2.3 Enzyme Cofactors 105Ca^2 ⊕ ions are weaker Lewis acids than
Mg^2 ⊕ions. Therefore, the replacement of Mg^2 ⊕
by Ca^2 ⊕may result in an inhibition of the kinase
enzymes. Enhancement of the activity of other
enzymes by Ca^2 ⊕is based on the ability of the
ion to interact with the negatively charged sites
of amino acid residues and, thus, to bring about
stabilization of the enzyme conformation (e. g.
α-amylase; cf. 4.4.4.5.1). The activation of the
enzyme may be also caused by the involvement
of the Ca^2 ⊕ion in substrate binding (e. g. lipase;
cf. 3.7.1.1).
The Zn^2 ⊕ion, among the series of transition met-
als, is a cofactor which is not involved in re-
dox reactions under physiological conditions. As
aLewisacid similar in strength to Mg^2 ⊕,Zn^2 ⊕
participates in similar reactions. Hence, substitut-
ing the Zn^2 ⊕ion for the Mg^2 ⊕ioninsomeen-
zymes is possible without loss of enzyme activity.
Both metal ions can function as stabilizers of
enzyme conformation and their direct partic-
ipation in catalysis is readily revealed in the
case of alcohol dehydrogenase. This enzyme
isolated from horse liver consists of two identical
polypeptide chains, each with one active site.
Two of the four Zn^2 ⊕ions in the enzyme readily
dissociate. Although this dissociation has no
effect on the quaternary structure, the enzyme ac-
tivity is lost. As described under section 2.3.1.1,
both of these Zn^2 ⊕ ions are involved in the
formation of the active site. In catalysis they
polarize the substrate’s C−O linkage and, thus,
facilitate the transfer of hydride ions from or
to the cosubstrate. Unlike the dissociable ions,
removal of the two residual Zn^2 ⊕ions is possible
only under drastic conditions, namely disruption
of the enzyme’s quaternary structure which is
maintained by these two ions.
2.3.3.2 Iron, Copper and Molybdenum
The redox system of Fe^3 ⊕
/
Fe^2 ⊕covers a wide
range of potentials (Table 2.5) depending on the
attached ligands. Therefore, the system is excep-
tionally suitable for bridging large potential dif-
ferences in a stepwise electron transport system.
Such an example is encountered in the transfer of
electrons by the cytochromes as members of the
respiratory chain (cf. textbook of biochemistry)
or in the biosynthesis of unsaturated fatty acids
(cf. 3.2.4), and by some individual enzymes.
Iron-containing enzymes are attributed either to
the heme (examples in 3.3.2.2) or to the non-
heme Fe-containing proteins. The latter case is
exemplified by lipoxygenase, for which the mech-
anism of activity is illustrated in section 3.7.2.2,
or by xanthine oxidase.
Xanthine oxidase from milk (Mr= 275 ,000) re-
acts with many electron donors and acceptors.
However, this enzyme is most active with sub-
strates such as xanthine or hypoxanthine as elec-
tron donors and molecular oxygen as the electron
acceptor. The enzyme is assumed to have two ac-
tive sites per molecule, with each having 1 FAD
moiety, 4 Fe-atoms and 1 Mo-atom. During the
oxidation of xanthine to uric acid:(2.16)oxygen is reduced by two one-electron steps
to H 2 O 2 by an electron transfer system in which
the following valence changes occur:(2.17)Under certain conditions the enzyme releases
a portion of the oxygen when only one electron
transfer has been completed. This yields O 2 ,
the superoxide radical anion, with one unpaired
electron. This ion can initiate lipid peroxidation
by a chain reaction (cf. 3.7.2.1.8).
Polyphenol oxidases and ascorbic acid oxi-
dase, which occur in food, are known to have
aCu^2 ⊕/Cu^1 ⊕redox system as a prosthetic group.
Polyphenol oxidases play an important role in
the quality of food of plant origin because they
cause the “enzymatic browning” for example in
potatoes, apples and mushrooms. Tyrosinases,
catecholases, phenolases or cresolases are en-
zymes that react with oxygen and a large range
of mono and diphenols.