Lysozyme bound to the D1.3 monoclonal antibody retains enzymatic activity in assays using N-acetylglucosamine oligomers as substrate

An enzymatic assay using Micrococcus lysodeikticus as substrate had shown that hen egg-white lysozyme (HEL), complexed to the specific monoclonal antibody D1.3, was enzymatically inactive. However, a crystallographic study of the HEL-Fab D1.3 complex at 2.8 Å resolution showed that Fab D1.3 is bound to the enzyme at a region remote from the catalytic site, and that no significant conformational change had occured in the bound lysozyme. To ressolve this apparent discrepancy, oligomers of N-acetylglucosamine, containing an average of six residues and which should not be sterically hindered by the monoclonal antibody, were used as substrate in enzyme assays. In these assays lysozyme complexed to antibody D1.3 retained enzymatic activity, thus supporting the results of the crystallographic study and indicating no major conformational change or rigidification of its structure.