The NMR structure of the activation domain isolated from porcine procarboxypeptidase B.

The three‐dimensional structure of the activation domain isolated from porcine pancreatic procarboxypeptidase B was determined using 1H NMR spectroscopy. A group of 20 conformers is used to describe the solution structure of this 81 residue polypeptide chain, which has a well‐defined backbone fold from residues 11–76 with an average root mean square distance for the backbone atoms of 1.0 +/‐ 0.1 A relative to the mean of the 20 conformers. The molecular architecture contains a four‐stranded beta‐sheet with the polypeptide segments 11–17, 36–39, 50–56 and 75–76, two well defined alpha‐helices from residues 20–30 and 60–70, and a 3(10) helix from residues 43–46. The three helices are oriented almost exactly antiparallel to each other, are all on the same side of the beta‐sheet, and the helix axes from an angle of approximately 45 degrees relative to the direction of the beta‐strands. Three segments linking beta‐strands and helical secondary structures, with residues 32–35, 39–43 and 56–61, are significantly less well ordered than the rest of the molecule. In the three‐dimensional structure two of these loops (residues 32–35 and 56–61) are located close to each other near the protein surface, forming a continuous region of increased mobility, and the third disordered loop is separated from this region only by the peripheral beta‐strand 36–39 and precedes the short 3(10) helix.