Crystallization and preliminary crystallographic studies of Schizolobium parahyba chymotrypsin inhibitor (SPCI) at 1.8 Å 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 Å resolution from a single crystal of SPCI under cryogenic conditions. The protein crystallized in space group P 2 1 2 1 2, with unit‐cell parameters a  = 40.01, b = 71.58, c = 108.68 Å 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.