Three-Dimensional Structure of Fab Fragment of Monoclonal Antibody LNKB-2 Complexed with Antigenic Nonaptide from Human Interleukin-2

The three-dimensional structure of the antigen-binding fragment (Fab) of the monoclonal antibody LNKB-2 in complex with the synthetic antigenic nonapeptide of human interleukin-2 (IL-2; Lys-Pro-Leu-Glu-Glu-Val-Leu-Asn-Leu-O) was determined by X-ray diffraction at a resolution of 2.6 angstrom in the crystal space group P2(1)2(1)2(1). The peptide adopts a somewhat distorted alpha-helical conformation, close to that of fragment 64-72 of the IL-2 antigen. Four out of the six hypervariable loops in the antigen-binding site of the Fab fragment are involved in nonapeptide association through hydrogen bonding, salt bridge formation, and hydrophobic interactions. Moreover, Tyr residues of an antibody play an important role in antigen-antibody recognition.