BioPHYSICAL chemistry

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).