second edition, information on iron – sulfur clusters in nitrogenase is found in
Section 3.6.4. Rubredoxin and ferredoxin iron – sulfur proteins are electron
transferases, as are the previously discussed cytochromes. Rubredoxins are
found in anaerobic bacteria and contain iron ligated to four cysteine – sulfur
ligands. Ferredoxins are found in plant chloroplasts and mammalian tissue and
contain spin - coupled [2Fe – 2S] clusters.
7.9.2.1 The Enzyme Aconitase
tion or addition of water in the second step of the citric acid (Krebs) cycle,
catalyzing the interconversion of citrate and isocitrate viacis - aconitate. See
reference 8 , pages 190 – 196, Figure 7.49 , and equation 7.13.
citrate HO -aconitate
HO
HO
HO
HO
2 2
2
2
2
⎯→⎯⎯←⎯⎯⎯⎯→⎯⎯ ←⎯⎯⎯→⎯⎯
−
+
+
−
cis ⎯⎯ isocitrate (7.13)
The aconitase superfamily contains fi ve phylogenetic groups: (1) mitochon-
drial aconitases (mAcn); (2) cytoplasmic aconitases (cAcn) and iron regula-
tory proteins (IRP1 and IRP2) of higher organisms and bacterial aconitase As
(AcnA); (3) homoaconitases; (4) fungal and bacterial isopropylmalate isom-
erases (IPMI); and (5) bacterial aconitase Bs (AcnB). Aconitase was one of
the fi rst iron – sulfur cluster - containing proteins to be studied in detail. Early
in its investigation, it was found that activation of aconitase required conver-
sion of a [3Fe – 4S] cluster within the protein into a [4Fe – 4S] cluster.^170 The
Fe(II) ion inserted into the [3Fe – 4S] cluster in the presence of a reductant,
termed Fe a or Fe 4 , remains in its Fe 2+ , high - spin, six - coordinate state. Fe 4
becomes directly involved in binding substrate via carboxyl and hydroxyl
ligands for citrate ( β - carboxyl) and isocitrate ( α - carboxyl) and via α - or β -
carboxyl ligands for cis - aconitate.^171 (See reaction 7.13, Figures 7.49 and 7.50 .)
M ö ssbauer spectroscopy indicates that the [Fe 3 S 4 ] + cluster is fi rst reduced. This
is followed by the slower uptake step of Fe(II) during enzyme rearrangement
with the formation of the active enzyme containing a [Fe 4 S 4 ] n + cluster. The
[Fe 4 S 4 ] n + cluster can exist with the iron ions (as tetrahedrally coordinated high -
spin d^6 or d^5 species) in a series of oxidation and overall spin states, [Fe 4 S 4 ] 3+
Figure 7.49 Formation of isocitrate from citrate as catalyzed by the enzyme
aconitase.
COO-
CH 2
HO C COO-
CH 2
COO-
COO-
CH 2
HO C COO-
CH
COO-
COO-
CH 2
H C COO-
CH
COO-
HO
Citrate cis-Aconitate Isocitrate
NON-HEME IRON-CONTAINING PROTEINS 455
