Subdomains in the F and G Helices of Bacteriorhodopsin Regulate Structural Changes

Replacement of aspartate 85 (in helix C) with an asparagine (D85N) isolates a subset of the conformational transitions accessed during proton translocation[3]. In the absence of photoexcitation, three spectrally distinct species exist in equilibrium and their relative concentrations are regulated by pH. At neutral pH, the protein exists in an O-like state (λmax = 615 nm, appears blue). As the pH is raised, an N(λmax = 570 nm, appears purple) and M-like state (λmax, 410 nm, appears yellow) become populated. The transitions between M, N, and O reset the major structural change and proton binding site pKa’s.