Mechanisms of inward transmembrane proton translocation

Proton Release Reactions in the Inward H+ Pump NsXeR

“…At the same time, no large-scale movements of helices are observed. The absence of large structural rearrangements was interpreted as a key step for inward proton transport, as it maintains a hydrophobic gate on the cytoplasmic side and prevents proton backflow . In Ns XeR, where F174 is located in this position, a similar flexibility may be observed, resulting in different structural changes as compared to Hs BR.…”

“…In this regard, our MD simulations on the F174W variant of NsXeR indeed show a larger shift on helix F (Figure S21) but also a conformational flip of tryptophan 174 (Figure S24), consistent with recent structural data. 55 Notably, we did not observe any changes in the spectrally broad continuum band feature after proton release, as it was found for HsBR in the frequency range above 1750 cm −1 and interpreted as an excess proton shared in a hydrogen-bonded network of protonatable residues including water molecules. 49 Close inspection of the spectral region below 1760 cm −1 reveals a broad positive absorption feature, which is, in the difference spectra of wild-type NsXeR, obscured by strong bands at 1741, 1725, and 1711 cm −1 (Figure 3).…”

“…In Ns XeR, where F174 is located in this position, a similar flexibility may be observed, resulting in different structural changes as compared to Hs BR. In this regard, our MD simulations on the F174W variant of Ns XeR indeed show a larger shift on helix F (Figure S21) but also a conformational flip of tryptophan 174 (Figure S24), consistent with recent structural data …”

“…Its reprotonation occurs during the MI/MII transition (Figure ). This is different as compared to other inward proton pumps, where the homologous residue is deprotonated in the ground state. , The elevated p K a of D220 of dark-state Ns XeR may be explained by the interaction with H48 (Figure S25 and Table S2). As a consequence of this observation, the RSB proton must be transferred to another primary binding site.…”

“…The absence of large structural rearrangements was interpreted as a key step for inward proton transport, as it maintains a hydrophobic gate on the cytoplasmic side and prevents proton backflow. 55 In NsXeR, where F174 is located in this position, a similar flexibility may be observed, resulting in different structural changes as compared to HsBR. In this regard, our MD simulations on the F174W variant of NsXeR indeed show a larger shift on helix F (Figure S21) but also a conformational flip of tryptophan 174 (Figure S24), consistent with recent structural data.…”