A LOCAL ELECTROSTATIC CHANGE IS THE CAUSE OF THE LARGE-SCALE PROTEIN CONFORMATION SHIFT IN BACTERIORHODOPSIN

During light-driven proton transport bacteriorhodopsin shuttles betwee n two protein conformations, A large-scale structural change similar t o that in the photochemical cycle is produced in the D85N mutant upon raising the pH, even without illumination, We report here that (i) the pK(a) values for the change in crystallographic parameters and for de protonation of the retinal Schiff base are the same, (ii) the retinal isomeric configuration is nearly unaffected by the protein conformatio n, and (iii) preventing rotation of the C-13-C-14 double bond by repla cing the retinal with an all-trans locked analogue makes little differ ence to the Schiff base pK(a). We conclude that the direct cause of th e conformational shift is destabilization of the structure upon loss o f interaction of the positively charged Schiff base with anionic resid ues that form its counter-ion.