Photocycle and Vectorial Proton Transfer in a Rhodopsin
from the Eukaryote <i>Oxyrrhis marina</i>
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Abstract
Retinylidene photoreceptors are ubiquitously
present in marine
protists as first documented by the identification of green proteorhodopsin
(GPR). We present a detailed investigation of a rhodopsin from the
protist <i>Oxyrrhis marina</i> (OR1) with respect to its
spectroscopic properties and to its vectorial proton transport. Despite
its homology to GPR, OR1’s features differ markedly in its
pH dependence. Protonation of the proton acceptor starts at pH below
4 and is sensitive to the ionic conditions. The mutation of a conserved
histidine H62 did not influence the p<i>K</i><sub>a</sub> value in a similar manner as in other proteorhodopsins where the
charged histidine interacts with the proton acceptor forming the so-called
His-Asp cluster. Mutational and pH-induced effects were further reflected
in the temporal behavior upon light excitation ranging from femtoseconds
to seconds. The primary photodynamics exhibits a high sensitivity
to the environment of the proton acceptor D100 that are correlated
to the different initial states. The mutation of the H62 does not
affect photoisomerization at neutral pH. This is in agreement with
NMR data indicating the absence of the His-Asp cluster. The subsequent
steps in the photocycle revealed protonation reactions at the Schiff
base coupled to proton pumping even at low pH. The main electrogenic
steps are associated with the reprotonation of the Schiff base and
internal proton donor. Hence, OR1 shows a different theme of the His-Asp
organization where the low p<i>K</i><sub>a</sub> of the
proton acceptor is not dominated by this interaction, but by other
electrostatic factors