the ferritin model, [Fe 12 (O) 2 (OMe) 18 (OAc) 6 (MeOH) 4.67 ], was published by the
Lippard group in 1994.^179
Hemerythrin is an oligomeric protein responsible for dioxygen (O 2 ) trans-
portation in the marine invertebrates. Myohemerythrin is a monomeric O 2 -
binding protein, found in the muscles of marine invertebrates. Hemerythrin
and myohemerythrin are essentially colorless when deoxygenated, but turn a
violet - pink in the oxygenated state. Neither enzyme contains a heme group.
In both hemerythrin and monohemerythrin, the dioxygen - binding site is a
binuclear iron center. The iron ions are coordinated to the protein through the
carboxylate side chains of one glutamate, one aspartate, and fi ve histidine
amino acid residues. Hemerythrin and myohemerythrin are often described
according to oxidation and ligation states of the iron center: (1) Fe(II) – OH –
Fe(II), reduced, deoxy; (2) Fe(II) – OH – Fe(III), semi - met; (3) Fe(III) – O –
Fe(III) – OOH − , oxidized, oxy; and (4) Fe(III) – OH – Fe(III) – other ligands,
oxidized, met. The X - ray crystallographic structures of oxy (PDB: 1HMO) and
deoxy (PDB: 1HMD) were determined at 2.00 Å resolution in the early
1990s.^180 More information on hemerythrin and associated proteins can be
found at the website http://www.sanger.ac.uk/cgi-bin/Pfam/getacc?PF01814.
Purple acid phosphatases (PAPs) occur widely in nature and are responsi-
ble for hydrolysis of orthophosphate monoethers to alcohols under acidic
conditions according to the reaction
RCHOPO 232 −−+→HO22 4RCHOH HPO+^2 (7.14)
In the plant form, PAPs contain a binuclear metal center containing hexaco-
ordinate Zn(II) and Fe(III) ions, while in the mammalian form, PAPs contain
binuclear iron centers having Fe(II) – Fe(II) or Fe(II) – Fe(III) oxidation states.
The Fe(III) – M(II) center constitutes the active form of the enzyme. Ligands
include histidine N δ or N ε atoms, mono - or bidentate O atoms from aspartate
(Oδ ) or glutamate (O ε ) residues, the hydroxy group of a tyrosine residue,
a bridging O(H) ligand, and/or water or hydroxide ions. The purple color of
PAPs is due to a tyr → Fe(III) charge transfer (CT) band around 560 nm.
Recent X - ray crystallographic structures have mostly characterized the cata-
lytically inactive Fe(III) – Fe(III) form of the mammalian enzyme.^181
7.9.3.1 Methane Monooxygenases. Methane monooxygenase (MMO) and
ribonucleotide reductases (RNR) belong to a class of enzymes that contain a
nonheme diiron(II) center with two bridging carboxylate ligands as well as
terminal carboxylate and histidine ligands. Only the MMO enzyme will be
discussed in any detail here. The enzyme methane monooxygenase (MMO) is
utilized by methanotrophic bacteria (bacteria capable of using methane as
both a carbon and an energy source) to oxidize methane (CH 4 ) to methanol
(CH 3 OH) with incorporation of one oxygen atom from O 2.
CH 42 ++O NADH H+→++CH OH H O NAD 3 + + 2 (7.15)
NON-HEME IRON-CONTAINING PROTEINS 459