Unidirectional proton transfer mechanism in the L→M→N sequence of bacteriorhodopsin

Calcium binding to the purple membrane: A molecular dynamics study

The purple membrane (PM) is a specialized membrane patch found in halophilic archaea, containing the photoreceptor bacteriorhodopsin (bR). It is long known that calcium ions bind to the PM, but their position and role remain elusive to date. Molecular dynamics simulations in conjunction with a highly detailed model of the PM have been used to investigate the stability of calcium ions placed at three proposed cation binding sites within bR, one near the Schiff base, one in the region of the proton release group, and one near Glu9. The simulations suggest that, of the sites investigated, the binding of calcium ions was most likely at the proton release group. Binding in the region of the Schiff base, while possible, was associated with significant changes in local geometry. Calcium ions placed near Glu9 in the interior of bR (simultaneously to a Ca(2+) near the Schiff base and another one near the Glu194-Glu204 site) were not stable. The results obtained are discussed in relation to recent experimental observations and theoretical considerations.

Computational analysis of the transient movement of helices in sensory rhodopsin II

MD simulation of sensory rhodopsin II was executed for three intermediates (ground-state, K-state, M-state) appearing in its photocycle. We observed a large displacement of the cytoplasmic side of helixF only in M-state among the three intermediates. This displacement was transmitted to TM2, and the cytoplasmic side of TM2 rotated clockwise. These transient movements are in agreement with the results of an EPR experiment. That is, the early stage of signal transduction in a sRII-HtrII complex was successfully reproduced by the in silico MD simulation. By analyzing the structure of the sRII-HtrII complex, the following findings about the photocycle of sRII were obtained: (1) The hydrogen bonds between helixF and other helices determine the direction of the movement of helixF; (2) three amino acids (Arg162, Thr189, Tyr199) are essential for sRII-HtrII binding and contribute to the motion transfer from sRII to HtrII; (3) after the isomerization of retinal, a major conformational change of retinal was caused by proton transfer from Schiff base to Asp75, which, in turn, triggers the steric collision of retinal with Trp171. This is the main reason for the movement of the cytoplasmic side of helixF.