Active site lysine backbone undergoes conformational changes in the bacteriorhodopsin photocycle

Results are presented demonstrating that the backbone of the active site lysine of bacteriorhodopsin undergoes light-induced structural alterations during bacteriorhodopsin-mediated light-induced proton pumping. This conclusion is based on difference Fourier transform infrared spectroscopy of isotopically labeled bacteriorhodopsin. The data demonstrate that the backbone carbonyl of lysine achieves an extremely low vibrational frequency during M₄₁₂ intermediate formation. This is preceded by a structural transition in the lysine backbone that leads to an active site lysine carbonyl with the observed low vibrational frequency, probably due to a high degree of solvation.