17.7 The initial three-dimensional crystals were not suitable for X-ray
diffraction but the two-dimensional crystals yield useful electron-
microscopy data.17.8 The phase can be determined directly since a real image can be
obtained with the diffraction image.17.9 The cubic lipid phases were used to crystallize the protein.17.10 Whereas the overall structure and the position of the retinal were
identified, the detailed mechanism of how the light-induced changes
lead to the transfer of a proton were still unknown.17.11 Water molecules are important in establishing the hydrogen-
bonding network in bacteriorhodopsin.17.12 The initial isomerization causes a proton to be transferred.17.13 The three-dimensional structures have the same overall fold with
a retinal in the center.17.14Rhodopsin has a complex photocycle, which is coupled to a variety
of membrane-associated proteins including phosphodiesterase and
transducin. Bacteriorhodopsin is a closely related bacterial protein
that also contains retinal, but performs a proton transfer rather
than undergoing structural changes that lead to alterations of the
membrane potential.17.15 Transducin is one of the components of the G-coupled process; it
binds to the light-induced structure of rhodopsin, resulting in the
release of GDP and uptake of GTP.17.16 The transfer process is coupled to a rearrangement of the protein,
which allows the movement of the chloride ion.17.17 The isomerization of retinal results in dipole movement of the
retinal, and this triggers the proton-transfer process.17.18 Whereas both cases represent light-induced switches, the
isomerization-induced dipole motion of the retinal results in the
positively charged proton and negatively charged chloride ion
moving in opposite directions.17.19 Melanopsin is a protein that is thought to have a fold similar to
rhodopsin and to bind trans-retinal. Melanopsin has an absorption
maximum at 480 nm. Light results in a trans-to-cisisomerization
that triggers a signal cascade as found for the other rhodopsins.
Despite this similarity of the photocycle, the cellular response is
probably very different as the protein is not in the rod or cone cells
but is rather located in special ganglion cells that are coupled to480 ANSWERS TO PROBLEMS
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