Combination of the various secondary structures in a protein results in its three-dimensional structure. Many proteins fold into a fairly compact, globular structure.
The folding of a protein molecule into a distinct three-dimensional structure determines its function.Enzyme activity requires the exact coordination of catalytically important residues in the three-dimen-sional space. Binding of antibody to antigen and binding of growth factors and cytokines to their receptors all require a distinct, specific surface for high affinity binding. These interactions do not occur if the tertiary structures of antibodies, growth factors and cytokines are altered.
A unique tertiary structure of a protein can often result in the assembly of the protein into a distinct quaternary structure consisting of a fixed stoichiometry of protein chains within the complex. Assembly can occur between the same proteins or between different polypeptide chains. Each molecule in the complex is called a subunit. Actin and tubulin self-associate into F-actin and microtubule, while hemoglobin is a tetra-mer consisting of two a and two b subunits. Among the cytokines and growth factors, interferon-g is a homo-dimer, while platelet-derived growth factor is a homo-dimer of either A or B chains or a heterodimer of the A and B chain. The formation of a quaternary structure occurs via non-covalent interactions or through dis-ulfide bonds between the subunits.