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 β-sheet with the polypeptide segments 11-17, 36-39, 50-56 and 75-76, two well defined α-helices from residues 20-30 and 60-70, and a 310 helix from residues 43-46. The three helices are oriented almost exactly antiparallel to each other, are all on the same side of the β-sheet, and the helix axes from an angle of ∼45° relative to the direction of the β-strands. Three segments linking β-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 β-strand 36-39 and precedes the short 310 helix.