Bacteriorhodopsin, Molecular Biology of
583
sequence is reversed. The light-driven pro-
ton pump is highly effective: the quantum
yield for producing the K intermediate is
about 0.6, and the protons transported is
close to one/photocycle. From calorimetry,
it appears that nearly 50% of the energy
gain in the K intermediate is conserved
in the form of a transmembrane electro-
chemical difference for protons. On the
other hand, feedback mechanisms, so far
not well de±ned, appear to limit the pump
at very high proton gradients, perhaps by
shunt reactions.
6
A Family of Retinal Proteins
Bacteriorhodopsin is one of three kinds
of similar retinal proteins in halobacte-
rial membranes. Their functions are all
based on the photoisomerization of all-
trans
retinal
to
13-cis,15-anti
and
the
protein reactions that accompany its ther-
mal reisomerization. Halorhodopsin is an
inward-directed light-driven chloride ion
pump. It lacks homologues for Asp85 and
Asp96 (containing threonine and alanine
at these locations respectively), and the
retinal Schiff base does not deprotonate
during the photocycle. Sensory rhodopsins
I and II are receptors for phototactic be-
havior. A profound similarity in the mech-
anisms of these proteins with different
functions is indicated by the fact that, for
the most part, their activities are intercon-
vertible with minimal perturbations. Thus,
the D85T mutant of bacteriorhodopsin
binds chloride, exhibits a photocycle simi-
lar to that of halorhodopsin, and transports
chloride from the extracellular to the cyto-
plasmic direction. Halorhodopsin, in turn,
transports protons when the weak acid,
azide, is added. The azide binds near the
Schiff base and functions as a proton ac-
ceptor like Asp85 in bacteriorhodopsin,
and another azide molecule is the proton
donor on the cytoplasmic side, as in the
D96N mutant of bacteriorhodopsin. Sen-
sory rhodopsin I transports protons such
as bacteriorhodopsin when the transduc-
ing protein that is normally tightly bound
to it is genetically deleted.
Bibliography
Books and Reviews
Ebrey, T.G. (1993) Light Energy Transduction
in Bacteriorhodopsin, in: Jackson, M. (Ed.)
Thermodynamics of Membranes, Receptors and
Channels
, CRC Press, New York, pp. 353–387.
Haupts, U.,
Tittor, J.,
Oesterhelt, D.
(1999)
Closing in on bacteriorhodopsin: progress
in understanding the molecule,
Annu. Rev.
Biophys. Biomol. Struct.
28
, 367–399.
Lanyi, J.K., V´ar´o, G. (1995) The photocycles of
bacteriorhodopsin,
Isr. J. Chem.
35
, 365–386.
Lanyi, J.K. (2000) Molecular mechanism of ion
transport in bacteriorhodopsin: Insights from
crystallographic, spectroscopic and mutational
studies,
J. Phys. Chem. B
104
, 11441–11448.
Lanyi,
J.K.
(Ed.)
(2000)
Bacteriorhodopsin,
Biochim. Biophys. Acta
1460
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Luecke, H., Lanyi, J.K. (2002) Structural clues
to
the
mechanism
of
ion
pumping
in
bacteriorhodopsin,
Adv.
Protein
Chem.
63
,
111–130.
Maeda, A.
(1995)
Application
of
FTIR
spectroscopy to the structural study on the
function of bacteriorhodopsin,
Isr. J. Chem.
35
, 387–400.
Mathies, R.A., Lin, S.W., Ames, J.B., Pollard,
W.T. (1991) From femtoseconds to biology:
mechanism
of
bacteriorhodopsin’s
light-
driven proton pump,
A
n
n
u
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Primary Literature
Balashov, S.P.,
Imasheva, E.S.,
Govindjee, R.,
Ebrey, T.G.
(1996)
Titration
of
aspartate-
85
in
bacteriorhodopsin:
what
it
says
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