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Figure 1.
Figure 1. Crystal structures of FdI mutants showing His 2 and
His 25 contacts, solvent accessibility and electron density.
Stereo diagrams showing contacts <4 Å from His N atoms to
[Fe−S] clusters and hydrogen bonds (green dotted lines).
Hydrogen bonds involving water molecules are gray dotted lines.
The corresponding Phe residues in native FdI are shown based on
least squares superposition of native FdI onto each mutant FdI.
Atoms are colored orange (Fe), yellow (S), red (O), blue (N),
purple (His C) and gray (C). a, The F2H mutant structure refined
at 1.62 Å resolution. Contacts <4 Å from His 2 atoms
to sulfur atoms of the [[4Fe−4S](Cys[4])]^2- center include N
 1−S
(3.64 Å) and C  −S
(3.63 Å). Both conformers of Glu 46 are shown. b, The F25H
mutant structure refined at 1.75 Å resolution. Contacts <4
Å from His 25 atoms to sulfur atoms of the
[[4Fe−4S](Cys[4])]^2- center are N  epsilon
2−S (3.80 Å) and C epsilon
1−S (3.68 Å), from His 25 atoms to Cys 20 N 1−S
(3.88
Å) and C epsilon
1−S (3.79
Å), and from His 25 atoms to Cys 16
([[3Fe−4S](Cys[3])]^2- center) N epsilon
2−S (3.74
Å) and C 2−S
(3.94
Å). The solvent-accessible surface of the c, FdI F2H and
d, F25H mutants. The electron density of each His residue is
also shown. Atoms and their corresponding surfaces are colored
as in (a,b). His 2 is completely buried except for a small
portion of C epsilon
1 (purple dots in (c)). His 25 is also completely buried except
for a contact of N 1
to a water molecule as shown in (b). Electron density maps are
calculated with  [A]
coefficients and contoured at 1.5 .
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