I.
INTRODUCTION
Enzyme,
any one of many
specialized
organic substances,
composed of
polymers of
amino acids,
that act as
catalysts to
regulate the
speed of the
many chemical
reactions involved
in the metabolism
of living organisms.
The name enzyme
was suggested
in 1867 by the
German physiologist
Wilhelm Kühne
(1837-1900);
it is derived
from the Greek
phrase en
zyme, meaning
"leavened" (an
element that
produces an
altering or
transforming
influence).
Those enzymes
identified now
number more
than 700.
Enzymes
are classified
into several
broad categories,
such as hydrolytic,
oxidizing, and
reducing, depending
on the type
of reaction
they control.
Hydrolytic enzymes
accelerate reactions
in which a substance
is broken down
into simpler
compounds through
reaction with
water molecules.
Oxidizing enzymes,
known as oxidases,
accelerate oxidation
reactions; reducing
enzymes speed
up reduction
reactions, in
which oxygen
is removed.
Many other enzymes
catalyze other
types of reactions.
Individual
enzymes are
named by adding
ase to
the name of
the substrate
with which they
react. The enzyme
that controls
urea decomposition
is called urease;
those that control
protein hydrolyses
are known as
proteinases.
Some enzymes,
such as the
proteinases
trypsin and
pepsin, retain
the names used
before this
nomenclature
was adopted.
II.
PROPERTIES OF
ENZYMES
As
the Swedish
chemist Jöns
Jakob Berzelius
suggested in
1823, enzymes
are typical
catalysts: they
are capable
of increasing
the rate of
reaction without
being consumed
in the process.
See Catalysis.
Some
enzymes, such
as pepsin and
trypsin, which
bring about
the digestion
of meat, control
many different
reactions, whereas
others, such
as urease, are
extremely specific
and may accelerate
only one reaction.
Still others
release energy
to make the
heart beat and
the lungs expand
and contract.
Many facilitate
the conversion
of sugar and
foods into the
various substances
the body requires
for tissue-building,
the replacement
of blood cells,
and the release
of chemical
energy to move
muscles.
Pepsin,
trypsin, and
some other enzymes
possess, in
addition, the
peculiar property
known as autocatalysis,
which permits
them to cause
their own formation
from an inert
precursor called
zymogen. As
a consequence,
these enzymes
may be reproduced
in a test tube.
As
a class, enzymes
are extraordinarily
efficient. Minute
quantities of
an enzyme can
accomplish at
low temperatures
what would require
violent reagents
and high temperatures
by ordinary
chemical means.
About 30 g (about
1 oz) of pure
crystalline
pepsin, for
example, would
be capable of
digesting nearly
2 metric tons
of egg white
in a few hours.
The
kinetics of
enzyme reactions
differ somewhat
from those of
simple inorganic
reactions. Each
enzyme is selectively
specific for
the substance
in which it
causes a reaction
and is most
effective at
a temperature
peculiar to
it. Although
an increase
in temperature
may accelerate
a reaction,
enzymes are
unstable when
heated. The
catalytic activity
of an enzyme
is determined
primarily by
the enzyme's
amino-acid sequence
and by the tertiary
structure—that
is, the three-dimensional
folded structure—of
the macromolecule.
Many enzymes
require the
presence of
another ion
or a molecule,
called a cofactor,
in order to
function.
As
a rule, enzymes
do not attack
living cells.
As soon as a
cell dies, however,
it is rapidly
digested by
enzymes that
break down protein.
The resistance
of the living
cell is due
to the enzyme's
inability to
pass through
the membrane
of the cell
as long as the
cell lives.
When the cell
dies, its membrane
becomes permeable,
and the enzyme
can then enter
the cell and
destroy the
protein within
it. Some cells
also contain
enzyme inhibitors,
known as antienzymes,
which prevent
the action of
an enzyme upon
a substrate.
III.
PRACTICAL USES
OF ENZYMES
Alcoholic
fermentation
and other important
industrial processes
depend on the
action of enzymes
that are synthesized
by the yeasts
and bacteria
used in the
production process.
A number of
enzymes are
used for medical
purposes. Some
have been useful
in treating
areas of local
inflammation;
trypsin is employed
in removing
foreign matter
and dead tissue
from wounds
and burns.
IV.
HISTORICAL REVIEW
Alcoholic
fermentation
is undoubtedly
the oldest known
enzyme reaction.
This and similar
phenomena were
believed to
be spontaneous
reactions until
1857, when the
French chemist
Louis Pasteur
proved that
fermentation
occurs only
in the presence
of living cells
(see Spontaneous
Generation).
Subsequently,
however, the
German chemist
Eduard Buchner
discovered (1897)
that a cell-free
extract of yeast
can cause alcoholic
fermentation.
The ancient
puzzle was then
solved; the
yeast cell produces
the enzyme,
and the enzyme
brings about
the fermentation.
As early as
1783 the Italian
biologist Lazzaro
Spallanzani
had observed
that meat could
be digested
by gastric juices
extracted from
hawks. This
experiment was
probably the
first in which
a vital reaction
was performed
outside the
living organism.
After Buchner's
discovery scientists
assumed that
fermentations
and vital reactions
in general were
caused by enzymes.
Nevertheless,
all attempts
to isolate and
identify their
chemical nature
were unsuccessful.
In 1926, however,
the American
biochemist James
B. Sumner succeeded
in isolating
and crystallizing
urease. Four
years later
pepsin and trypsin
were isolated
and crystallized
by the American
biochemist John
H. Northrop.
Enzymes were
found to be
proteins, and
Northrop proved
that the protein
was actually
the enzyme and
not simply a
carrier for
another compound.
Research
in enzyme chemistry
in recent years
has shed new
light on some
of the most
basic functions
of life. Ribonuclease,
a simple three-dimensional
enzyme discovered
in 1938 by the
American bacteriologist
René
Dubos and isolated
in 1946 by the
American chemist
Moses Kunitz,
was synthesized
by American
researchers
in 1969. The
synthesis involves
hooking together
124 molecules
in a very specific
sequence to
form the macromolecule.
Such syntheses
led to the probability
of identifying
those areas
of the molecule
that carry out
its chemical
functions, and
opened up the
possibility
of creating
specialized
enzymes with
properties not
possessed by
the natural
substances.
This potential
has been greatly
expanded in
recent years
by genetic engineering
techniques that
have made it
possible to
produce some
enzymes in great
quantity (see
Biochemistry).
The
medical uses
of enzymes are
illustrated
by research
into L-asparaginase,
which is thought
to be a potent
weapon for treatment
of leukemia;
into dextrinases,
which may prevent
tooth decay;
and into the
malfunctions
of enzymes that
may be linked
to phenylketonuria,
diabetes, and
anemia and other
blood disorders.
Contributed
By: John H.
Northrop,
M.A., Ph.D.
Late
Professor
Emeritus of
Bacteriology
and Physiology,
University
of California,
Berkeley.
Recipient,
Nobel Prize
in Chemistry
(1946)