The dynamic feature of the proton collecting antenna of a protein surface

We are now in an exciting era of molecular bioenergetics. High-resolution x-ray structures have been determined for several of the key components of bioenergetics systems, including the respiratory enzymes cytochrome oxidase (1-4) and the bc 1 complex (5, 6), the F 1 component of the F 1 F o -ATPase (7), the light-driven proton pump bacteriorhodopsin (8, 9), and the bacterial photosynthetic reaction center (10-12). Each of these membrane proteins generates (or can use) a protonmotive force across the membrane by using unique mechanisms. In cytochrome oxidase the complex oxygen chemistry is somehow linked at different stages to driving protons about 50 Å across the membrane against an electrochemical gradient. In this issue of the Proceedings, Hartmut Michel (45) offers a new model of how this process might work. He presents a challenge to a paradigm that has been generally accepted by researchers in the field for the past decade. To be sure, his proposal will spark healthy debate and a new round of experiments designed to test the predictions of the new model that distinguish it from other models that have been offered.

Section: Introductionmentioning

confidence: 99%

How does cytochrome oxidase pump protons?

Gennis

1998

“…In these systems, after protons are taken up from solution, they are transferred through specific pathways composed of protonatable residues and water molecules, often coordinated by polar residues. Results from earlier studies have shown that proton uptake from solution is assisted by acidic groups near the orifice of a proton pathway, forming a proton-collecting antenna (1)(2)(3)(4). Such groups may act to funnel protons toward the proton-pathway entry point.…”

mentioning

confidence: 99%

Role of aspartate 132 at the orifice of a proton pathway in cytochromecoxidase

Johansson

Högbom

Carlsson

et al. 2013

Proton transfer across biological membranes underpins central processes in biological systems, such as energy conservation and transport of ions and molecules. In the membrane proteins involved in these processes, proton transfer takes place through specific pathways connecting the two sides of the membrane via control elements within the protein. It is commonly believed that acidic residues are required near the orifice of such proton pathways to facilitate proton uptake. In cytochrome c oxidase, one such pathway starts near a conserved Asp-132 residue. Results from earlier studies have shown that replacement of Asp-132 by, e.g., Asn, slows proton uptake by a factor of ∼5,000. Here, we show that proton uptake at full speed (∼10 4 s −1 ) can be restored in the Asp-132–Asn oxidase upon introduction of a second structural modification further inside the pathway (Asn-139–Thr) without compensating for the loss of the negative charge. This proton-uptake rate was insensitive to Zn 2+ addition, which in the wild-type cytochrome c oxidase slows the reaction, indicating that Asp-132 is required for Zn 2+ binding. Furthermore, in the absence of Asp-132 and with Thr at position 139, at high pH (>9), proton uptake was significantly accelerated. Thus, the data indicate that Asp-132 is not strictly required for maintaining rapid proton uptake. Furthermore, despite the rapid proton uptake in the Asn-139–Thr/Asp-132–Asn mutant cytochrome c oxidase, proton pumping was impaired, which indicates that the segment around these residues is functionally linked to pumping.

“…ligand binding to membrane proteins) has been demonstrated in several studies and explained in terms of initial nonspecific binding of the solute molecules to the membrane followed by diffusion along the surface to their target molecules (1)(2)(3)(4)(5)(6)(7)(8). Studies on some specific membrane-bound proton pumps, for example cytochrome c oxidase or bacteriorhodopsin (9)(10)(11)(12), have revealed higher than diffusion-limited rates of proton uptake (9,(13)(14)(15). It has been hypothesized that these proteins have a surface proton-collecting antenna, consisting of negative and buffering groups, aiding the proton uptake (12,16,17).…”

mentioning

confidence: 99%

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

Sandén

Salomonsson

Brzezinski

et al. 2010

Proton-transfer reactions across and at the surface of biological membranes are central for maintaining the transmembrane proton electrochemical gradients involved in cellular energy conversion. In this study, fluorescence correlation spectroscopy was used to measure the local protonation and deprotonation rates of single pHsensitive fluorophores conjugated to liposome membranes, and the dependence of these rates on lipid composition and ion concentration. Measurements of proton exchange rates over a wide proton concentration range, using two different pH-sensitive fluorophores with different pK a s, revealed two distinct proton exchange regimes. At high pH (>8), proton association increases rapidly with increasing proton concentrations, presumably because the whole membrane acts as a proton-collecting antenna for the fluorophore. In contrast, at low pH (<7), the increase in the proton association rate is slower and comparable to that of direct protonation of the fluorophore from the bulk solution. In the latter case, the proton exchange rates of the two fluorophores are indistinguishable, indicating that their protonation rates are determined by the local membrane environment. Measurements on membranes of different surface charge and at different ion concentrations made it possible to determine surface potentials, as well as the distance between the surface and the fluorophore. The results from this study define the conditions under which biological membranes can act as proton-collecting antennae and provide fundamental information on the relation between the membrane surface charge density and the local proton exchange kinetics.biomembrane | diffusion | electrostatic potential | fluorescence correlation spectroscopy (FCS) | proton transfer E nergy conversion in living cells typically involves proton translocation across a membrane, via proton transporters. These transporters maintain a proton electrochemical gradient utilizing free energy provided, for example by electron transfer or light. The free energy stored in this gradient is used, e.g., for transmembrane transport, motility, or synthesis of ATP by the ATP synthase. Results from a range of studies indicate that the membrane plays an important role in these processes, in addition to serving as a barrier. The membrane surface may also provide a proton link between the various membrane-embedded proteins, where the mere two-dimensional confinement of the reactants can also play a role. Enhancement of reaction rates between solute molecules and their target molecules on the surface and in the vicinity of biological membrane interfaces (e.g. ligand binding to membrane proteins) has been demonstrated in several studies and explained in terms of initial nonspecific binding of the solute molecules to the membrane followed by diffusion along the surface to their target molecules (1-8). Studies on some specific membrane-bound proton pumps, for example cytochrome c oxidase or bacteriorhodopsin (9-12), have revealed higher than diffusion-limited rates of proton upta...