THE 2.5 Å STRUCTURE OF FAB JEL42-HPR COMPLEX L. Prasad, J.S. Lee, E.B. Waygood and L.T.J. Delbaere. Department of Biochemistry,University of Saskatchewan, 107 Wiggins Road, Saskatoon, Saskatchewan, S7N 5E5 CANADA

We have reported the comparison of epitope predictions from site-directed mutagenesis experiments with those from the 2.8 Å structure of the Fab Jel42 antibody-HPr complex (Prasad et al. 1993. J. Biol. Chem., 268, 10705-10708). We now report the 2.5 Å structure of the Jel42-HPr complex which provides further details of antibody-antigen recognition.

The refinement of the structure was extended to 2.5 Å with x-ray intensity data collected on a large crystal (1.2 x 0.4 x 0.2 mm) with an Enraf-Nonius FAST Area Detector on an Enraf-Nonius FR571 rotating anode generator with copper radiation and graphite monochromator at 15deg.C. The refinement was carried out using the molecular dynamics program X-PLOR. Cycles of refinement were interspersed with manual model building and adjustments using the program TURBO-FRODO. The structure refined to an R-value of 0.22 for the resolution range 10 to 2.5 Å. The refined structure includes 102 solvent molecules and 2 sulfate ions.

The elbow angle for Fab Jel42 in the complex is 154deg.. The buried surface area at the antibody-antigen combining site, calculated by the method of Connolly using a probe radius of 1.7 Å is 665 Å2 for the Fab and 718 Å2 for HPr. Antibody-antigen binding is mediated by 9 hydrogen bonds and 79 van der Waals contacts ; 16 residues of HPr and 21 residues of the Fab are involved in the binding. An additional salt bridge contact involves the CDR loop (CDRL2) which previously appeared not to be involved in antigen binding. In addition, there are 18 contacts between the Fab and HPr, mediated through 11 water molecules.

This research was funded by the Medical Research Council of Canada.