Long-Range Conformational Changes in Monoclonal Antibodies Revealed Using FPOP-LC-MS/MS

Differences in conformational dynamics between two full-length monoclonal antibodies have been probed in detail using Fast Photochemical Oxidation of Proteins (FPOP) followed by proteolysis and LC-ESI-MS/MS analyses. FPOP uses hydroxyl radical labeling to probe the surface-accessible regions of proteins and has the advantage that the resulting covalent modifications are irreversible, thus permitting optimal downstream analysis. Despite the two monoclonal antibodies (mAbs) differing by only three amino acids in the heavy chain complementarity determining regions (CDRs), one mAb, MEDI1912-WFL, has been shown to undergo reversible self-association at high concentrations and exhibited poor pharmacokinetic properties in vivo, properties which are markedly improved in the variant, MEDI1912-STT. Identifying the differences in oxidative labeling between the two antibodies at residue level revealed long-range effects which provide a key insight into their conformational differences. Specifically, the amino acid mutations in the CDR region of the heavy chain resulted in significantly different labeling patterns at the interfaces of the C_L-C_H1 and C_H1-C_H2 domains, with the nonaggregating variant undergoing up to four times more labeling in this region than the aggregation prone variant, thus suggesting a change in the structure and orientation of the C_L-C_H1 interface. The wealth of FPOP and LC-MS data obtained enabled the study of the LC elution properties of FPOP-oxidized peptides. Some oxidized amino acids, specifically histidine and lysine, were noted to have unique effects on the retention time of the peptide, offering the promise of using such an analysis as an aid to MS/MS in assigning oxidation sites.