BIOINORGANIC CHEMISTRY A Short Course Second Edition

384 IRON-CONTAINING PROTEINS AND ENZYMES

[Fe 2 S 2 ] cluster - binding subdomains have the topology of an incomplete anti-
parallelβ - barrel. One iron atom of the Rieske [Fe 2 S 2 ] cluster is coordinated
by two cysteine residues through the S γ atom and the other is coordinated by
two histidine residues through the N δ atoms. The ligands coordinating the
cluster originate from two loops, with each loop contributing one cysteine and
one histidine ligand. See Figure 7.25 for the structures of the cytochrome b,
cytochrome f, and heme “ x ” hemes.

Figure 7.25 Structures of (A) heme b, (B) heme c and heme f, and (C) heme c i or
heme x.

N

N N

CO 2 H

N

Fe

CO 2 H

N

N

CO 2 H

N

N

Scys

Fe

Scys

protein

CO 2 H

N

N N

CO 2 H

N

Fe

Scys

protein

CO 2 H

AB

C

heme b, protoporphyrin IX, Fe(II)

and Fe(III) distal and proximal

histidine ligands

heme c, covalent connection through

two cysteine S atoms to protein chain.

heme f, covalent connection through two

cysteine S atoms (cys22, cys25) to protein chain in

b(6)f, Fe coordination through his26 Nε 2 and

backbone amide N of tyr1. Residue numbering

from PDB: 1VF5.

heme ci

heme x, covalent connection through one

cysteine S atom (cys35) to protein chain in b(6)f, no

aa residue axial ligands, water at 2.53 Å.