BioPHYSICAL chemistry

CHAPTER 15 X-RAY DIFFRACTION AND EXAFS 341

SSR

Fe

S SR

RS

Fe

RS

SR

SR

RS

Fe

RS

123

EXAFS signal

Distance (Å)

123

EXAFS signal

Distance (Å)

Fe–S

Fe–S

Fe–Fe

Figure 15.27EXAFS data of rubredoxin and ferredoxin. Also shown are the three-dimensional
and chemical structures of the iron–sulfur clusters. Modified from Yachandra (1995).

proteins that also contain sulfur. Typically, the metal cofactor of iron–
sulfur proteins can be either a single iron, a Fe 2 S 2 cluster, or a Fe 4 S 4 cluster
with the ligands all being provided by cysteine residues. For a single-iron
protein, such as rubredoxin, only one peak is seen at 2.26 Å in the EXAFS
spectrum, associated with the nearly identical Fe–S distances of the iron
and the sulfur atoms of the coordinating residues (Figure 15.27). For a Fe 2 S 2
cluster, two peaks are observed, typically at 2.25 and 2.73 Å, arising from
the Fe–S and Fe–Fe distances respectively.