Proton nuclear magnetic resonance studies of human immunoglobulins: conformation of the hinge region of the IgG1 immunoglobulin.

The conformation of the hinge region of the human IgG1 immunoglobulin has been investigated by making use of His-224 in the hinge region as a built-in proton nuclear magnetic resonance (NMR) probe. Human myeloma IgG1 (kappa) proteins Ogo and Yot and human polyclonal IgG were used along with their Fab and F(ab')2 fragments for the assignment of the His-224 signals. The titration behavior of His-224 of the intact IgG and the fragments was compared. It was shown that the titration curves for the intact IgG and the F(ab')2 fragments are identical and quite similar to those for the histidine residue in small peptides. By contrast, the Fab fragments give titration curves which are quite different from those for the intact IgG and the F(ab')2 fragments. Conclusions derived may be summarized as follows: (1) in the intact IgG1, the hinge peptide is fully exposed to the solvent and exhibits internal motion which is much more rapid than the Fab segmental motion with respect to Fc: (2) at the loss of the Fc portion of the IgG, the conformation of the hinge peptide in the F(ab')2 fragments remains unchanged; (3) the heavy--heavy interchain interactions involving the two disulfide bridges do not play the primary role in determining the conformation of the hinge region in the intact IgG as well as in the F(ab')2 fragments; (4) the existence of a small stretch of peptide fragment Thr-225--Leu-234 is essential in maintaining the conformation of the hinge region of the intact IgG and the F(ab')2 fragments; (5) in the Fab fragments, as a result of cleavage of a major portion of the hinge peptide, the C-terminal part of the heavy chain including His-224 is partially folded back toward the globular portion of the polypeptide chains; and (6) the hinge peptide in the Fab fragments still retains a degree of flexibility which is similar to that in the intact IgG and the F(ab')2 fragments.