BIOINORGANIC CHEMISTRY A Short Course Second Edition

histidine ligations effectively crosslink the B and D helices. The unusually
connected heme x (see Figure 7.25C ) is covalently linked to the cytochrome
b protein via a thioether bond to an invariant (among many cytochrome b(6)f
species) cys35, a residue in helix A of cytochrome b 6. Heme x differs in at least
two ways from c - type cytochromes: (1) It features only one rather than the
normal two thioether linkages to protein cysteines, and (2) it has no orthogonal
histidine ligand to the heme ’ s iron center but rather appears to be linked via
a water molecule hydrogen - bonded to a propionate side chain of heme b n and
to the backbone amide nitrogen of invariant gly38 within cytochrome b 6.
Heme x, having a vacant sixth coordination position, is believed to carry a
ferric, high - spin iron ion that should yield an EPR signal. Of the four hemes,
heme x is positioned nearest the stromal (n) side of the membrane in a nearly
perpendicular orientation to b n above it. Selected bond distances are collected
in Table 7.5.

TABLE 7.5 Bond Distances in Cytochrome b(6)f Hemes for PDB: 1VF5

Heme

Complex Subunit/Heme -

Protein Connection

Complex Subunit/Fe

Ligands (in Addition to

Porphyrin N Atoms)

Bond

Distance

( Å )

Heme f Cyto f subunit/Covalent
bonds between S γ of cys22
(Sγ – C = 1.83 Å ) and cys25
(Sγ – C = 1.84 Å ) to carbon
atoms of the porphyrin ’ s
C = CH substituents

Cyto f subunit

his26 N ε 2 2.44

tyr1 backbone amide N 2.34

Rieske
[2Fe – 2S]

Rieske iron – sulfur protein
(RISP or ISP)
cys108 S γ – Fe 1 2.32
cys126 S γ – Fe 1 2.32
his110 N δ 1 – Fe 2 2.27
his129 N δ 1 – Fe 2 2.25
Heme b p No covalent connection
from protein to porphyrin
ligand

Cyto b 6 subunit
his86 (helix B) N ε 2 – Fe 2.08
his187 (helix D) N ε 2 – Fe 2.29
Heme b n No covalent connection
from protein to porphyrin
ligand

Cyto b 6 subunit
his100 (helix B) N ε 2 – Fe 2.51
his202 (helix D) N ε 2 – Fe 2.30
Heme x Cyto b 6 subunit helix A
cys35 S γ bonded to carbon
atom of the porphyrin ’ s
C = CH substituent
(Sγ – C = 1.78 Å )

H 2 O – Fe 2.53

Fe axial H 2 O ligand

hydrogen - bonded to

propionate side chain of

heme b n (OH 2 · · · O =

3.50 Å ) and cyto b 6 gly38

backbone amide N – H

(OH 2 · · · N – H = 2.70 Å )

CYTOCHROME b(6)f: A GREEN PLANT CYTOCHROME 387