PARTICIPATION OF BACTERIORHODOPSIN ACTIVE-SITE LYSINE BACKBONE IN VIBRATIONS ASSOCIATED WITH RETINAL PHOTOCHEMISTRY

Bacteriorhodopsin (bR) has been biosynthetically prepared with lysine deuterated at its alpha-carbon (C-alpha-H). The labeled membranes containing bR were investigated by difference Fourier transform infrared (FTIR) spectroscopy. It has been derived from K/bR and M/bR difference spectra (K and M are photocycle intermediates) that several bands previously assigned to the retinal chromophore are coupled to the C-alpha-H. The vibrational modes that exhibit this coupling are principally associated with C15-H and N-H vibrations. [C-alpha-H-2]Lysine-labeled bR was fragmented enzymatically, and bR structures were regenerated with the C-alpha-H-2 label either on lysine-216 and -172 or on the remaining five lysine residues of the protein. FTIR studies of the regenerated bR system, together with methylation of all lysines except the active-site lysine, reveal that the changes observed due to backbone labeling arise from the active-site lysine. The intensity of the C15-H out-of-plane wag is interpreted as a possible indication of a twist around the C15 = N bond.