Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

22 Chapter 2
maintaining protein tertiary structure and, as we shall see below, they are
central to the behaviour of biological membranes.
Having bound the substrate, the enzyme then proceeds to catalyse its
speciWc chemical reaction using active site catalytic groups, and Wnally releases
its product back into solution. Enzyme catalysis will be discussed in the next
chapter. However, before Wnishing the discussion of enzyme structure three
special classes of enzyme structural types will be introduced.
2.8 Metallo-enzymes
Although the polypeptide backbone of proteins is made up only of the 20
common l-amino acids, many proteins bind one or more metal ions. Enzymes
which bind metal ions are known as metallo-enzymes: in these enzymes the
metal cofactor is usually found at the active site of the enzyme, where it may
have either a structural or a catalytic role.
A brief summary of the more common metal ions is given in Table 2.1.
Magnesium ions are probably the most common metal ion cofactor: they are
found in many enzymes which utilise phosphate or pyrophosphate substrates,
since magnesium ions eVectively chelate polyphosphates (see Figure 2.20).
Zinc ions are used structurally to maintain tertiary structure, for example in
the ‘zinc Wnger’ DNA-binding proteins by co-ordination with the thiolate side
chains of four cysteine residues, as shown in Figure 2.21a. In contrast, zinc is
also used in a number of enzymes as a Lewis acid to co-ordinate carbonyl
groups present in the substrate and hence activate them towards nucleophilic
attack, as shown in Figure 2.21b.
Table 2.1 Metallo-enzymes.
Metal Types of enzyme Role of metal Redox active
Mg Kinases, phosphatases, phosphodiesterases Binding of phosphates/polyphosphates
Zn Metalloproteases, dehydrogenases Lewis acid carbonyl activation
Fe Oxygenases (P 450 , non-haem) Binding and activation of oxygen ü
[FeS] Clusters
Electron transport, hydratases
Cu Oxygenases Activation of oxygen ü
Mn Hydrolases, hydratases Lewis acid ü
Co Vitamin B 12 coenzyme Homolysis of Co–carbon bond ü
Mo Nitrogenase Component of Mo/Fe cluster ü