MECHANISMS OF ENZYMATIC REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 63

aforementioned hypothesis, which suggests that the forced protonation of a functional
group accounts for the GTP phosphate bond hydrolysis: the hydrolytic water is positioned
by the

2.5. Concerted reactions

2.5.1. SYNCHRONIZATION FACTOR

In order to explain the high efficiency of many chemical and enzymatic processes, wide
use is made of the concepts of energetically favorable, concerted mechanisms. In a
concerted reaction a substrate is simultaneously attacked by different active reagents with
acid and basic groups, nucleophyle and electrophyle, or reducing and oxidizing agents. It
may however be presumed, that certain kinetic limitations exists on the realization of
reactions which are accompanied by a change in the configuration of a large number of
nuclei (Bordwell, 1970, Likhtenshtein, 1974, 1976a, 1977a,b, 1988a; Bernasconi, 1992).
According a simplified theory (Alexandrov, 1976), a concerted reaction occurs as a
result of the simultaneous transition (taking approximately of a system of
independent oscillators, with the mean displacement of nuclei from the ground state, to
the activated state in which this displacement exceeds for each nuclei a certain critical
value

and the activation energy of the concerted process the
theory gives the following expression for the synchronization factor which is the ratio of
the pre-exponential factors of the synchronous and simple processes:

If

where n is the number of vibrational degrees of freedom of the nuclei participating in the
concerted transition.
At and

In fact, in the frame of the Alexandrov model, when the average thermal energy of the
system exceeds the energy of the activation barrier, the process can be
considered as activationless. Analysis of Eqs. 2.44 and 2.45 provides a clear idea of the
scale of the synchronization factor, and the dependence of this factor on the number of n
and therefore on the number of broken bonds and the energy activation (Fig. 2.12). For
example, at moderate energy activation 20-40 kJ/mole, typical for enzymatic reactions, the
incorporation of each new nucleus into the transition state can lead to a ten-fold decrease in
the rate of the process.