Enzyme action is the result of a large number
of discrete steps involving a great variety of
processes such as cooperative conformational
changes, acid-base catalysis, nucleophylic and/or
electrophylic attack from properly positioned
groups, etc.; it is widely recognized that in order
to be useful for catalysis, the various elementary
processes must be space- and time-controlled
during enzyme function. In the past decade great
progresses have been made in understanding the
chemistry and the stereochemistry of enzyme
action, with particular emphasis on the role of the
spatial effects. Obviously, an analysis of the
temporal aspects of enzyme action is equally
important. The ultimate goal is the description of
the concomitance and/or sequence of individual
elementary steps in the catalytic act. This ambitious
but difficult goal can be approached by
focusing the attention on the time constants of the
various elementary processes and assessing their
microscopic mechanism by comparative studies on
representative model systems. This approach was introduced in enzymology with the development
of fast relaxation methods and will be followed in
this paper, with the understanding that it suffers
from the same intrinsic limitations as an analysis
of a musical piece restricted to a list of the sound
frequencies occurring in it but devoid of any
information about their temporal sequence and
relative intensity.
Our aims are:
l. To review time events detected in
enzymes using a proper physical framework, i.e.,
the theory of the random processes.
2. To identify these events at a molecular
level by comparison with processes occurring in
appropriate model systems.
3. To discuss the statistical significance of
the detected events.
We shall start with the simpler model systems
and shall then proceed to analyze situations of
increasing complexity and eventually consider
enzyme-substrate complexes. For each class of
events some data will be critically reviewed and
their relevance to enzyme catalysis stressed. All data will then be comparatively discussed according
to their time scale and some mechanistic
conclusions will be derived.
The representative enzymes considered in this
review were chosen among those which can work
as separate entities in an aqueous medium because
they are simpler and better known.
