dergoing reaction (the substrate)
binds to a speciÜc *active site on the
enzyme molecule to form a shortlived intermediate (see enzyme–
substrate complex): this greatly increases (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 concentration and by temperature and pH,
which must lie within a certain
range. Other molecules may compete
for the active site, causing *inhibition of the enzyme or even irreversible destruction of its catalytic
properties.
Enzyme production is governed by
a cell’s genes. Enzyme activity is further controlled by pH changes, alterations in the concentrations of
essential cofactors, feedback inhibition 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 kinetics.
Enzymes are classiÜed into six
major groups, according to the type
of reaction they catalyse: (1) oxidoreductases; (2) transferases; (3) hydrolases; (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 lactose; it is classiÜed as a hydrolase.
A
• Information about IUPAC nomenclature
enzyme inhibition See inhibition.
enzyme kinetics The study of the
rates of enzyme-catalysed reactions.
Rates of reaction are usually measured by using the puriÜed enzyme in
vitro with the substrate and then observing the formation of the product
or disappearance of the substrate. As
the concentration of the substrate is
increased the rate of reaction increases proportionally up to a certain
point, after which any further increase 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 further increase in the rate of reaction
will occur only if more enzyme is
added. Reaction rates are also affected by the presence of inhibitors
(see inhibition), temperature, and pH
(see enzyme).
enzyme–substrate complex The
intermediate formed when a substrate molecule interacts with the
*active site of an enzyme. Following
the formation of an enzyme–
substrate complex, the substrate
molecule undergoes a chemical reaction and is converted into a new
product. Various mechanisms for the
formation of enzyme–substrate complexes have been suggested, including the *induced-Üt model and the
*lock-and-key mechanism.
ephedrine An 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 treatment of asthma. It is also used as a
stimulant and appetite suppressant.
Structurally, it is a phenylethylamine
and is similar to amphetamines, although less active. It is, however,
widely used in the illegal synthesis of
methamphetamine. The molecule
has two chiral centres. If the stereo207
ephedrine
e
N
H
CH 3
OH
CH 3
Ephedrine
www.AzShimi.ir www.AzShimi.com
binds to a speciÜc *active site on the
enzyme molecule to form a shortlived intermediate (see enzyme–
substrate complex): this greatly increases (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 concentration and by temperature and pH,
which must lie within a certain
range. Other molecules may compete
for the active site, causing *inhibition of the enzyme or even irreversible destruction of its catalytic
properties.
Enzyme production is governed by
a cell’s genes. Enzyme activity is further controlled by pH changes, alterations in the concentrations of
essential cofactors, feedback inhibition 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 kinetics.
Enzymes are classiÜed into six
major groups, according to the type
of reaction they catalyse: (1) oxidoreductases; (2) transferases; (3) hydrolases; (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 lactose; it is classiÜed as a hydrolase.
A
• Information about IUPAC nomenclature
enzyme inhibition See inhibition.
enzyme kinetics The study of the
rates of enzyme-catalysed reactions.
Rates of reaction are usually measured by using the puriÜed enzyme in
vitro with the substrate and then observing the formation of the product
or disappearance of the substrate. As
the concentration of the substrate is
increased the rate of reaction increases proportionally up to a certain
point, after which any further increase 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 further increase in the rate of reaction
will occur only if more enzyme is
added. Reaction rates are also affected by the presence of inhibitors
(see inhibition), temperature, and pH
(see enzyme).
enzyme–substrate complex The
intermediate formed when a substrate molecule interacts with the
*active site of an enzyme. Following
the formation of an enzyme–
substrate complex, the substrate
molecule undergoes a chemical reaction and is converted into a new
product. Various mechanisms for the
formation of enzyme–substrate complexes have been suggested, including the *induced-Üt model and the
*lock-and-key mechanism.
ephedrine An 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 treatment of asthma. It is also used as a
stimulant and appetite suppressant.
Structurally, it is a phenylethylamine
and is similar to amphetamines, although less active. It is, however,
widely used in the illegal synthesis of
methamphetamine. The molecule
has two chiral centres. If the stereo207
ephedrine
e
N
H
CH 3
OH
CH 3
Ephedrine
www.AzShimi.ir www.AzShimi.com