Half channels mediating H+ transport and the mechanism of gating in the Fo sector of Escherichia coli F1Fo ATP synthase

Abstract

H(+)-transporting F1Fo ATP synthase catalyzes the synthesis of ATP via coupled rotary motors within Fo and F1. H(+) transport at the subunit a-c interface in trans-membranous Fo drives rotation of the c-ring within the membrane, with subunit c being bound in a complex with the γ and ε subunits extending from the membrane. Finally, the rotation of subunit γ within the α3β3 sector of F1 mechanically drives ATP synthesis within the catalytic sites. In this review, we propose and provide evidence supporting the route of proton transfer via half channels from one side of the membrane to the other, and the mechanism of gating H(+) binding to and release from Asp61 of subunit c, via conformational movements of Arg210 in subunit a. We propose that protons are gated from the inside of a four-helix bundle at the periplasmic side of subunit a to drive protonation of cAsp61, and that this gating movement is facilitated by the swiveling of trans-membrane helices (TMHs) 4 and 5 at the site of interaction with cAsp61 on the periphery of the c-ring. Proton release to the cytoplasmic half channel is facilitated by the movement of aArg210 as a consequence of this proposed helical swiveling. Finally, release from the cytoplasmic half channel is mediated by residues in a complex of interacting extra-membraneous loops formed between TMHs of both subunits a and c. This article is part of a Special Issue entitled: 18th European Bioenergetic Conference.

A chemiosmotic mechanism of symport

“…Interestingly, a similar mechanism has been suggested for H + transport through the F o portion of F 1 /F o -ATPase, where an Arg residue in subunit a is postulated to facilitate deprotonation of an Asp residue in the c subunit (reviewed in refs. 61,62).…”

Fo-driven Rotation in the ATP Synthase Direction against the Force of F1 ATPase in the FoF1 ATP Synthase

“…The interior of subunit a TMHs 2-5 has been shown to provide an aqueous accessible channel for ions to enter from the periplasm to protonate cAsp-61 during ATP synthesis (17). Cross-linking experiments (18,44) indicate that protonation of this half-channel via aAsn-214 and aGln-252 promotes the rotation of TMHs 4 and 5 relative to other subunit a helices and also relative to the c-ring (Fig.…”

“…8C is believed to move aAsn-214 and aGln-252 into a position that enables the transfer of the proton to c 1 Asp-61 (17) so that c 1 Asp-61 reverts to the closed-locked position (19,20). Because this half-channel also includes His-245, Glu-219, and perhaps other residues (1), there is probably a complex network for proton movement so that the proton transferred to c 1 Asp-61 is probably not the same one that entered the half-channel to induce formation of the PPC conformation.…”

“…As the result of extensive mutational analysis and cross-linking studies (11)(12)(13)(14), subunit a is believed to fold into five TMHs, of which TMHs 2-5 form a four-helix bundle (15,16). The center of TMHs 2-5 provides an aqueous accessible half-channel in subunit a for ions to enter from the periplasm to protonate cAsp-61 during ATP synthesis (17). Cross-linking evidence shows that protonation of this half-channel causes TMHs 4 and 5 to swivel in a manner thought to facilitate proton transfer to cAsp-61 (18).…”

Interacting cytoplasmic loops of subunits a and c of Escherichia coli F 1 F 0 ATP synthase gate H + transport to the cytoplasm

“…The Ag + -sensitive substitutions on the periplasmic side of TMHs 2-5 cluster at the interior of the four-helix bundle predicted by cross-linking and could interact to form a continuous aqueous pathway extending from the periplasmic surface to the central region of the lipid bilayer (11,13,19,20). We have proposed that the movement of H + from the periplasmic half-channel and binding to the single ionized Asp61 in the c-ring is mediated by a swiveling of TMHs at the a-c subunit interface (16,(21)(22)(23)(24). This gating is thought to be coupled with ionization of a protonated cAsp61 in the adjacent subunit of the c-ring and with release of the H + into the cytoplasmic halfchannel at the subunit a-c interface.…”