STRUCTURE OF PORCINE PANCREATIC SPASMOLYTIC POLYPEPTIDE AT 1.95-ANGSTROM RESOLUTION

The structure of a trigonal crystal form of porcine pancreatic spasmol ytic polypeptide (PSP) has been solved by molecular replacement and re fined to 1.95 Angstrom resolution. Three heavy-atom derivatives were p repared, giving unbiased phase information, which was used in the mode l building of the protein molecules. The final conventional R value is 19.8% with the inclusion of 183 water molecules. PSP crystallizes as a dimer in space group P3(1)21 with a non-crystallographic twofold axi s relating the monomers. The monomer consists of two very similar doma ins each composed of three loop regions. Two clefts are found in the m onomer, one in each domain, that are proposed as possible substrate-bi nding sites. Important interactions have been identified in the propos ed substrate-binding sites, where conserved water molecules probably m imic the hydrophilic positions of the substrate. The estimated cleft s ize is 9 x 9 x 12 Angstrom. Analysis of the charge distribution within the clefts, by an electrostatic potential calculation, shows the clef ts to be essentially non-charged.