MECHANISMS OF ENZYMATIC REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 67

electron transport may cause significant changing of local charges and, therefore, does not
violate significantly the reaction complex nuclear frame.
The multi-electron nature of the energetically favorable process does not evidently
impose any new, additional restriction on its velocity. Within a coordination sphere the
orbital overlap is effective and, therefore the resonance integral V is high. The strong
delocalization of electrons in clusters, polynuclear complexes in clusters and polynuclear
complexes reduces to a minimum the reconstitution of the nuclear system during electronic
transitions and, therefore, provides a high value for the synchronization factor.
An important feature of polynuclear transition metal complexes in redox enzymes and
its chemical models is their ability to evolve inert molecules, such as and into

inner-sphere chemical conversion under ambient condition to and

correspondingly. According to thermodynamic estimations the formation of
and HO as intermediates in the above mentioned processes is energetically strongly
unfavorable. Therefore, these reactions include multi-electron elementary steps. It is
necessary to stress that realization of elementary four-electron redox reaction is provided
by a simultaneous transport of additional number electrons from the nearest electron
donating or electron-accepting centers, that is to say, metal clusters or polynuclear
complexes.
As an example, a four-electron transfer from two metal atoms in a binuclear

complex may be visualized:

Here, the longer arrow indicates the direction of the preferred electron transfer from the
metal to the substrate (S), and the shorter arrow indicates the direction of the reverse
transfer. It is obvious that four protons accompanied by the water molecule rearrangement
cannot be transferred in one synchronous step. Owing to the high degree of electron
delocalization in the polynuclear metal complexes, these complexes are more suitable for
multi-electron processes.
In real situations (Sections 3.1 and 3.5) sequential one-electron transfers precede the
formation of electron-rich or electron deficient multi-electron catalytic complexes. Thus,
such systems may be considered as devices for switching processes from the multistep
one-electron mechanism to the multi-electron mechanism.

2.7. Stabilization of enzyme reactions transition states

The fundamental concept of the transition state stabilization was introduced to Linus
Pauling in 1948 who said: “I think that enzymes are molecules that are complementary in
structure to the activated complex of the reactions that they catalyze, that is, the molecular
configuration that is intermediate between the reacting substances and the product of the
reaction”. This concept was widely accepted and used for the interpretation of
experimental structural and kinetics data on enzyme catalysis, for the design of new
substrates and inhibitors and for chemical mimicking of enzyme reactions. Decisive
contributions in this area have been made by structural physical methods, X-ray analysis,
in particular, and site-directed mutagenesis.