MECHANISMS OF CHOSEN ENZYME REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 94

of the system and to minimize reorganization energy due to electron transfer, the
following requirements should be fulfilled: a) multi-orbital binding of dinitrogen in a
metalcluster when a transfer of electrons from the filled d-orbitals to the anti-binding of
the ligand is accompanied by the reverse transfer of electrons from the binding orbitals
of the ligand to empty d-orbitals of the metal, b) essential delocalization of the
electrostatic charge among the electron donor atoms with only a slight change of local
charge on each atom (only a polynuclear metalo-complex fits such requirements) and c)
the decisive four-electron elementary steps of the nitrogen reduction to be preceded by
consecutive accumulation of electrons in the catalytic cluster accompanied by proton
transfer.
5. It was suggested (Likhtenshein and Shilov, 1976; Likhtenshtein et al., 1980;
Likhtenshtein, 1988a) that the role of ATP hydrolysis might eventually consist of a
forced non-equilibrium protonation of a protein group, which increases the iron- sulfur
cluster positive charge. This increase contributes in turn to the transfer of an extra
electron and, after removal of the proton to the consequent formation of a super-reduced
form of the cluster. Thus, ATP hydrolysis energy may be consumed for producing a
center with high reducing power.
6. According to the principle of dynamic adaptation (Likhtenshtein, 1976a), the
multi-orbital interaction between a substrate and metal atoms in a bi- or polynuclear
center and the consequent chemical conversion require a certain optimum flexibility of
metal atoms involved in the catalytic process. Such flexibility would allow the space
provision for each step of the consecutive chemical reaction, i.e. complexation, product
formation and release.
Studies on model polynuclear catalytic systems have confirmed that for the catalytic
reduction of dinitrogen under mild conditions, it is necessary to use a polynuclear
transition metal complex capable of donating four electrons to form the hydrazine
derivative.
Recent quantum mechanical calculations
A simplified model of FeMoco as was theoretically examined by
Deng and Hoffman (1993). An energy level diagram was calculated and on this basis
various models of dinitrogen complex were considered. A model with bridged
between two iron atoms inside the cluster was found to be best for electron occupation of
of dinitrogen and, therefore, for weakening the N-N bond and negatively charging
the nitrogen atoms. This model is similar to those suggested earlier by Ohrme-Johnson
(Ohrme-Johnson 1972, 1992; Ohrme-Johnson et al. 1972). This calculation has led to an
important conclusion: to be activated, dinitrogen should accept at least three electrons
and be protonated.