Enzyme
Human glyoxalase I. Two zinc ions that are
needed for the enzyme to catalyze its reac-
tion are shown as purple spheres, and an en-
zyme inhibitor called S-hexylglutathione is
shown as a space-filling model, filling the two
active sites.
Enzymes are biomolecules that catalyze (i.e.,
increase
the
rates of)
chemical
reac-
tions.[1][2] Almost all enzymes are proteins.
In enzymatic reactions, the molecules at the
beginning of the process are called sub-
strates, and the enzyme converts them into
different molecules, called the products. Al-
most all processes in a biological cell need
enzymes to occur at significant rates. Since
enzymes are selective for their substrates
and speed up only a few reactions from
among many possibilities, the set of enzymes
made in a cell determines which metabolic
pathways occur in that cell.
Like all catalysts, enzymes work by lower-
ing the activation energy (Ea or ΔG‡) for a re-
action, thus dramatically increasing the rate
of the reaction. Most enzyme reaction rates
are millions of times faster than those of
comparable un-catalyzed reactions. As with
all catalysts, enzymes are not consumed by
the reactions they catalyze, nor do they alter
the equilibrium of these reactions. However,
enzymes do differ from most other catalysts
by being much more specific. Enzymes are
known to catalyze about 4,000 biochemical
reactions.[3] A few RNA molecules called
ribozymes catalyze reactions, with an import-
ant example being some parts of the ribo-
some.[4][5] Synthetic molecules called artifi-
cial enzymes also display enzyme-like catalys-
is.[6]
Enzyme activity can be affected by other
molecules. Inhibitors are molecules that de-
crease enzyme activity; activators are mo-
lecules that increase activity. Many drugs
and poisons are enzyme inhibitors. Activity is
also affected by temperature, chemical envir-
onment (e.g., pH), and the concentration of
substrate. Some enzymes are used commer-
cially, for example, in the synthesis of antibi-
otics. In addition, some household products
use enzy