Selectivity of retinal photoisomerization in proteorhodopsin is controlled by aspartic acid 227.
Similarly to bacteriorhodopsin, proteorhodopsin that normally contains all-trans and 13-cis retinal is transformed at low pH to a species containing 9-cis retinal under continuous illumination at lambda > 530 nm. This species, absorbing around 430 nm, returns thermally in tens of minutes to initial pigment and can be reconverted also with blue-light illumination. The yield of the 9-cis species is negligibly small at neutral pH but increases manyfold (>100) at acid pH with a pK(a) of 2.6. This indicates that protonation of acidic group(s) alters the photoreaction pathway that leads normally to all-trans --> 13-cis isomerization. In the D97N mutant, in which one of the two acidic groups in the vicinity of the retinal Schiff base is not ionizable, the yield of 9-cis species at low pH shows a pH dependence similar to that in the wild-type but with a somewhat increased pK(a) of 3.3. In contrast to this relatively minor effect, replacement of the other acidic group, Asp227, with Asn results in a remarkable, more than 50-fold, increase in the yield of the light-induced formation of 9-cis species in the pH range 4-6. It appears that protonation of Asp227 at low pH is what causes the dramatic increase in the yield of the 9-cis species in wild-type proteorhodopsin. We conclude that the photoisomerization pathways in proteorhodopsin to 13-cis or 9-cis photoproducts are controlled by the charge state of Asp227.
Imasheva
Eleonora S
ES
Department of Physiology and Biophysics, University of California, Irvine, California 92697, USA.
Balashov
Sergei P
SP
Wang
Jennifer M
JM
Dioumaev
Andrei K
AK
Lanyi
Janos K
JK
D019943:Amino Acid Substitution / Q000235:genetics; D001216:Asparagine / Q000235:genetics; D001224:Aspartic Acid / Q000737:chemistry* / Q000235:genetics; D001436:Bacteriorhodopsins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D001665:Binding Sites / Q000235:genetics; D003624:Darkness; D020563:Gammaproteobacteria; D006863:Hydrogen-Ion Concentration; D007536:Isomerism; D008027:Light; D012172:Retinaldehyde / Q000737:chemistry*; D012243:Rhodopsin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D025604:Rhodopsins, Microbial; D012995:Solubility; D017550:Spectroscopy, Fourier Transform Infrared; D013696:Temperature
D025604:Rhodopsins, Microbial; C413548:proteorhodopsin; D001224:Aspartic Acid; D001436:Bacteriorhodopsins; D001216:Asparagine; D012243:Rhodopsin; D012172:Retinaldehyde