1: Adv Biophys  1988;24:123-75 

Structure and function of bacteriorhodopsin.

Kouyama T, Kinosita K Jr, Ikegami A.

Institute of Physical and Chemical Research, Saitama, Japan.

Bacteriorhodopsin (bR) is a powerful light-driven proton pump. We have developed
a procedure to prepare bR-containing membrane vesicles in which a pH gradient as
large as 4 pH units can be generated and maintained in the light. Using such a
system, we have demonstrated that bR exhibits a high proton pump activity in a
wide pH region; it works well at least between pH 4 and 9.5. For the large
light-induced pH change in the external medium, the presence of a high
concentration (approximately 0.1 M) of magnesium or transition metal ion is
required. It is suggested that the influx of magnesium ion, electrically coupled
with the proton release, takes place in the light. The three-dimensional
structure of bR was studied by fluorescence energy transfer techniques. It was
shown that the retinal chromophore is located 10 A below a surface of purple
membrane. The in-plane location and orientation of retinal was also determined;
it exists in a pocket surrounded by the helices 3, 4, 5, and 6. The position of
a fluorescent probe labeled to Lys 41 was determined to be near the helix 7.
Based on the results obtained, we propose a model of the bR structure. The
dynamic structure of bR was investigated by fluorescence depolarization
techniques. It was shown that the retinal chromophore is tightly buried in a
pocket within the protein. In the presence of detergents like octylglucoside,
its tertiary structure can be stable near the electric isosbestic point. The
rate of dissociation/association process of bR molecules is sensitive to the pH
of the medium. Dimeric and/or trimeric bR can exist stably if the concentration
of detergent and other solvent conditions are adequately controlled. The
photoreaction of bR in purple membrane, including the dark/light adaptation, the
trans photocycle and the primary photoreaction, was reported. With respect to
the trans photocycle, we found that, at alkaline pH, an M-like photoproduct (NM)
is generated by excitation of a long-lived photoproduct N560 which has a major
absorption maximum near 560 nm. We suggest that, at alkaline pH, the overall
photoreaction of bR under steady illumination is approximated by the two-photon
cycle: bR570 approximately greater than M412----N560 approximately greater than
NM----bR570. With respect to the primary photoreaction, we found that the
fluorescence quantum yield of the near-infrared emission of bR was greatly
enhanced at acidic pH (approximately pH 2).(ABSTRACT TRUNCATED AT 400 WORDS)

Publication Types:
Review
Review, Academic

PMID: 3077237 [PubMed - indexed for MEDLINE]