Two progressive substrates of the M-intermediate can be identified in glucose-embedded, wild-type bacteriorhodopsin.
Glucose-embedded bacteriorhodopsin shows M-intermediates with different Amide I infrared bands when samples are illuminated at 240 or 260 K, in contrast with fully hydrated samples where a single M-intermediate is formed at all temperatures. In hydrated, but not in glucose-embedded specimens, the N intermediate is formed together with M at 260 K. Both Fourier transform infrared and electron diffraction data from glucose-embedded bacteriorhodopsin suggest that at 260 K a mixture is formed of the M-state that is trapped at 240 K, and a different M-intermediate (MN) that is also formed by mutant forms of bacteriorhodopsin that lack a carboxyl group at the 96 position, necessary for the M to N transition. The fact that an MN species is trapped in glucose-embedded, wild-type bacteriorhodopsin suggests that the glucose samples lack functionally important water molecules that are needed for the proton transfer aspartate 96 to the Schiff base (and, thus, to form the N-intermediate); thus, aspartate 96 is rendered ineffective as a proton donor.
Vonck
J
J
Life Sciences Division, Donner Laboratory, Lawrence Berkeley Laboratory, Berkeley, California 94720.
Han
B G
BG
Burkard
F
F
Perkins
G A
GA
Glaeser
R M
RM
D001436:Bacteriorhodopsins / Q000737:chemistry* / Q000235:genetics / Q000528:radiation effects; D055592:Biophysical Phenomena; D001703:Biophysics; D005947:Glucose / Q000737:chemistry; D019617:Halobacterium salinarum / Q000737:chemistry / Q000235:genetics / Q000528:radiation effects; D015394:Molecular Structure; D010777:Photochemistry; D012545:Schiff Bases / Q000737:chemistry; D017550:Spectroscopy, Fourier Transform Infrared
D012545:Schiff Bases; D001436:Bacteriorhodopsins; D005947:Glucose