BioPHYSICAL chemistry

The transition from the dark state to the initial K state happens when
light is absorbed. During the transition from K to L, proton transfer occurs
due to change in the retinal-binding site and a small movement of helix
C that shifts Asp-85 toward the retinal. The proton associated with the
retinal is transferred to Asp-85 as facilitated by Tyr-89 and Asp-212. In
the transition from L to M, the deprotonation causes the retinal to shift,
consequently moving helices F and G. During the M-to-N transition, the
retinal is reprotonated by Asp-96 and a movement of helices F and G
opens a channel that causes Asp-96 to be reprotonated. The protein then
relaxes back to the dark state.
The structure then acts as a proton switch, with one side of the
membrane well coupled to the retinal, leading to the early proton release

CHAPTER 17 SIGNAL TRANSDUCTION 383

402

401

HN

Protonated Schiff base,

all-trans-retinal

(a) (b)

Thr-89

Tyr-185

Tyr-57

Asp-212

Asp-85

OO

O

HO

Glu-194

Glu-204

HO

O

OH

C

C





407

406

403

404 405

Arg-82 NH 2

NH

HOO

NH 2

C

C

O

OH

H

C N

OO

C





401

N ̈

Unprotonated Schiff base,

13-cis-retinal

Trp-86 Tyr-185

Tyr-57

Asp-212

Asp-85

O

O

HO

HO

 C

407

(^405404)
Arg-82
Glu-194
Tyr-83
NH 2
NH
NH 2
C
OH O O
C

 Glu-204
OO
C

Figure 17.9The structure of bacteriorhodopsin, as determined by X-ray diffraction, revealed many
details about the proton pathway, including the presence of bound water molecules that complete
the pathway. The positions of the amino acid residues and waters change in response to the
isomerization. Modified from Belrhali et al. (1999).