the gas constant (J⅐K Ϫ1 ⅐mol Ϫ1 ), T is the absolute temperature (K), F is the Faraday constant (C mol Ϫ1 ), and ⌬pH is the pH difference between the two bulk solutions on either side of the membrane. The proton-motive force is used, e.g., by ATP synthase to produce ATP. It originally was proposed that ⌬p only depends on the solution properties and that the membrane only acts as a passive barrier isolating the two compartments (1). Over the years, evidence has accumulated indicating that the scenario is more complicated and also that the membrane surface and the water layer near the surface play major roles in the process (for a recent review, see ref.2). One important finding in this respect is the alkaliphilic bacteria, which live in environments having 2-3 units higher pH than that of the bacterial cytosol. Assuming a passive role of the membrane, these bacteria would in principle not be able to maintain a ⌬p large enough to account for their ATP production (3). To resolve this apparent paradox, the idea of a localized proton circuit along the membrane surface, originally proposed by Williams (4), has been exploited (2, 5-7) (see also ref. 8 for a theoretical analysis as well as refs. 9 and 10). According to this idea, protons ejected by a membrane-spanning proton transporter are transferred along the membrane to the proton acceptor (e.g., ATP synthase) faster than they are released to the bulk solution. Most likely, this process is facilitated by surface-b...

Ion Dynamics in Cationic Lipid Bilayer Systems in Saline Solutions

“…Hopping occurs rapidly on a subnanosecond time scale, and is fast considering the distance covered, thus making an important contribution to the Na + diffusion along the membrane. We are not aware of any other studies reporting the lateral hopping motion of Na + ions we describe here, but the diffusional behavior and transport of protons close to and along bilayer surfaces has proven to be very rich 93 and the behavior observed for positive Na + ions here could be related to some degree to the behavior of protons.…”

Surface-coupled proton exchange of a membrane-bound proton acceptor

“…More recently, using fluorescence correlation spectroscopy (FCS), increased protonation rates were observed at membrane interfaces for single fluorophores conjugated to small unilaminar vesicles (SUVs) (21) and for single fluorophores conjugated to membrane-reconstituted cytochrome c oxidase (22). Proton exchange measurements using FCS have several advantages.…”

Studying Ion Exchange in Solution and at Biological Membranes by FCS

“…The recent work by our group (Brändén et al 2006), (Öjemyr et al, 2009), (Persson et al, 2009), (Sandén et al, 2010) …”

“…As for lipid-labeled fluorescein (Brändén et al, 2006), it was found that the proton association rate, k +1 , was more than two orders of magnitude larger for fluorescein-CytcO reconstituted into lipid vesicles (DOPG), than for detergent-solubilized fluorescein-CytcO, which in turn was slightly larger (by a factor of two) than the k +1 rate measured for free fluorescein. The minor increase of k +1 for the detergent-solubilized fluorescein-CytcO, in the absence of a membrane, compared to that of free fluorescein indicates that the particular site of labelling of fluorescein on CytcO was not surrounded by a proton-collecting antenna on the protein surface.…”

“…Based on these selective advantages of the FCS approach we have investigated the principal role of biological membranes for proton uptake of membrane incorporated proteins (Brändén et al, 2006).…”