THE 1.6-ANGSTROM STRUCTURE OF KUNITZ-TYPE DOMAIN FROM THE ALPHA-3 CHAIN OF HUMAN TYPE-VI COLLAGEN

The C-terminal Kunitz-type domain from the alpha 3 chain of human type VI collagen (C5), a single 58 amino acid residue chain with three dis ulfide bridges, uas cloned, expressed and crystallized in a monoclinic form, space group P2(1), with a = 25.7 Angstrom, b = 38.2 Angstrom, c = 28.8 Angstrom and beta = 109 degrees. The structure was resolved by molecular replacement, using Alzheimer's protein precursor inhibitor and bovine pancreatic trypsin inhibitor three-dimensional structures a s search models. The molecule with one sulfate ion and 43 associated w ater molecules was refined by XPLOR to an X-factor of 18.9 % at 1.6 An gstrom. The molecule was not degraded by trypsin and did not inhibit t rypsin or tested serine proteases. As opposed to the other Kunitz fami ly members, C5 demonstrates left-handed chirality of the Cys14-Cys38 d isulfide bond. Inversion of the Thr13 carbonyl and bulky side-chains a t the interface with trypsin in a model of the C5-trypsin complex may explain the lack of inhibition of trypsin.