Principles of Protein Structure - H. Schulz 1982

Covalent Protein Structure
Chain Assemblies
Immunoglobulins and HL-A Proteins

The covalent bond between immunoglobulin chains is essential for their function. Plasma IMMUNOGLOBULINS perfectly illustrate many aspects of the correlations between Cell/13.html">Protein Structure, function, and environment [81]. In an immunoglobulin (Fig. 4.2, c), each of the two antigen-binding centers is formed by polypeptide chains of two types. The association of two identical light chains and two identical heavy chains is crucial for the molecule's function. Uncontrolled chain exchange among thousands of different plasma immunoglobulins would hinder the effective interaction of these Antibodies with the Antigens that induced their synthesis. The covalent joining of the four chains, which is essential for Specificity, protects immunoglobulins from accidental structural disruption [81].

HL-A Proteins must be oligomeric. Unlike immunoglobulins, the biological specificity of an HL-A protein (Fig. 4.2, b) is determined by only two (identical) heavy chains; the light chains of all HL-A proteins in the body are identical [86, 87]. Therefore, there is no need for a covalent bond between the light and heavy chains; such a bond should exist only between the heavy chains. Dissociation and reassociation of the light chains cannot disrupt the molecular specificity. The three-dimensional structure of HL-A proteins was predicted based on data regarding their specificity and the known structure of immunoglobulins. Thus, physiological data facilitate The Development of working hypotheses in protein structure research.

On the one hand, the oligomeric STRUCTURE OF THE HL-A protein can in principle be considered a drawback, as it leads to the filtration and degradation of the light chain ($eta_2$-microglobulin, Fig. 4.2, b) in the Kidneys [88]. However, a compensating advantage is that the light chain of HL-A proteins is synthesized by specialized Cells, and since these cells are distributed throughout the body much like Hormones, partial losses in the kidneys are of no major significance. It seems advisable to initiate systematic studies of the cells that produce the light chain. In this way, the results of protein structure studies may also lead to working hypotheses in general physiology.