Crystal structure of the O intermediate of the Leu93→Ala mutant of bacteriorhodopsin.

The lifetime of the O intermediate of bacteriorhodopsin (BR) is extended by a factor of ∼250 in the Leu93-to-Ala mutant (BR_L93A). To clarify the structural changes occurring in the last stage of the proton pumping cycle of BR, we crystallized BR_L93A into a hexagonal P622 crystal. Diffraction data from the unphotolyzed state showed that the deletion of three carbon atoms from Leu93 is compensated by the insertion of four water molecules in the cytoplasmic vicinity of retinal.