the velocity
of the reaction is greatly altered, usually accelerated; (2) the catalytic
agent does not appear as one of the initial substances, or end-products, of
the reaction, and is not itself altered by the chemical change which is
taking place; (3) the accelerating effect is directly proportional to the
amount of the catalyst which is present; (4) relatively small amounts of
the catalyst produce very large results in the reacting mixture; and (5)
the catalysts cannot themselves initiate reactions, but only influence the
velocity of reactions which would otherwise take place at a different rate
(usually much more slowly) in the absence of any catalytic agent.

Enzymes conform to all of these properties of catalysts, and are commonly
defined as the "catalysts of living matter." They are almost universally
present in living organs of every kind, and perform exceedingly important
functions, both in the building-up of synthetic materials and in the
rendering soluble of the food of both plants and animals, so that it can be
translocated from place to place through the tissues of the organism.

Enzymes differ from inorganic catalysts in being destroyed by heat, in not
always carrying the reaction to the same stage as does the inorganic
catalyst which may accelerate the same reaction, and in producing
different changes in the same substance by different enzymes.

The name "enzyme" comes from Greek words meaning "in yeast," as the nature
and effect of the enzyme involved in the alcoholic fermentation of sugars
by yeast were those which were first recognized and understood. It was at
first thought, by Pasteur and his students, that fermentation is the direct
result of the life activities of the yeast plant. Later, it was found that
water extracts from sprouted barley, from almond seeds, and from the
stomach, pancreas, etc., were able to bring about the decomposition of
starch, of amygdalin, and of proteins, respectively, in a way which seemed
to be quite comparable to the fermentative action of yeasts. Hence, it was
thought that there were two varieties of active agents of this kind, one
composed of living cells and the other non-living chemical compounds, and
these were called the "organized ferments" and the "unorganized ferments,"
respectively. However, in 1897, Buechner found that by grinding yeast cells
with sharp sand until they were completely disintegrated and then
submitting the mass to hydraulic pressure, he could