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Thermodynamic Stability of Immunoglobulins and Allosteric Interactions with Ferritin and Protein A: Distinct Properties of the Two Antibodies of IgG2a Subclass

A. A. Chumanevich1, Z. I. Kravchuk1, A. P. Vlasov1, O. V. Zhorov2, and S. P. Martsev1*

1Institute of Bioorganic Chemistry, National Academy of Sciences of Belarus, ul. Zhodinskaya 5/2, Minsk, 220141 Belarus; fax: (0172) 63-5128; E-mail: martsev@ns.iboch.ac.by

2Experimental Production Company of the Institute of Bioorganic Chemistry, National Academy of Sciences of Belarus, ul. Zhodinskaya 5/3, Minsk, 220141 Belarus; fax: (0172) 63-5128

* To whom correspondence should be addressed.

Received October 15, 1997
The two anti-ferritin monoclonal antibodies of mouse IgG2a subclass, G10 and F11, are described that have similar affinity to human spleen ferritin and identical protein A-binding affinity. Antigen binding was shown to change significantly the protein A-binding parameters of the IgG2a antibodies. Antigen-induced conformational changes result in enhanced protein A-binding affinity of the G10 antibody while reduced affinity of the F11 antibody. Antigen binding does not change inherently low affinity of the anti-ferritin IgG1 antibody C5 to protein A. Differential scanning calorimetry revealed that the enthalpy and Gibbs free energy of denaturation for G10 was respectively by 19 and 29% higher than the corresponding parameters for F11. The lower structural energetics of F11 is associated with the lack of a calorimetrically revealed folding unit that may be responsible for distinct interaction between the antigen-binding and protein A-binding sites. This work provides experimental demonstration of the fact that functionally significant interactions between the two spatially remote recognition sites in antibodies of the same heavy chain isotype can be modulated by relatively small structural variations that also result in different thermodynamic stability.
KEY WORDS: IgG2a antibodies, protein A, differential scanning calorimetry, stability, activity