Weakened coupling of conserved arginine to the proteorhodopsin chromophore and its counterion implies structural differences from bacteriorhodopsin

In wild-type proteorhodopsin (pR), titration of the chromophore's counterion Asp97 occurs with a pKa of 8.2±0.1. R94C mutation reduces this slightly to 7.0±0.2, irrespective of treatment with ethylguanidinium. This contrasts with the homologous archaeal protein bacteriorhodopsin (bR), where R82C mutation was previously shown to elevate the pKa of Asp85 by ∼5 units, while reconstitution with ethylguanidinium restores it nearly to the wild-type value of 2.5. We conclude there is much weaker electrostatic coupling between Arg94 and Asp97 in the unphotolyzed state of pR, in comparison to Arg82 and Asp85 in bR. Therefore, while fast light-driven H+ release may depend on these two residues in pR as in bR, no tightly conserved pre-photolysis configuration of them is required.