Energy transduction in transmembrane ion pumps

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Recent crystallographic structures of three different ion pumps provide a first view of the mechanisms by which these molecular machines transfer ions across cell membranes against an electrochemical gradient. Each of the structures reinforces the concept that several buried counter ions have central roles in substrate recruitment, substrate binding and energy transduction during ion pumping. The spatial organization of the counter ions suggests that, initially, one or more counter ions lowers the Born energy cost of binding a substrate ion in the low-dielectric interior of the membrane. Subsequently, a ligand-induced conformational change seems to close a charged access gate to prevent backflow from a subsequent, low-affinity state of the pump. A final role of the buried counter ions might be to couple the input of external energy to a small charge separation between the substrate ion and the buried counter ions, thereby decreasing the binding affinity for the substrate ion in preparation for its release on the high-energy side of the membrane.

Section snippets

Experimental description of bacteriorhodopsin

The bacteriorhodopsin photocycle generates a proton-motive force 21, 22 over a wide range of ambient pH values. When Asp85 is protonated at low pH, however, the wild-type protein acts as a Cl− ion pump [23], as does its homologous membrane protein, halorhodopsin, under physiological conditions [24]. Indeed, when Asp85 is replaced by the polar but uncharged residues threonine or serine, the resulting single-mutant proteins also function as Cl− ion pumps 25, 26, 27. These observations thus

Anion binding in the bR(D85S) mutant and halorhodopsin

Binding of a substrate anion by bR(D85S) produces a complex arrangement of counter ions that resembles the arrangement found in the wild-type protein 2, 28. It is therefore not surprising that anion binding also induces a repacking of helices on the extracellular side of the protein that brings their hydrogen-bond interactions and tertiary structure closer to that of the resting state of bacteriorhodopsin. Substrate binding thus seems to induce an autonomous conformational change that closes

The Born energy for substrate binding is prepaid by buried counter ions

It makes intuitive sense for the substrate-binding site of a membrane pump to be located at a relatively deep, internal site of the protein 3, 4, 11, 12, 46. For an ion pump, however, this design poses a potential problem in terms of the high energy cost of burying the substrate ion in a low-dielectric environment. As explained further in Box 1, however, the formation of an ion pair between the substrate ion and a previously buried counter ion gives back the same amount of energy as it costs to

Access to the internal binding pocket is controlled by a counter ion

In the anion-bound state of halorhodopsin [3], as well as in the anion-bound state of bR(D85S) [2], there is no solvent-accessible tunnel that connects the binding site to the aqueous surface of the pump. Instead, access to the binding site is physically blocked by a conserved arginine residue (Figure 3a,b), as it is in the resting state of wild-type bacteriorhodopsin (not shown). What is surprising, however, is the fact that there still is no solvent-accessible tunnel that extends into the

Charge separation is likely to raise the potential of ionic substrates

Because as yet there is no high-resolution crystal structure of a photoproduct of the anion-bound bR(D85S) mutant, we can construct only a preliminary model of how photoisomerization is likely to raise the energy of the anionic substrate in the binding pocket. We feel that it is reasonable to assume that the reaction coordinate for the photocycle of the anion-bound mutant protein, when expressed in structural terms, cannot change substantially owing to the substitution of serine for aspartate

Concluding remarks

We have discussed recent advances in the macromolecular crystallization of transmembrane ion pumps and have shown how these structures both provide a detailed understanding of the molecular nature of previously abstract concepts, such as gates, intramembrane binding sites, and open and closed conformations, and suggest new concepts such as that of ‘Born energy chaperones’.

Specifically, the structural information that is now available for bacteriorhodopsin and some of its mutants has been used

Note added in proof

Two new crystal structures for the Ca2+-ATPase appeared [57] while this review was in proof. These structures add information about the rather large conformational change that occurs (i) when a nonhydrolyzable ATP analog is bound (a change that is described as closing the door against Ca2+-substrate backflow) and (ii) the very small, but important, further structural change that occurs upon binding a presumed transition state analog for the formation of the high-energy phosphorylated

Acknowledgements

This work has been supported, in part, by a grant (GM51487) from the National Institutes of Health.

References (57)

  • H. Belrhali

    Protein, lipid and water organization in bacteriorhodopsin crystals: a molecular view of the purple membrane at 1.9 Å resolution

    Structure Fold. Des.

    (1999)
  • M.T. Facciotti

    Structure of an early intermediate in the M-state phase of the bacteriorhodopsin photocycle

    Biophys. J.

    (2001)
  • J. Lanyi et al.

    Crystallographic structure of the retinal and the protein after deprotonation of the Schiff base: the switch in the bacteriorhodopsin photocycle

    J. Mol. Biol.

    (2002)
  • J.K. Lanyi et al.

    Mechanism of proton transport in bacteriorhodopsin from crystallographic structures of the K, L, M1, M2, and M2′ intermediates of the photocycle

    J. Mol. Biol.

    (2003)
  • K. Edman

    Deformation of helix C in the low temperature L-intermediate of bacteriorhodopsin

    J. Biol. Chem.

    (2004)
  • F.M. Betancourt et al.

    Chemical and physical evidence for multiple functional steps comprising the M state of the bacteriorhodopsin photocycle

    Biochim. Biophys. Acta

    (2000)
  • B.G. Han

    The bacteriorhodopsin photocycle: direct structural study of two substrates of the M-intermediate

    Biophys. J.

    (1994)
  • J. Vonck

    Two progressive substrates of the M-intermediate can be identified in glucose-embedded, wild-type bacteriorhodopsin

    Biophys. J.

    (1994)
  • F.M. Hendrickson

    Structural characterization of the L-to-M transition of the bacteriorhodopsin photocycle

    Biophys. J.

    (1998)
  • S. Rouhani

    Crystal structure of the D85S mutant of bacteriorhodopsin: model of an O-like photocycle intermediate

    J. Mol. Biol.

    (2001)
  • T.A. Steitz

    Quantitative application of the helical hairpin hypothesis to membrane proteins

    Biophys. J.

    (1982)
  • B. Forbush

    Rapid release of 42K or 86Rb from two distinct transport sites on the Na,K-pump in the presence of Pi or vanadate

    J. Biol. Chem.

    (1987)
  • M. Kolbe

    Structure of the light-driven chloride pump halorhodopsin at 1.8 Å resolution

    Science

    (2000)
  • C. Toyoshima

    Crystal structure of the calcium pump of sarcoplasmic reticulum at 2.6 Å resolution

    Nature

    (2000)
  • C. Toyoshima et al.

    Structural changes in the calcium pump accompanying the dissociation of calcium

    Nature

    (2002)
  • D. Stock

    Molecular architecture of the rotary motor in ATP synthase

    Science

    (1999)
  • G. Chang et al.

    Structure of MsbA from E. coli: a homolog of the multidrug resistance ATP binding cassette (ABC) transporters

    Science

    (2001)
  • K.P. Locher

    The E. coli BtuCD structure: a framework for ABC transporter architecture and mechanism

    Science

    (2002)

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