MECHANISMS OF CHOSEN ENZYME REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 89

al., 1997). Theoretical considerations and experimental data allow for rapid long-
distance electron transfer between these donor-acceptor sites (Section 2.1). For instance,
Eq. (Fig. 2.6) predicts that at this distance the maximum rate constants for long distance
ET would be about The latter values are even higher than correspondent
experimental rate constants of electron transfer from FeP to the P-cluster of FeMoP
(Burgess and Lowe, 1996).
The helix, which binds the ligand of the P cluster of with in the
region of FeMoco, is assumed to be the way for the transfer of an electron directly from
the P cluster to FeMoco (Christiansen et al., 2000). When nitrogenase reduces the
oxidation of D-clusters has been independently proved by the finding that, during the
transfer of an electron from Av2 to Av1, changes in the absorption of Av1 occur before
the transfer of an electron to FeMoco (Duyvis et al., 1997).
The P cluster undergoes redox-dependent structural rearrangement, which can be
coupled with the transfer of an electron or a proton to FeMoco. (Peters et al., 1997). The
oxidation of the P cluster is accompanied by the coordination of and the amide
nitrogen of with the Fe atoms of the P cluster. Redox titration of P-cluster
indicates that the redox potential of transition is pH dependent (0.053 V/pH
unit) (Lanzilotta et al., 1998). It was suggested that electron transfer from P-cluster to
FeMoco at physiological pH values is accompanied by coupled proton transfer.

3.1.4. ATP CENTERS AND ATP HYDROLYSIS

The isolated Fe-protein can bind MgATP and MgADP at a stoichiometry of two
nucleotides per dimer (Schindelin et al., 1997; Rees and Howard, 2000; Chiu et al.,
2001). It was shown that an ATP analog, forms a stable and non-active
complex with A2 component of nitrogenase (Av1-Av2), in which it is located between
two domains at a distance about 15 Å from the

cluster. The cluster and

are separated by a region, which includes Asp 1 25, Glu 1 28, Asp 1 29 and Cys 1 32 (Fig.

3.3).
Although various biological systems of energy transformation (the system of
oxidative phosphorylation, the actin—myosin complex, nitrogenase)
have different structures and perform different biological functions, the main regularities
of ATP hydrolysis in these enzymes are similar (Rees and Howard, 2000; Syrtsova and
Timofeeva, 2001; and references therein). All ATPases hydrolyze ATP at the
phosphoanhydride bond, catalyze direct and intermediate exchange, and
have at least two regions of MgATP binding. One of the regions of conformational
variability in FeP is a switch region, which includes Asp 1 25, Glu 1 28, Asp 1 29 and

Cys132.
In a complex with which is considered to be the structural analog of
ATP, Av2 undergoes a large conformational change relative to its structure in the
nucleotide -free, autonomous state in the absence of A1 (Howard and Rees, 1994;
Shindelin et al., 1997, Rees and Howard, 1999). The conformational change in A2
results in about a 13° rotation of each monomer toward the subunit interface and about a
4 Å closer approach of the A2 to the A1 P-cluster. Because electron
transport between FeP to FeMoP occurs as a long-distance process, it was suggested that