MECHANISMS OF CHOSEN ENZYME REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 110

determined at various temperatures (18°, 4°, -18° and -160°) and redox states [Fe(III)-
Fe(III)] and [Fe(II)-Fe(II)] ( Rosenwieg et al., 1993; Waller and Lipscomb, 1996; and
references therein). According to the crystallographic model (Fig.3.12), two iron atom
are bridged with two protonated oxygen atoms ([Fe(III)-Fe(III)]) or oxygen atoms of two
carboxylates [Fe(II)-Fe(II)]. Histidine, the carboxylate group and water ligands form the
six-ligand coordination sphere of both atoms in the complex oxidized state and mixed
six

- five coordination in its reduced state.

No obvious evidence concerning substrate entry to the diiron cluster have been
revealed indicating that the entry channel may be opened due to the proteins spontaneous
flexibility or may be created by binding MMOB or MMOR (Wallar and Lipscomb,
1996). Recent data on crystal structure of MMOH from M. capsulatus demonstrate the
geometric variability of the enzyme active site (Whittington et al., 2001). It is shown,
that ferrous atoms, adjacent and the Asn214 group have a certain pliability,
which enables small molecules to penetrate into the active site.