BioPHYSICAL chemistry

during steps 1 and 2 (Figure 17.11). The protein then switches, due to

conformational changes, to be coupled to the other side of the membrane,

leading to the proton uptake during steps 3 and 4. The system then

resets with the reprotonation of Glu-194 and the re-isomerization of the

retinal in step 5.

Comparison of rhodopsins from different organisms

The three-dimensional structure of rhodopsin has also been solved using

X-ray diffraction and was found to be closely related to that of bacterior-

hodopsin, as expected based on the spectroscopic studies and sequence

comparisons. Whereas the 348 amino acid residues form seven long helices

384 PART 3 UNDERSTANDING BIOLOGICAL SYSTEMS USING PHYSICAL CHEMISTRY

N

O

(a) Ground state

all-trans-retinal protonated

Inside

Outside

Membrane

Light



A

B

C

E D

F

G

C

G

F

F C

G

(b) L intermediate

13-cis-retinal protonated

H

O

H

O

O

N

O

(d) N intermediate

13-cis-retinal protonated

O

H

H



O

O

N

O

Asp 96

Asp 85

Arg 82

H

OH

O

O

H

H

C

G

F

N

O

(c) Late M intermediate

13-cis-retinal neutral

H

O

H



O

O

Figure 17.10A summary of the structural changes associated with each of the intermediate states
of bacteriorhodopsin. Modified from Kuhlbrandt (2000).