Channels proton translocation channel

A hydrophobic complex of three or four proteins (also called proteolipids), or Fq complex, located in the inner membrane, which is thought to contain a proton-translocating channel and... 

Protons reentering the cell via a reverse proton flow through the same proton-translocating channel... 

Figure 5.22 Components of the proton-translocating unit of ATP synthase (a, b) and the proton path through the membrane (c). Each proton enters the cytosolic half-channel, follows a complete rotation of the c ring, and exits through the other half channel into the matrix. (From Berg et al., 2002. Reproduced with permission from W.H. Freeman and Co.)...

Suzuki, T., Ueno, H., Mitome, N., Suzuki, J., and Yoshida, M. (2002). F(0) of ATP synthase is a rotary proton channel Obligatory coupling of proton translocation with rotation of c-subunit ring./. Biol. Chem. 277, 13281-13285. 

The compound dicyclohexylcarbodiimide (DCCD) inhibits proton translocation through F0 by reacting with the carboxyl group of a single glutamate residue in the channel-forming loop of the proteolipid. 

Tight coupling between ATP synthesis and proton translocation is dependent on the impermeability of the membrane to protons, so that the F0 channel and ATP synthesis provide the only way for protons to reenter the mitochondrial matrix. Physical damage to the membranes, or chemicals that allow the dissipation of the proton or electrical potential gradient, will allow alternative pathways for reentry of protons, and will uncouple respiration from ATP synthesis (see Problem 14.5). 

There are three main reasons to suggest a specific function of subunit III in proton translocation. First, Casey et al. [171] showed that modification of this subunit with dicyclohexylcarbodiimide (DCCD) blocks proton translocation, but has little effect on electron transfer. Similar results have been obtained with the reconstituted oxidase from the thermophilic bacterium PS3 [164]. Prochaska et al. [160] showed that DCCD binds mainly to Glu-90 of the bovine subunit III, which is predicted to lie within the membrane domain and hence to be a site analogous to the DCCD binding site in the membranous fj, sector of the ATP-synthase (Fig. 3.8 see also Ref. 85). Since the latter is a part of a proton-conducting channel in ATP synthase, subunit III was thought to have the same function. However, there is one essential difference between the two phenomena. Modification of the membranous glutamic residue in by DCCD leads also to inhibition of ATP hydrolysis in the complex, as expected for two linked reactions. In contrast, DCCD has little or no effect on electron transfer in cytochrome oxidase under conditions where H translocation is abolished. Hence, DCCD cannot simply be judged to block a proton channel in the oxidase. More appropriately, it decouples proton translocation from electron transfer. 

Two major ATP synthesizing reactions in living organisms are oxidative phosphorylation and photophosphorylation. Both reactions take place in H -ATPase (FqF,), which is driven by an electrochemical potential difference of protons across the biomembrane, as predicted by Mitchell [1]. In Racket s laboratory, ATPases related to oxidative phosphorylation were prepared, but their relationship to Mitchell s chemiosmotic hypothesis [1] was not described [2], Later, an insoluble ATPase (H -ATPase) was shown to translocate protons across the membrane when it was reconstituted into liposomes [3], H -ATPase was shown to be composed of a catalytic moiety called F, (coupling factor 1) [4], and a membrane moiety called Fq [5], which confers inhibitor sensitivity to F,. F was shown to be a proton channel, which translocates down an electrochemical potential gradient across the membrane when Fg is reconstituted into liposomes (Fig. 5.1) [6]. Thus, -ATPase was called FqFj or ATP synthetase. 

The subunit domain a b2 Ci2 in Fq is considered to be the site for proton translocation, but the details of the mechanism remain to be explored. It is generally agreed, however, that the a- and c-subunits mediate proton translocation, while the b-subunit only acts as a structural element to link F, (a3 p3) with Fq (the a-subunit). It is also known from the study of Schneider and Altendorf that all three subunits, a, b and c, are required for an active proton channel reconstituted in E. coli ATP synthase. In their experiment, these authors first dissociated, separated and purified the individual subunits and then integrated the subunits in all possible combinations into phospholipid vesicles. Each assembly was then tested for proton-translocation activity as well as its ability to bind to Fi, and it was found that functional activity could only be achieved by the combination a b2 Cio, the same combination that exists in native Fq. 

Fig. 41. Model for proton translocation and generation of torque in Fq. a ring of 12 c-subunits is in contact with the large a-subunit. The a-subunIt carries two access channels for protons on the two sides of the membrane. Figure source Junge, Lill and Engelbrecht(1997) A TP synthase an electrochemical transducer with rotatory mechanics. Trends in Blochem Sci 22 422.

The third alternative is proton exchange along hydrogen-bonded water molecules (33-35). In bacteriorhodopsin, for example, a recent structural model at 3.5-A resolution strongly suggests that water molecules form a narrow channel and are involved in proton delivery to the chromophore (36). The remainder of this review will discuss chains of hydrogen-bonded water molecules as potential proton translocators and describe some initial tests of the concept. 

---