Structural changes in bacteriorhodopsin during proton translocation revealed by neutron diffraction.

A neutron diffraction study of spectroscopic states for the light-energized proton pump bacteriorhodopsin (BR) is presented. The photocycle states BR-568 and M were generated at temperatures above 4 degrees C and were measured after trapping at--180 degrees C. In the BR-568 to M-state transition, which is known to be a key step in transmembrane proton pumping, reversible structural changes of the protein were detected. These structural alterations occur in the neighborhood of the cyclohexene ring and at the Schiff's base end of the chromophore retinal. They are interpreted as a 1-2 degree tilt of three or four of the transmembrane alpha-helices or as positional changes of four or five amino acids. The structural changes observed are inherent in the transport mechanism of bacteriorhodopsin.