MECHANISMS OF CHOSEN ENZYME REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 96

between the cubanes and simultaneous it placing one hydrogen atom on a sulfur atom
bridging the cubanes, the molecule interaction with the cofactor is found to be attractive.
The calculations (Rod and Norskov, 2000; Rod et al., 1999) were based on density
function theory, with plane wave expansion of the Kohn-Sham wave functions and a
generalized approximation for the exchange correlation term. Two different clusters to
mimic the central part of the FeMoco have included the effect of the surrounding by
invoking a proton donor in the vicinity of the cofactor. The authors have come to the
following conclusions: 1) can adsorb in and end-on fashions, 2) binding is
strongest during turn over: an electron needs to be transferred to the FeMoco and a
proton to the vicinity in order for to spend an appropriate time in the adsorbed state,
3) NNH state is not stable and quickly decays, 4) If there are three H atoms on the
cluster, the system can transfer into adsorbed hydrazine immediately and this state is
irreversible. In fact, only the forth electron/proton transfer will make the reaction
irreversible (Fig. 3.9).
This conclusion appears to agree fully with the concept of the aforementioned
thermodynamically favorable four-electron mechanism of reduction (Likhtenshtein
and Shilov, 1970) and with the evolution of free hydrazine at the acid or base treatment
of nitrogenase during turn-over (Lowe et al., 1993), (5) Histidine is the only aniino acid
side chain capable of donating protons in neutral pH. The Fe atoms are not saturated.
Though aforementioned theoretical calculations are based on simplified truncated
models of FeMoco and use an approximate computational approach, they allow the