Comparison of the Fc fragment from a human IgG1 and its CH2, pFc', and tFc' subfragments. A study using reductive methylation and 13C NMR.

The Fc fragment of a human monoclonal IgG1 was compared with subfragments containing (a) the intact CH2 domain (CH2 fragment) or (b) the intact CH3 domain (pFc' and tFc' fragments). All fragments were reductively 13C-methylated and their resulting dimethyllysyl resonances characterized in 0.1 M KC1 as a function of pH by 13C NMR spectroscopy. Seven resonances were characterized for the 18 lysine residues of the Fc fragment, eight for the 12 lysines of the CH2 fragment, and five each for the 18 lysine residues of the Fc fragment, eight for the 12 lysines of the CH2 fragment, and five each for the 9 lysines of the pFc' and the 6 lysines of the tFc' fragments, respectively. The multiplicity of resonances indicates that the lysine residues in each fragment exist in a variety of microenvironments and that the fragments are all highly structured. The correspondence between 6 of the 12 or 13 perturbed lysine residues in the Fc fragment and the smaller subfragments indicates that the conformation of the CH2 and CH3 domains is largely unchanged in the smaller fragments. However, in addition to three lysines at the CH2-CH3 domain interface, whose environments were known to be disrupted in the smaller fragments, three or four lysine residues have somewhat different properties in the Fc fragment and in the subfragments, indicating that some local perturbations are induced in the domain structure in the subfragments.(ABSTRACT TRUNCATED AT 250 WORDS)