Light-driven proton translocation by bacteriorhodopsin reconstituted with the phenyl analog of retinal

The properties of bacteriorhodopsin in which the natural cofactor, retinal (Fig. 1, I), has been replaced by the synthetic analog, phenyl retinal (Fig. 1, II), have been studied. Phenyl retinal binds at the same site as retinal and supports light-dependent transmembrane proton translocation in phospholipid vesicles that contain bacterio-opsin. This result allows us to rule out proton abstraction from the beta-ionone ring of retinal as a step in the catalytic cycle. Furthermore, phenyl retinal, a planar molecule, binds to the apomembrane more rapidly than retinal, which suggests that the rate-determining step in the binding of retinal itself may be the attainment of a planar molecular configuration. The new chromophore undergoes a pH-dependent spectral shift that is not observed in the native membrane.