BioPHYSICAL chemistry

which grows in briny waters such as the Dead
Sea. This bacterium has a distinctive purple
color due to the presence of the extremely high
concentrations of retinal found in bacteri-
orhodopsin. The bacteriorhodopsin is present,
not for a visual response such as phototaxis,
but rather to convert light energy into chemical
energy in the form of proton gradients across the
cytoplasmic membrane.
Both rhodopsin and bacteriorhodopsin are pro-
teins with a simple polypeptide chain formed
by about 250 amino acid residues. Many of the
amino acid residues of these polypeptides are
conserved between rhodopsin and bacteriorho-
dopsin. As was found for rhodopsin, a retinal
binds through a Schiff base to a lysine, which is
residue 216 in bacteriorhodopsin. The photo-
cycle of bacteriorhodopsin has similarities to
that of rhodopsin, with the presence of differ-
ent spectral states that are generated on varying
lifetimes; however, there are two critical differ-
ences (Figure 17.7). First, the isomerization in
bacteriorhodopsin proceeds from transto cisin a reversible process rather
than the irreversible cis-to-transisomerization of rhodopsin that requires
the reincorporation of a new cisisomer of retinal before the protein becomes
active again. Second, the absorption of light by bacteriorhodopsin leads
to the transfer of a protein across the cell membrane to be used for the
generation of ATP from the buildup of a proton gradient.
The photocycles of rhodopsin and bacteriorhodopsin were established
with the use of transient optical spectroscopy, Fourier transform infrared
spectroscopy, and resonance Raman studies. Although the protein is large
with many overlapping vibrational bands, the vibrations associated with

CHAPTER 17 SIGNAL TRANSDUCTION 379

All-trans-retinal

N

Schiff base

Lys

Schiff base

Lys

H



Light

absorption

Proton

pumping

(a)

13-cis-retinal

N

H

13

13

15

(BR 568 nm)

Protonated

all-trans

Deprotonated

13-cis

Light

absorption

K (590 nm)

L (550 nm)

Early

M (412 nm)

Late

M (412 nm)

N (550 nm)

O (640 nm)

(b)

0.5 ms^1 μs

2 ms^70 μs

H

H

Figure 17.7Photocycle of a bacteriorhodopsin.

(a) In response to light the retinal of

bacteriorhodopsin undergoes a trans-to-cis

isomerization. (b) The isomerization process

proceeds through a number of intermediate

steps at different timescales. Unlike rhodopsin,

the cycle is fully reversible and coupled to the

transfer of a proton across the membrane.