Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

All Enzymes are Proteins 23
O
O −
P O −
2+
O Mg
O P O−
O
R
Figure 2.20 Binding of polyphosphate by magnesium.
Figure 2.21 Uses of zinc (a) in a structural role and (b) as a Lewis acid.
The other common role of metal ions is as redox reagents. Since none of
the 20 common amino acids are able to perform any useful catalytic redox
chemistry, it is not surprising that many redox enzymes employ redox-active
metal ions. We shall meet a number of examples of these redox-active metalloenzymes
in Chapter 6. For a more detailed discussion of the role of metal ions
in biological systems the reader is referred to several excellent texts in
bio-inorganic chemistry.
2.9 Membrane-associated enzymes
Although the majority of enzymes are freely soluble in water and exist in the
aqueous cytoplasm of living cells, there is a substantial class of enzymes that are
associated with the biological membranes which encompass all cells. Biological
membranes are made up of a lipid bilayer composed of phospholipid molecules
containing a polar head group and a hydrophobic fatty acid tail. The phospholipid
molecules assemble spontaneously to form a stable bilayer in which the
hydrophilic head groups are exposed to solvent water and the hydrophobic tails
are packed together in a hydrophobic interior.
Enzymes that are associated with biological membranes fall into two
classes, as illustrated in Figure 2.22:
(1) extrinsic membrane proteins which are bound loosely to the surface of the
membrane, often by a non-speciWc hydrophobic interaction, or in some
cases by a non-peptide membrane ‘anchor’ which is covalently attached to
the protein;