International Union of Crystallography, Acta Crystallographica Section F: Structural Biology and Crystallization Communications, 10(64), p. 914-917, 2008
DOI: 10.1107/s1744309108026870
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A binary complex of the Schizolobium parahyba chymotrypsin inhibitor (SPCI) with chymotrypsin was purified by size-exclusion chromatography and crystallized by the sitting-drop vapour-diffusion method with 100 mM MES-NaOH pH 5.5, 20%(w/v) PEG 6000, 200 mM LiCl as precipitant and 200 mM nondetergent sulfobetaine molecular weight 201 Da (NDSB-201) as an additive. SPCI is a small protein with 180 amino-acid residues isolated from S. parahyba seeds and is able to inhibit chymotrypsin at a 1:1 molar ratio by forming a stable complex. X-ray data were collected to 2.8 A resolution from a single crystal of the SPCI-chymotrypsin binary complex under cryogenic conditions. The crystal belongs to space group P2(1)2(1)2(1), with unit-cell parameters a = 45.28, b = 64.57, c = 169.23 A, and the R(merge) is 0.122 for 11 254 unique reflections. A molecular-replacement solution was found using the preliminary crystal structure of SPCI and the structure of chymotrypsin (PDB code 4cha) independently as search models.