Biomedical Engineering Reference
In-Depth Information
17.1.2
Bacteriorhodopsin: Structure and Biological Function
Bacteriorhodopsin is the light-harvesting protein present in the plasma membrane of
Halobacterium salinarium
(2). Under anaerobic conditions the cell membrane grows purple
membrane (PM) patches in the form of a hexagonal two-dimensional (2D) crystalline lat-
tice of uniformly oriented bR trimers. These patches roughly consist of a 10:1 molar ratio
of lipids to bR (3). They are irregular in shape and have a constant thickness of 5 nm with
lateral dimensions up to 5
m. PM is stable against sunlight exposure in the presence of
oxygen for several years. In water, its stability holds for temperatures over 80°C, pH val-
ues ranging from 0 to 12, and in the presence of high ionic concentrations (up to 3 M NaCl)
(4). PM preserves its color and photochemical activity under dry conditions and can with-
stand temperatures up to 140°C (5).
The crystalline structure is the root of bR's chemical and thermal stability. The bR mol-
ecules convert sunlight into chemical energy by transporting protons from the cytoplasm
to the cell exterior. Each bR molecule consists of 248 amino acid residues that are arranged
in seven
-helices (6) (Figure 17.1). A retinal residue is bound to lysine-216 (K216) via a
protonated Schiff-base linkage (8). All bR molecules within the patch are uniformly ori-
ented, where their carboxy termini are located in the cytoplasm and amino termini are
located in the periplasm. The seven transmembrane helices enclose an interhelical region
that contains the retinylidene residue, key amino-acid residues, and water molecules (9).
The amino acids in this region influence absorption properties of the chromophore and
control the proton transfer pathway.
Cytoplasmic side
G
Asp
96
A
F
C
B
Schiff
base
D
Asp
85
E
Arg
82
Asp
212
FIGURE 17.1
Structural model of bacteriorhodopsin. The
seven transmembrane helices are labeled A
to G. The retinal Schiff base and the amino
acids most relevant for proton transport,
Asp96, Asp85, Asp212, and Arg82, are shown
explicitly. (From http:// www.ks. uiuc. edu/
Research/newbr/br_fig.html (accessed Nov-
ember 10, 2005.)
Extracellular side
