linking different chains (inter-chain). An individual polypeptide chain in an oligo-
meric protein is referred to as a subunit. The subunits in a protein may be identical or
different: for example, haemoglobin consists of twoa- and twob-chains, and lactate
dehydrogenase of four (virtually) identical chains.
Traditionally, proteins are classified into two groups – globular and fibrous. The
former are approximately spherical in shape, are generally water soluble and may
contain a mixture ofa-helix,b-pleated sheet and random structures. Globular pro-
teins include enzymes, transport proteins and immunoglobulins. Fibrous proteins are
structural proteins, generally insoluble in water, consisting of long cable-like struc-
tures built entirely of either helical or sheet arrangements. Examples include hair
keratin, silk fibroin and collagen. The native state of a protein is its biologically
active form.
The process of protein denaturation results in the loss of biological activity,
decreased aqueous solubility and increased susceptibility to proteolytic degradation.
It can be brought about by heat and by treatment with reagents such as acids and
alkalis, detergents, organic solvents and heavy-metal cations such as mercury and
lead. It is associated with the loss of organised (tertiary) three-dimensional structure
and exposure to the aqueous environment of numerous hydrophobic groups previ-
ously located within the folded structure.
In enzymes, the specific three-dimensional folding of the polypeptide chain(s)
results in the juxtaposition of certain amino acid residues that constitute the active
site or catalytic site. Oligomeric enzymes may possess several such sites. Many
enzymes also possess one or more regulatory site(s). X-ray crystallography studies
have revealed that the active site is often located in a cleft that is lined with
hydrophobic amino acid residues but which contains some polar residues. The binding
of the substrate at the catalytic site and the subsequent conversion of substrate to
product involves different amino acid residues.
Some oligomeric enzymes exist in multiple forms called isoenzymes or isozymes
(Section 15.1.2). Their existence relies on the presence of two genes that give similar
but not identical subunits. One of the best-known examples of isoenzymes is lactate
dehydrogenase, which reversibly interconverts pyruvate and lactate. It is a tetramer
and exists in five forms (LDH1 to 5) corresponding to the five permutations of
arranging the two types of subunits (H and M), which differ only in a single amino
acid substitution, into a tetramer:Two cysteine sulphydryl
groups in juxtaposition
in the same or different
peptide chain(s)OxidationSH
SHS
SDisulphide bridgeFig. 8.1The formation of a disulphide bridge.306 Protein structure, purification, characterisation and function analysis