Structure of a monoclonal 2E8 Fab antibody fragment specific for the low-density lipoprotein-receptor binding region of apolipoprotein E refined at 1.9 Å

The crystal structure of the Fab fragment of 2E8, the monoclonal IgG1,κ antibody specific for the low-density lipoprotein (LDL) receptor-binding region of apolipoprotein E (apoE), has been solved by molecular replacement and refined at 1.9 Å resolution (PDB entry 12E8). Two 2E8 Fab molecules in the asymmetric unit are related by noncrystallographic symmetry and are hydrogen bonded through a β-­sheet-like intermolecular contact between the heavy-chain complementarity-determining regions 3 (CDRH3) of each molecule. The structure has been refined to an R value of 0.22 (R_free = 0.27). The initially ill-defined heavy-chain constant domain (C_H1) of 2E8 has been retraced with the aid of automatic refinement, confirming the β-sheet tracing independently of any starting models. As a resolution better than 2 Å is not common for Fab fragments, this model represents a well defined Fab structure and should prove useful in MR solution of other Fab fragments. Furthermore, in the absence of an LDL-receptor structure, the homology of the 2E8 CDRH2 to the ligand-binding domain of the LDL receptor has been exploited to model the apoE–LDL-receptor interaction.