an exterior surface that has significant electrostatic character, which serves
to interact with a chloride ion, allowing putative identification of the
chloride channel. Unlike ion channels, there is no open channel in the
protein but rather the pathway opens during the photocycles.
Retinal binds halorhodopsin at a site very similar to that of bacteri-
orhodopsin (Figure 17.13). Nine of the surrounding residues are conserved
and water is found, as was true for bacteriorhodopsin. A single chloride
ion was found near the retinal at a location corresponding to that of
Asp-85. Ion translocation is proposed to occur through a mechanism
related to that of bacteriorhodopsin. The ion is proposed to be driven by
ion–dipole interactions involving the NH group of the retinal before it is
released toward the cytoplasm and a new chloride ion enters the transport
site. Only the replacement of the negatively charged Asp-85 as a proton
acceptor in bacteriorhodopsin by chloride in halorhodopsin changes the
kinetic preference and therefore the ion specificity. Thus in both cases
the retinal serves as a switch for the movement of the proton or ion.
386 PART 3 UNDERSTANDING BIOLOGICAL SYSTEMS USING PHYSICAL CHEMISTRY
His kinase
Regulator
P
Regulator
Regulator
Methylation
helices
CYTOPLASM
H
BR HR
SRI
Htrl
SRII
HtrII
Cl
P
Flagellar motor
His kinase
Figure 17.12Four archad rhodopsins: bacteriorhodopsin (BR), halorhodopsin (HR), and the
phototaxis receptors sensory rhodopsins (SRI and SRII). The sensory rhodopsins are complexed to
the cognate transducer proteins (HtrI and HtrII). Modified from Spudrich et al. (2000).