The 1.6 A 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 disulfide bridges, was cloned, expressed and crystallized in a monoclonic form, space group P2(1), with a = 25.7 A, b = 38.2 A, c = 28.8 A 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 as search models. The molecule with one sulfate ion and 43 associated water molecules was refined by XPLOR to an R-factor of 18.9% at 1.6 A. The molecule was not degraded by trypsin and did not inhibit trypsin or tested serine proteases. As opposed to the other Kunitz family members, C5 demonstrates left-handed chirality of the Cys14-Cys38 disulfide bond. Inversion of the Thr13 carbonyl and bulky side-chains at the interface with trypsin in a model of the C5-trypsin complex may explain the lack of inhibition of trypsin.