Bound water in the proton translocation mechanism of the haem‐copper oxidases

Abstract

We address the molecular mechanism by which the haem-copper oxidases translocate protons. Reduction of O2 to water takes place at a haem iron-copper (CUB) centre, and protons enter from one side of the membrane through a 'channel' structure in the enzyme. Statistical-mechanical calculations predict bound water molecules within this channel, and mutagenesis experiments show that breaking this water structure impedes proton translocation. Hydrogen-bonded water molecules connect the channel further via a conserved glutamic acid residue to a histidine ligand of CuB. The glutamic acid side chain may have to move during proton transfer because proton translocation is abolished if it is forced to interact with a nearby lysine or arginine. Perturbing the CuB ligand structure shifts an infrared mode that may be ascribed to the O-H stretch of bound water. This is sensitive to mutations of the glutamic acid, supporting its connectivity to the histidine. These results suggest key roles of bound water, the glutamic acid and the histidine copper ligand in the mechanism of proton translocation.

Proton-pumping mechanism of cytochrome c oxidase: A kinetic master-equation approach

“…But the observed gating mechanism matches the predictions of the water-gated model of proton pumping by CcO [21] and more recent analyses of the pump mechanism [9, 65, 66]. In the water-gated model, a branched chain of water molecules inside a hydrophobic cavity next to the BNC [19, 67, 68] is thought to serve as an efficient proton conducting wire [69]. With heme a reduced, in molecular dynamics (MD) simulations the water-chain was found to be oriented from Glu 242 to the pump site [21], so that a proton from Glu 242 is easily transferred to the pump site.…”

Expanding the view of proton pumping in cytochrome c oxidase through computer simulation

“…Glu286, which lies at the end of the D-pathway [95, 96] and is near to the hydrophobic cavity in subunit I, is highly conserved in different forms of C c O. Mutations of this residue also significantly inhibit, if not block completely, enzymatic activity [4, 26, 41, 97]. Thus, it is considered that Glu286 is an essential residue for proton pumping in C c O.…”

“…Glu286 has been thought to transport the proton from the D-pathway to the hydrophobic cavity by a dihedral angle rotation of its side chain [52, 95]. From the PMF for this rotation, the reorientation of Glu286 from the “down” state (~75°) to the “up” state (~295°) crosses a 6 kcal/mol barrier.…”

Cytochrome c oxidase: exciting progress and remaining mysteries

“…2). Water is a critical component of each of the proton pathways leading from Glu286 to the proton acceptor in the pump pathway and to the enzyme active site (chemical protons), and it is postulated that either the presence/absence of water or the orientation of the water molecules determines which pathway is “open” and which is “closed” (Riistama et al 1997; Wikström et al 2003; Zheng et al 2003). It appears that the pathways for pumped protons and chemical protons alternate in being open and closed, providing a clear temporal separation for the flow of pumped protons and chemical protons.…”