The shape of immunoglobulin G molecules in solution

Area of Science:

• Biochemistry
• Molecular Biology
• Immunology

Background:

• Rabbit Immunoglobulin G (IgG) is a key component of the immune system.
• Understanding the three-dimensional structure of IgG in solution is crucial for elucidating its function.
• Previous studies suggested varying conformations of IgG, but precise distances between binding sites were not well-defined.

Purpose of the Study:

• To determine the minimum distance between the two hapten binding sites of rabbit IgG molecules in solution.
• To investigate the conformational flexibility and spatial arrangement of IgG molecules.
• To establish the typical solution-state structure of rabbit IgG.

Main Methods:

• Utilized singlet-singlet energy transfer (Förster Resonance Energy Transfer - FRET) between a donor (epsilon-dansyl-lysine) and an acceptor (fluorescein) labeled hybrid antibody.
• Measured the fluorescence lifetime of the donor in the presence and absence of the acceptor.
• Calculated the distance between binding sites based on energy transfer efficiency and known FRET parameters (R(0)).

Main Results:

• No significant energy transfer was detected, indicating the distance between binding sites exceeded the FRET measurement range (8.2 nm).
• Estimated the average distance between the two hapten-binding sites to be 9.2-10 nm.
• Calculated a minimum angle (alpha(M)) between the Fab moieties of 80-95 degrees, suggesting an open Y- or T-shaped conformation.

Conclusions:

• Rabbit IgG molecules in solution exhibit an open, flexible conformation.
• Hapten binding sites are located at the extremities of the Fab fragments.
• Observed conformations in antibody-antigen complexes with smaller angles likely result from specific conformational changes upon binding.