Research has shown that C-terminal lysine residues of the heavy chain (present on all IgG isotypes) are cleaved off once the IgG is in circulation. Studies have also shown that there is no functional impact of the lysine once it’s cleaved and it’s presence is actually inhibitory to allowing the antibody to reach full cytotoxicity potential: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4622059/