The word enzyme comes from Greek: "in ferment". As early
as the late 1700s and early 1800s the digestion of meat by stomach secretions and the conversion
of starch to sugars by plant extracts and saliva were observed.
Studying the fermentation of sugar to alcohol by yeast, Louis Pasteur came
to the conclusion that this fermentation was catalyzed by "ferments" in the yeast.
In 1879, yeast extracts were used to ferment sugar independent from the living
yeast cells by Eduard Buchner, proving that the fermentation was caused by
molecules that functioned even outside the cells. He called these molecules enzymes.
An enzyme (in Greek en = in
and zyme = blend) is a protein, or protein complex, that catalyses a chemical reaction and also controls
the 3D orientation of the catalyzed substrates. Like any catalyst, enzymes work by lowering the activation energy of
a reaction, thus allowing the reaction to proceed to its steady state or completion much faster than it otherwise would; the
enzyme (again, as with any catalyst) remains unaltered by the completed reaction and can therefore continue catalysis. It
is important to note that, as with all catalysts, all reactions catalyzed by enzymes must be 'spontaneous' i.e. with
the enzyme, they run in the same direction as they would without the enzyme, just more quickly; the concept is similar to
the likelihood of a ball rolling down a hill versus the likelihood of it rolling up the hill. Catalysis by an enzyme is analogous
to removing a pebble that is stopping the ball from rolling down the hill; the reaction goes to completion more quickly, but
the final product is identical. Given a particular starting set of conditions, the end products of a particular reaction (including
net energy), once steady state is reached, must always be identical, independent of the specific individual pathway taken
from beginning point to end point. This is required by the Law of Conservation of Energy, which would be violated by the possibility
of a cycle of moving down a pathway releasing less net energy and back up a different pathway with higher net energy, or vice
versa. An enzyme can, however, run a normally non-spontaneous reaction 'backwards by coupling it to a spontaneous one, as
long as the net free energy from the total of both reactions is negative.
Enzymes are necessary within biological cells to
control molecular shapes and because many chemical reactions would occur too slowly to sustain life; oxidation of organic
food compounds to provide energy, for instance. Enzymes may speed up biochemical reactions by a factor of one thousand times
or more. They also provide a means to control the reaction rates by modulating enzymatic activity