monitored to aid diagnosis of the organ damage and to make a prognosis for the
patient’s future recovery (Section 16.3).
Enzymes are believed to catalyse over 4 000 different reactions but individual
enzymes are characterised by theirspecificityfor a particular type of chemical
reaction. As a generalisation, enzymes involved in biosynthetic or signalling reactions
show a higher specificity than ones involved in degradation reactions.Bond specifi-
cityis characteristic of enzymes such as peptidases and esterases that hydrolyse
specific bond types. The specificity of these enzymes is determined by the presence
of specific functional groups within the substrate adjacent to the bond to be cleaved.
Group specificityis characteristic of enzymes that promote a particular reaction on a
structurally related group of substrates. As an example, the kinases catalyse the
phosphorylation of substrates that have a common structural feature such as a
particular amino acid (e.g. the tyrosine kinases, see Section 17.4.4) or sugar
(e.g. hexokinase). DNA polymerase has a high specificity not only copying the base
sequence of the DNA but also checking the product for accuracy afterwards. Enzymes
may also displaystereospecificityand be able to distinguish between optical and
geometrical isomers of substrates. Enzymes have ahigh capacity for regulationin
that the activity of enzymes that control the rate of a particular metabolic or signal-
ling pathway can be enhanced or reduced in response to changing intracellular and
extracellular demands. A range of regulatory mechanisms operates to allow short,
medium and long-term changes in activity (Section 15.5.2).Nomenclature and classification
By international convention, each enzyme is classified into one of six groups on the
basis of the type of chemical reaction that it catalyses. Each group is divided into
subgroups according to the nature of the chemical group and coenzymes involved in
the reaction. In accordance with theEnzyme Commission(EC)rules, each enzyme can
be assigned a unique four-figure code and an unambiguous systematic name based
upon the reaction catalysed. The six groups are:- Group 1:Oxidoreductases, which transfer hydrogen or oxygen atoms or electrons
from one substrate to another. This group includes the dehydrogenases, reductases,
oxidases, dioxidases, hydroxylases, peroxidases and catalase. - Group 2:Transferases, which transfer chemical groups between substrates.
The group includes the kinases, aminotransferases, acetyltransferases and
carbamyltransferases. - Group 3:Hydrolases, which catalyse the hydrolytic cleavage of bonds. The group
includes the peptidases, esterases, phosphatases and sulphatases. - Group 4:Lyases,which catalyse elimination reactions resulting in the formation
of double bonds. The group includes adenylyl cyclase (also known an adenylate
cyclase), enolase and aldolase. - Group 5:Isomerases, which interconvert isomers of various types by intramolecular
rearrangements. The group includes phosphoglucomutase and glucose-6-phosphate
isomerase.
582 Enzymes