Page 97 - Color Atlas of Biochemistry

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88 Metabolism

Enzymes: basics
C. Enzyme classes
Enzymes are biological catalysts—i. e., sub- More than 2000 different enzymes are cur-
stances of biological origin that accelerate rently known. A system of classification has
chemical reactions (see p. 24). The orderly been developed that takes into account both
course of metabolic processes is only possible their reaction specificity and their substrate
because each cell is equipped with its own specificity. Each enzyme is entered in the En-
genetically determined set of enzymes. It is zyme Catalogue with a four-digit Enzyme
only this that allows coordinated sequences Commission number (EC number). The first
of reactions (metabolic pathways;see p.112). digit indicates membership of one of the six
Enzymes are also involved in many regulatory major classes. The next two indicate sub-
mechanisms that allow the metabolism to classes and subsubclasses. The last digit indi-
adapt to changing conditions (see p.114). Al- cates where theenzymebelongs in thesub-
most all enzymes are proteins.However, subclass. For example, lactate dehydrogenase
there are also catalytically active ribonucleic (see pp. 98–101) has the EC number 1.1.1.27
acids, the “ribozymes” (see pp. 246, 252). (class 1, oxidoreductases; subclass 1.1, CH–OH
group as electron donor; sub-subclass 1.1.1,
+
NAD(P) as electron acceptor).
A. Enzymatic activity
Enzymes with similar reaction specificities
The catalytic action of an enzyme, its activity, are grouped into each of the six major classes:
is measured by determining the increase in The oxidoreductases (class 1) catalyze the
the reaction rate under precisely defined con- transfer of reducing equivalents from one re-
ditions—i. e., the difference between the turn- dox system to another.
over (violet) of the catalyzed reaction (or- The transferases (class 2) catalyze the
ange) and uncatalyzed reaction (yellow) in a transfer of other groups from one molecule
specific time interval. Normally, reaction rates to another. Oxidoreductases and transferases
are expressed as the change in concentration generally require coenzymes (see pp.104ff.).
–1
per unit of time (mol 1 –1 s ;see p. 22). The hydrolases (class 3) are also involved in
Since the catalytic activity of an enzyme is group transfer, but the acceptor is always a
independent of the volume, the unit used water molecule.
for enzymes is usually turnover per unit time, Lyases (class 4, often also referred to as
–1
expressed in katal (kat, mol s ). However, “synthases”) catalyze reactions involving ei-
the international unit U is still more com- ther the cleavage or formation of chemical
–1
monly used (µmol turnover min ;1 U = bonds, with double bonds either arising or
16.7 nkat). disappearing.
The isomerases (class 5)movegroups
within a molecule, without changing the
B. Reaction and substrate specificity
gross composition of the substrate.
The action of enzymes is usually very specific. The ligation reactions catalyzed by ligases
This applies not only to the type of reaction (“synthetases,” class 6) are energy-dependent
being catalyzed (reaction specificity), but also and are therefore always coupled to the hy-
to the nature of the reactants (“substrates”) drolysis of nucleoside triphosphates.
that are involved (substrate specificity;see
p. 94). In Fig. B, this is illustrated schemati- In addition to the enzyme name, we also
cally using a bond-breaking enzyme as an usually give its EC number. The annotated
example. Highly specific enzymes (type A, enzyme list (pp. 420ff.) includes all of the en-
top) catalyze the cleavage of only one type zymes mentioned in this book, classified ac-
of bond, and only when the structure of the cording to the Enzyme Catalog system.
substrate is the correct one. Other enzymes
(type B, middle) have narrow reaction specif-
icity, but broad substrate specificity. Type C
enzymes (with low reaction specificity and
low substrate specificity, bottom) are very
rare.

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