All enzymes contain a protein backbone.
In some enzymes this is the only component in the structure. However there are
additional non-protein moieties usually present which may or may not participate
in the catalytic activity of the enzyme. Covalently attached carbohydrate groups
are commonly encountered structural features which often have no direct bearing
on the catalytic activity, although they may well effect an enzyme's stability
and solubility. Other factors often found are metal ions
(cofactors) and low molecular weight organic molecules
(coenzymes). These may be loosely or tightly bound by
noncovalent or covalent forces. They are often important constituents
contributing to both the activity and stability of the enzymes. This requirement
for cofactors and coenzymes must be recognised if the enzymes are to be used
efficiently and is particularly relevant in continuous processes where there may
be a tendency for them to become separated from an enzyme's protein moiety.
Enzymes are classified according the report of a Nomenclature Committee
appointed by the International Union of Biochemistry (1984). This enzyme
commission assigned each enzyme a recommended name and a 4-part distinguishing
number. It should be appreciated that some alternative names remain in such
common usage that they will be used, where appropriate, in this text. The enzyme
commission (EC) numbers divide enzymes into six main groups
according to the type of reaction catalysed:
which involve redox reactions in which hydrogen or oxygen atoms or electrons are
transferred between molecules. This extensive class includes the dehydrogenases
(hydride transfer), oxidases (electron transfer to molecular oxygen), oxygenases
(oxygen transfer from molecular oxygen) and peroxidases (electron transfer to
peroxide). For example: glucose oxidase (EC 126.96.36.199, systematic name, b-D-glucose:oxygen 1-oxidoreductase).
b-D-glucose + oxygen
D-glucono-1,5-lactone + hydrogen peroxide
(2) Transferases which catalyse the transfer of
an atom or group of atoms (e.g., acyl-, alkyl- and glycosyl-), between two
molecules, but excluding such transfers as are classified in the other groups (e.g.
oxidoreductases and hydrolases). For example: aspartate aminotransferase (EC 188.8.131.52,
systematic name, L-aspartate:2-oxoglutarate aminotransferase; also called
glutamic-oxaloacetic transaminase or simply GOT).
L-aspartate + 2-oxoglutarate
oxaloacetate + L-glutamate
which involve hydrolytic reactions and their reversal. This is presently the
most commonly encountered class of enzymes within the field of enzyme technology
and includes the esterases, glycosidases, lipases and proteases. For example: chymosin (EC
184.108.40.206, no systematic name declared; also called rennin).
k-casein + water
+ caseino macropeptide
(4) Lyases which involve elimination
reactions in which a group of atoms is removed from the substrate. This includes
the aldolases, decarboxylases, dehydratases and some pectinases but does not
include hydrolases. For example: histidine ammonia-lyase (EC 220.127.116.11, systematic name, L-histidine
ammonia-lyase; also called histidase).
(5) Isomerases which
catalyse molecular isomerisations and includes the epimerases, racemases and
intramolecular transferases. For example: xylose isomerase (EC 18.104.22.168, systematic name,
D-xylose ketol-isomerase; commonly called glucose isomerase).
(6) Ligases, also known as synthetases, form a relatively small group of enzymes which involve the
formation of a covalent bond joining two molecules together, coupled with the
hydrolysis of a nucleoside triphosphate. For example: glutathione synthase (EC 22.214.171.124,
systematic name, g-L-glutamyl-L-cysteine:glycine ligase (ADP-forming);
also called glutathione synthetase).
ATP + g-L-glutamyl-L-cysteine
+ glycine ADP
+ phosphate + glutathione
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This page was established in 2004 and last updated by Martin
6 August, 2014