tion in plants.

THE NATURE OF ENZYME ACTION

The mechanism by which an enzyme accomplishes its catalytic effects has
been the object of extensive studies during recent years, especially since
the discovery by Buechner that enzymes could be isolated in solutions
entirely free from the disturbing influence of growing cells. Several
theories concerning the mode of this catalytic action have been advanced.
The earliest and simplest of these was that the enzyme simply creates an
environment favorable for the particular chemical reaction to take place,
as by exposing large surfaces of the substance in question to the action of
the hydrolytic, or other effective, agent, by means of surface adsorption
of the substrate material on the colloidal enzyme.

However, more recent investigations clearly indicate that there is an
actual combination between the substrate material and the enzyme, which
combination then breaks down with a resultant change in the substrate
material and a freeing of the enzyme for repeated recombination with
additional substrate, with the net result that the chemical change in the
substrate material is enormously accelerated. That such a combination
between substrate and enzyme actually exists has been demonstrated in two
different ways: (_a_) experimentally, by mixing together solutions of an
enzyme and of its substrate, each of which is filterable through paper or
through a porous clay filter, with the result that the active material in
the combined solutions will not pass through these same filters; and (_b_)
mathematically, by a study of the curves representing the reaction
velocities of typical reactions which are proceeding under the influence of
an enzyme, which show that so long as there is a large excess of substrate
material present, the accelerating influence of the catalyst is uniform
over given successive periods of time, but that when the quantity of
substrate material becomes smaller than that which permits the maximum
combining power of the enzyme to be exercised, the reaction velocity
immediately slows up.

Again, the fact that the specificity of the action of an enzyme, i.e., the
limitation of the action of that enzyme to a specific single compound or
group of similar compounds, is definitely related to the molecular
configuration of the molecule of the substrate, as has been found to be
true in all those cases where the molecular configuration of the substrate