BioPHYSICAL chemistry

Rhodopsin proteins in visual response

Much of the characterization of the visual
process has centered in rhodopsin in rod
cells. Less is known about the structure of
the photoreceptors in the cone cells. Color
vision is provided by three receptors that are
each responsive to a different color: blue,
green, and red. These receptors have been
sequenced and were found to be highly
homologous to rhodopsin. These cone re-
ceptors are modeled as having a structure
similar to rhodopsin, with a central retinal
molecule bound to a Schiff base. The absorp-
tion is tuned to different wavelengths by
differences in the amino acid residues in
the retinal-binding site. Together the photo-
receptors in the rod and cone cells provide
the information for the initial stages of visual
processing. The eyes also provide another
function; namely, they serve as light sensors
to regulate circadian rhythms of the body.
Melanopsin is a protein that is thought
to have a fold similar to rhodopsin and to
bind trans-retinal, which has an absorp-
tion maximum at 480 nm. Light results
in a trans-to-cisisomerization that triggers
a signal cascade, as found for the other
rhodopsins. Despite this similarity of the
photocycle, the cellular response is prob-
ably much different as the protein is not in
the rod or cone cells but rather is located
in special ganglion cells that are coupled to
the optical nerve. In addition to questions
concerning the response mechanism, the
reason why vertebrates respond to blue
light remains an open question.

CHAPTER 17 SIGNAL TRANSDUCTION 387

RET N

Halorhodopsin Bacteriorhodopsin

hν

Cl

δ

H

δ

Lys



H

RET

δ

N

δ

Lys

HO S115



Cl

Cl

H

RET

δ

N

δ

Lys



RET N

D85 hν

O O

δ

H

δ

Lys





H

RET

δ

N

N

OOH

δ

Lys



RET

Lys

D85

O  O

D85

Figure 17.13Comparison of the light-induced

changes for bacteriorhodopsin and halorhodopsin.

Modified from Kolbe et al. (2000)

References and further reading

Belrhali, H., Nollert, P., Royant, A. et al. (1999) Protein,
lipid, and water organization in bacteriorhodopsin
crystals: a molecular view of the purple membrane
at 1.9 Å resolution. Structure 7 , 909 –17.

Blurner, K.J. (2004) The need for speed. Nature

427 , 20 –1.

Edman, K., Nollert, P., Royant, A. et al. (1999) High-

resolution X-ray structure of an early intermediate