576
Bacteriorhodopsin, Molecular Biology of
Photocycle
Reaction sequence initiated by photoisomerization of the retinal of bacteriorhodopsin.
Photointermediates
States of bacteriorhodopsin that arise transiently in the photocycle, identiFable through
their distinct spectral properties and from the structure of the protein and the retinal.
Retinal Schiff Base
Covalent bond between the C
15
atom of the retinal and the side-chain amino nitrogen
(NZ) of Lys216, with the nitrogen normally in the protonated state.
Schiff Base Counterion
Complex of ionic side chains and bound water with a net negative charge that
compensates the positive charge of the protonated Schiff base.
Transmembrane Helix
An
α
-helical polypeptide segment that traverses the lipid bilayer, a typical structural
motif in membrane proteins.
Ion Pump
Membrane protein that utilizes a chemical or photochemical reaction to drive the
energetically uphill translocation of an ion across the membrane.
Chemiosmotic Coupling
General biological principle in which pumps generate a transmembrane proton
gradient, and the protons moving down the gradient through speciFc membrane
proteins provide energy for a variety of metabolically useful reactions.
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Bacteriorhodopsin is a light-driven pump that generates an electrochemical gradient
for protons across the membrane. It is unique among such transporters, widespread
in biology, in being a small, seven-transmembrane helical protein. Its remarkably
simple transport mechanism is driven by the thermal reisomerization of the
photoisomerized 13-
cis
retinal to all-trans. The ease of performing static and
time-resolved spectroscopy of many kinds, as well as the propensity of the
protein for crystallization, have generated a large amount of valuable information
about the transformations of the retinal and the protein during the transport
cycle.
The
data
describe
how
the
motions
of
the
retinal
cause
structural
changes that result in proton transfers between donor and acceptor groups
inside the protein and in the release and uptake of a proton at the opposite
membrane surfaces. As a result, today, bacteriorhodopsin is the best-understood
ion pump.
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