Mediation mechanism of tyrosine 185 on the retinal isomerization equilibrium and the proton release channel in the seven-transmembrane receptor bacteriorhodopsin

• Direct contact of tyrosine 185 with the Ret chromophore in the DA purple membrane has been observed for the first time by ssNMR.
• Tyrosine 185 both maintains the isomerization equilibrium of the Ret chromophore and retains aspartic acid 212 in a favorable conformation.
• Removal of the phenolic hydroxyl group in the Y185F mutant causes a change in the M state, loss of the O state and elongation of the pumping cycle time.

Electrostatic coupling leading to conformational changes in proteins is challenging to demonstrate directly, it requires that both the local, discrete electronic details and dynamic information relevant to the functional descriptions are probed. Here, as a novel study to address this challenge, the roles of an aromatic residue in influencing the functional conformational changes of a membrane receptor in its natural membrane environment are reported. Previously intractable discrete electronic details have been obtained using 2D solid-state NMR of specifically labelled receptor, reinforced with molecular dynamic simulations, mutational analysis and functional assays, supported by and compared with rigid-atom crystal structural models. Hydrogen bonding and hydrophobic interactions are identified as the mechanistic origin for direct electromechanical coupling to the dynamics of conformational changes within the receptor.