Complementarities and Convergence of Results in Bacteriorhodopsin Trimer Simulations

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The results published by Kandt, Schlitter, and Gerwert in a recent issue of Biophysical Journal (Kandt et al., 2004) on molecular dynamics (MD) simulations of bacteriorhodopsin (bR) trimer in a water/lipid environment represent a new and valuable step in the field of bacteriorhodopsin modeling and, more generally, in the developing field of simulations of fully integrated and functional membrane biosystems. In their work, Kandt et al. report on water dynamics in and around bacteriorhodopsin trimer as a function of time and as a function of the protonation state of the retinal moiety. Their findings complement our previously published results on simulation of bR in monomers as well as of the purple membrane (PM), comprising bacteriorhodopsin trimers explicitly hydrated in their complete native functional lipid environment (Baudry et al., 2001). The two studies exhibit differences in technical and structural respects in the type of lipids (POPC bilayer in Kandt et al., native PM lipids in Baudry et al., including squalene molecules, the removal of which leads to modified photocycle kinetics), in the force field (GROMACS in Kandt et al., CHARMM in Baudry et al.), in simulation time (5 ns in Kandt et al., 1 ns in Baudry et al.), in the MD engine used (GROMACS in Kandt et al., NAMD2 for MD simulations and CHARMM for free energy calculations in Baudry et al.), as well as in the starting structures for ground-state bR. Despite these differences, we find it extremely interesting that several of the findings previously published in Baudry et al., in particular the role of Asp-96, Arg-82, and retinal isomerization on internal water movement and water exchange with the bulk, are very similar to those reported in Kandt et al. As the results of Baudry et al. were not cited nor commented on in Kandt et al., we believe it is of interest to discuss the similarities of the two articles in this letter to exemplify the convergence, reliability, and maturity of recent advances in the field of bR molecular modeling