CRYSTAL STRUCTURE OF HUMAN FACTOR XIII BOUND TO YTTERBIUM: PROBING CALCIUM-BINDING. Brian A. Fox, Paul D. Bishop, Ronald E. Stenkamp, David C. Teller,and Vivien C. Yee, Departments of Biochemistry and Biological Structure, University of Washington, Seattle, WA, 98195 and ZymoGenetics Inc, Seattle, WA, 98102

We have determined the structure of the recombinant human blood coagulation protein, factor XIII, with ytterbium bound in its calcium binding site. In the final step of blood clotting, activated factor XIIIa crosslinks polymerized fibrin and renders it mechanically and proteolytically stable. The factor XIII active site bears a striking resemblance to that of the cysteine proteases. This structural similarity supports a transglutaminase mechanism based on the reverse of the proteolysis carried out by the papain family of cysteine proteases.

Factor XIII is activated by thrombin cleavage in the presence of physiological concentrations of calcium. In the absence of proteolytic cleavage, factor XIII can be activated by high, non-physiological levels of calcium, in the upper millimolar range. In efforts to identify the protein ligands necessary for calcium binding, and to understand the role of calcium in the activation of the enzyme, we have determined the structure of factor XIII bound to the calcium analog ytterbium.

A crystal of factor XIII zymogen soaked in 2mM YbCl₃ for 48 hours was used for data collection. The unit cell dimensions for this P2₁ crystal were a=101.06 Å , b=72.39 Å, c=135.99 Å, and beta=106.09 deg. The structure is being refined against diffraction data from 10.0-2.5 Å resolution. This work has been funded by NIH grant HL-50355.