Formation of a Long-Lived Photoproduct with a Deprotonated Schiff Base in Proteorhodopsin, and Its Enhancement by Mutation of Asp227

Proteorhodopsin, a retinal protein of marine proteobacteria similar to bacteriorhodopsin of the archaea, is a light-driven proton pump. Absorption of a light quantum initiates a reaction cycle (turnover time of ca. 50 ms), which includes photoisomerization of the retinal from the all-trans to the 13-cis form and transient deprotonation of the retinal Schiff base, followed by recovery of the initial state. We report here that in addition to this fast cyclic conversion, illumination at high pH results in accumulation of a long-lived photoproduct absorbing at 362 nm. This photoconversion is much more efficient in the D227N mutant in which the anionic Asp227, which together with Asp97 constitutes the Schiff base counterion, is replaced with a neutral residue. Upon illumination at pH 8.5, most of the D227N pigment is converted to the 362 nm species, with a quantum efficiency of ca. 0.2. The pK a for this transition in the wild type is 9.6, but decreased to 7.5 after mutation of Asp227. The short wavelength of the absorption maximum of the photoproduct indicates that it has a deprotonated Schiff base. In the dark, this photoproduct is converted back to the initial pigment with a time constant of 30 min (in D227N, at pH 8.5), but it can be reconverted more rapidly by illumination with near-UV light. Experiments with “locked” retinal analogues which selectively exclude rotation around either the C9C10, C11C12, or C13C14 bond show that formation of the 362 nm species involves isomerization around the C13C14 bond. In agreement with this, retinal extraction indicates that the 362 nm photoproduct is 13-cis whereas the initial state is predominantly all-trans. A rapid shift of the pH from 8.5 to 4 greatly accelerates thermal reconversion of the 362 nm species to the initial pigment, suggesting that its recovery involving the thermal isomerization of the chromophore is controlled by ionizable residues, primarily the Schiff base and Asp97. The transformation to the long-lived 362 nm photoproduct is apparently a side reaction of the photocycle, a response to high pH, caused by alteration of the normal reprotonation and reisomerization pathway of the Schiff base.