16.27 The oxidation state determines whether there are five or six elec-
trons in the d orbitals. The five electronic levels are divided in energy
into a lower group of three and an upper group of two depending
upon coordination. The relative energies of these five levels deter-
mines the distributions of the electrons.16.28 The signals should be centered around g=2 with a width deter-
mined by the hyperfine coupling. For tyrosines the conjugated
system leads to a broad and complex signal.16.29 The EPR signal would probably be centered around g= 2 and
would depend upon the redox state of each Fe^2 +; the presence of
two electronic spins would lead to the presence of multiple peaks
in the spectrum.16.30 The EPR signal has the characteristic gvalue and linewidth of a
tyrosyl radical that is present in the protein near the iron cluster.16.31 Deoxyribonucleotides are derived from the corresponding ribo-
nucleotides by direct reduction at the 2′carbon atom of the ribose.
The reaction is driven by the action of two cysteines, one of which
forms a transient radical before formation of a disulfide bond between
the two cysteines (which is subsequently reduced by thioredoxin).
The tyrosyl radical does not play a direct role in this mechanism,
but may be involved in a regulatory or initiation role for the enzyme.CHAPTER 17
17.1 They have the membrane protein rhodopsin, which contains a
pigment that responds to light.17.2 Transducin binds to rhodopsin, releases GDP, and takes up GTP; then
transducin binds to phosphodiesterase. The phosphodiesterase–
transducin complex is active, causing cGMP to convert into GMP.
Both reactions deplete cGMP, closing channels regulated by cGMP.17.3 The initial light-induced structural change in bacteriorhodopsin is
isomerization of the retinal.17.4 The absorption spectrum of the retinal changes as it undergoes the
conformational changes of the photocycle.17.5 The retinal in both cases undergoes an isomerization but the
direction differs: cis to trans in rhodopsin and trans to cis in
bacteriorhodopsin.17.6 The isomerization and other conformational changes resulted in
characteristic shifts of the infrared spectrum.ANSWERS TO PROBLEMS 479
9781405124362_5_end.qxd 4/29/08 9:17 Page 479