Proteolytic dissection of a hapten binding site.

Area of Science:

• Immunology
• Structural Biology
• Biochemistry

Background:

• Immunoglobulin G (IgG) Gar is a human myeloma protein with high affinity for riboflavin.
• Understanding the structural basis of antibody-antigen interactions is crucial for antibody engineering and therapeutics.

Purpose of the Study:

• To investigate the role of specific antibody domains and regions in maintaining riboflavin binding affinity.
• To determine if variable heavy (V(H)) and variable light (V(L)) chain segments can reconstitute a functional riboflavin binding site.

Main Methods:

• Derivation of variable region fragments from IgG Gar heavy and light chains.
• Comparison of riboflavin binding affinity of F(ab) fragments, V(H) + light chain, and V(H) + V(L) constructs.
• Quantification of binding constants to assess affinity changes.

Main Results:

• The F(ab) fragment retained the same riboflavin binding ability as the intact IgG Gar molecule.
• The active site formed by V(H) and the light chain showed a 100- to 1000-fold decrease in binding affinity.
• Further removal of the light chain's third hypervariable region and constant domain reduced affinity by an additional order of magnitude.

Conclusions:

• The V(H) and V(L) segments of IgG Gar can reconstitute a riboflavin binding site.
• The decrease in affinity suggests that the constant domains (C(H1)) and specific light chain regions are critical for optimal riboflavin binding.
• Reduced affinity is likely due to decreased rigidity of hypervariable loops, affecting optimal positioning of amino acid residues for hapten contact.