Crystallization and preliminary crystallographic studies of Schizolobium parahyba chymotrypsin inhibitor (SPCI) at 1.8 A resolution.

SPCI, a Kunitz-type chymotrypsin inhibitor, is a 180-amino-acid polypeptide isolated from Schizolobium parahyba seeds. This inhibitor has been characterized as a highly stable protein over a broad pH and temperature range. SPCI was crystallized using a solution containing 0.1 M sodium acetate trihydrate buffer pH 4.6, 33%(v/v) PEG 2000 and 0.2 M ammonium sulfate. Data were collected to 1.80 A resolution from a single crystal of SPCI under cryogenic conditions. The protein crystallized in space group P2(1)2(1)2, with unit-cell parameters a = 40.01, b = 71.58, c = 108.68 A and an R(merge) of 0.052. The structure of SPCI has been solved by molecular replacement using the known structure of the Kunitz-type trypsin inhibitor from Delonix regia (PDB code 1r8n) as the search model.