18.21 The filter region has several positions that each select for a particu-
lar ion by providing a coordination from the protein that is specific
for a given ion through the outermost electrons of the ion.18.22 Different parts of the ion channel are labeled with a donor and
acceptor and the distance between these is calculated by measuring
the efficiency of energy transfer from the donor to the acceptor,
which has a dependence of 1/r^6. These data are then measured
for voltage differences to determine the voltage-induced structural
changes.18.23This receptor is a pentamer with two binding sites for acetylcholine.
The channel has approximately 5-fold symmetry and roughly
forms a cylinder with a channel 10 –20 Å in diameter. The trans-
port of ions through the channel is determined by the presence
of several negatively charged amino acids in the center of the
channel, which prevent the transport of anions and limit the trans-
port of cations by size exclusion. A structure has been determined
for a water-soluble protein that binds acetylcholine and serves as
a model for the receptor. This structure shows that acetylcholine
binds at the interface between two subunits formed by several
loops.CHAPTER 19
19.1 GFP emits a green fluorescence when irradiated with ultraviolet light.
Digestion of GFP shows that the cofactor is formed by cyclization
of residues Ser-65, Tyr-66, and Gly-67.19.2 The chromophore is formed by a modification of three residues,
65, 66, and 67.19.3 Measurement of cells grown under either aerobic or anaerobic
conditions showed that the GFP polypeptide is expressed under
both conditions but that the cofactor is formed only when oxygen
is present.19.4 The cofactor has different interactions with the surrounding pro-
tein and the molecular structure of the cofactor changes when the
amino acids (residues 65 – 67) that form the cofactor change.19.5 The two absorption bands arise from two states of the pigment as
it is formed.19.6 With Ser-65 and Gly-67, this residue forms the chromophore. Any
changes of this residue results in an altered pigment, if it is formed
at all.484 ANSWERS TO PROBLEMS
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