material has been established (see pages 56 to 58), is an added indication
that there is some kind of a union between the enzyme and the substrate as
a first step in the catalytic process.

As to the nature of this supposed combination of substrate and enzyme, two
theories are held. The first is that this union is in the form of an actual
molecular combination, or chemical compound, and the other is that it is a
purely physical, or colloidal complex. The latter view has by far the
greater weight of theoretical and experimental evidence in its support. The
relation of electrolytes to the catalytic effect of enzymes, the appearance
of the reacting masses under the ultramicroscope, and the effect of heat
upon the reacting mixtures, all point to the conclusion that the phenomenon
is colloidal rather than molecular in character. This view also makes the
remarkable catalytic effects which take place in living protoplasm, which
undoubtedly exists in the colloidal condition, much more easily understood.
This phase of the matter will be much more apparent after the chapter
dealing with the physical chemistry of the protoplasm has been studied.

A further indication that the mechanism of enzyme activity is colloidal in
character lies in the fact that, so far as is known, all reactions which
are catalyzed by specific enzymes are reversible and the same enzyme will
accelerate the velocity of the reaction in either direction, the direction
in which the reaction goes being determined by the conditions surrounding
the reacting material at the time. It was formerly supposed that enzymes
catalyze only decomposition reactions and that the synthetic reactions of
living tissues are produced by means of some other force or agency. This
view supported the idea of a chemical union of the enzyme with the
substrate which, when it breaks down, breaks the molecule of the substrate
material into some simpler form, or forms. But it is now known that the
reaction which is influenced by the enzyme will be catalyzed in either
direction by the specific enzyme which "fits" the particular substrate
material at every point of its molecular configuration, as the glove fits
the hand. The contrast between this fitting of the enzyme to the entire
configuration of the molecule, and the union at a single point or group
which is characteristic of chemical linkages, is apparent. As examples of
the synthetic action of the same enzyme which, under other conditions,