- Group 6:Ligases(also calledsynthases), which catalyse covalent bond formation with
the concomitant breakdown of a nucleoside triphosphate, commonly ATP. The group
includes carbamoyl phosphate synthase and DNA ligase.
As an example of the operation of these rules, consider the enzyme alcohol
dehydrogenase which catalyses the reaction:alcoholþNADþ ! aldehyde or ketoneþNADHþHþ
It has the systematic name alcohol : NAD oxidoreductase and the classification number
1:1:1:1. The first 1 indicates that it is an oxidoreductase, the second 1 that it acts on
a CH–OH donor, the third 1 that NADþor NADPþis the acceptor and the fourth 1 that
it is the first enzyme named in the 1:1:1 subgroup. Systematic names tend to be user-
unfriendly and for day-to-day purposes recommended trivial names are preferred.
When correctly used they give a reasonable indication of the reaction promoted by
the enzyme in question but they fail to identify fully all the reactants involved. For
example, glyceraldehyde-3-phosphate dehydrogenase fails to identify the involvement
of orthophosphate and NADþand phosphorylase kinase fails to convey the information
that it is thebform of phosphorylase that is subject to phosphorylation involving ATP.Cofactors
The catalytic properties of an enzyme are often dependent upon the presence of non-
peptide molecules calledcofactorsorcoenzymes. These may be either weakly or
tightly bound to the enzyme; in the latter case they are referred to as aprosthetic
group. Examples of coenzymes include NADþ, NADPþ, FMN and FAD, whilst
examples of prosthetic groups include haem and oligosaccharides, and simple metal
ions such as Mg^2 þ,Fe^2 þand Zn^2 þ. DNA and RNA polymerases and many nucleases,
for example, require two divalent cations for their active site. The cations correctly
orientate the substrate and promote acid–base catalysis.15.1.2 Isoenzymes and multienzyme complexes
Isoenzymes
Some enzymes exist in multiple forms calledisoenzymesorisoformsthat differ in
amino acid sequence. An example is lactate dehydrogenase (LD) (EC 1:1:1:27) which
exists in five isoforms. LD is a tetramer which can be assembled from two subunits,
H (for heart) and M (for muscle). The five forms are therefore H4, H3M, H2M2, HM3
and M4 which can be separated by electrophoresis and shown to have different
affinities for their substrates, lactate and pyruvate, and for analogues of these two
compounds. They also have different maximum catalytic activities and tissue distri-
butions, and as a consequence are important in diagnostic enzymology (Section 16.3).Multienzyme complexes
Some enzymes that promote consecutive reactions in a metabolic pathway associate
to form amultienzyme complex.Examples include the fatty-acid synthase (EC 2:3:1:86)583 15.1 Characteristics and nomenclature