The crystal structure of the Fab fragment of a rat monoclonal antibody against the main immunogenic region of the human muscle acetylcholine receptor

The crystal structure of the Fab fragment of a rat monoclonal antibody, num ber 192, with a very high affinity (K-d = 0.05 nM) for the main immunogenic region of the human muscle acetylcholine receptor (AChR), has been determi ned and refined to 2.4 Angstrom resolution by X-ray crystallographic method s. The overall structure is similar to a Fab (NC6.8) from a murine antibody , used as a search model in molecular replacement. Structural comparisons w ith known antibody structures showed that the conformations of the hypervar iable regions H1, H2, L1, L2, L3 of Fab192 adopt the canonical structures 1 , 1, 2, 1, and 1, respectively. The surface of the antigen-binding site is relatively planar, as expected for an antibody against a large protein anti gen, with an accessible area of 2865 Angstrom(2). Analysis of the electrost atic surface potential of the antigen-binding site shows that the bottom of the cleft formed in the center of the site appears to be negatively charge d. The structure will be useful in the rational design of very high affinit y humanized mutants of Fab192, appropriate for therapeutic approaches of th e model autoimmune disease myasthenia gravis.