dergoing reaction (the substrate)
binds to a speciÜc active site on the
enzyme molecule to form a short-
lived intermediate (see enzyme–
substrate complex): this greatly in-
creases (by a factor of up to 10^20 ) the
rate at which the reaction proceeds
to form the product. Enzyme activity
is inÛuenced by substrate concentra-
tion and by temperature and pH,
which must lie within a certain
range. Other molecules may compete
for the active site, causing inhibi-
tion of the enzyme or even irre-
versible destruction of its catalytic
properties.
Enzyme production is governed by
a cell’s genes. Enzyme activity is fur-
ther controlled by pH changes, alter-
ations in the concentrations of
essential cofactors, feedback inhibi-
tion by the products of the reaction,
and activation by another enzyme,
either from a less active form or an
inactive precursor (zymogen). Such
changes may themselves be under
the control of hormones or the
nervous system. See also enzyme ki-
netics.
Enzymes are classiÜed into six
major groups, according to the type
of reaction they catalyse: (1) oxidore-
ductases; (2) transferases; (3) hydro-
lases; (4) lyases; (5) isomerases; (6)
ligases. The names of most individual
enzymes also end in -ase, which is
added to the names of the substrates
on which they act. Thus lactase is the
enzyme that acts to break down lac-
tose; it is classiÜed as a hydrolase.
A
- Information about IUPAC nomenclature
enzyme inhibitionSee inhibition.
enzyme kineticsThe study of the
rates of enzyme-catalysed reactions.
Rates of reaction are usually meas-
ured by using the puriÜed enzyme in
vitro with the substrate and then ob-
serving the formation of the product
or disappearance of the substrate. As
the concentration of the substrate is
increased the rate of reaction in-
creases proportionally up to a certain
point, after which any further in-
crease in substrate concentration no
longer increases the reaction rate (see
michaelis–menten curve). At this
point, all active sites of the enzyme
are saturated with substrate; any fur-
ther increase in the rate of reaction
will occur only if more enzyme is
added. Reaction rates are also af-
fected by the presence of inhibitors
(see inhibition), temperature, and pH
(see enzyme).enzyme–substrate complexThe
intermediate formed when a sub-
strate molecule interacts with the
*active site of an enzyme. Following
the formation of an enzyme–
substrate complex, the substrate
molecule undergoes a chemical reac-
tion and is converted into a new
product. Various mechanisms for the
formation of enzyme–substrate com-
plexes have been suggested, includ-
ing the *induced-Üt model and the
*lock-and-key mechanism.
ephedrineAn alkaloid,
C 6 H 5 CH(OH)CH(CH 3 )NHCH 3 found in
plants of the genus Ephedra, once
used as a bronchodilator in the treat-
ment of asthma. It is also used as a
stimulant and appetite suppressant.
Structurally, it is a phenylethylamine
and is similar to amphetamines, al-
though less active. It is, however,
widely used in the illegal synthesis of
methamphetamine. The molecule
has two chiral centres. If the stereo-207 ephedrine
e
NHCH 3OHCH 3Ephedrine