Amino acid sequencing of a trypsin inhibitor by refined 1.6 Å X-ray crystal structure of its complex with porcine β-trypsin

AbstractThe stoichiometric complex formed between porcine β-trypsin and the Momordica charantia, Linn. Cucurbitaceae trypsin inhibitor-A (MCTI-A) was crystallized and its X-ray crystal structure determined using molecular replacement method. The primary sequence and topology of the inhibitor was determined by recognizing the electron density and refined to a final R value of 0.167 (7.0—1.6 Å) with RMS deviation of bond lengths from standard values 0.012 Å. The sequence was compared with those obtained by other groups and was found to be similar to the squash proteinase inhibitor. Its spatial structure and the conformation of its primary binding segment from Cys-31 (P3) to Glu-71 (P3′) which contains the reactive scissile bond Arg-51 C-Ile-61 N were also very similar with other squash family proteinase inhibitors