The 1.6 Å structure of Kunitz-type domain from the α3 chain of human type VI collagen

The C-terminal Kunitz-type domain from the α3 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 monoclinic form, space group P 2 1 , with a = 25.7 Å, b = 38.2 Å, c = 28.8 Å and β = 109°. 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 Å. 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 Cyauthor4-Cy8 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.