The refined 2.0 Å X‐ray crystal structure of the complex formed between bovine β‐trypsin and CMTI‐I, a trypsin inhibitor from squash seeds (Cucurbita maxima) Topological similarity of the squash seed inhibitors with the carboxypeptidase A inhibitor from potatoes

The stoichiometric complex formed between bovine β‐trypsin and the Cucurbita maxima trypsin inhibitor I (CMTI‐I) was crystallized and its X‐ray crystal structure determined using Patterson search techniques. Its structure has been crystallographically refined to a final R value of 0.152 (6.0 — 2.0 Å). CMTI‐I is of ellipsoidal shape; it lacks helices or β‐sheets, but consists of turns and connecting short polypeptide stretches. The disulfide pairing is CYS‐3I‐20I, Cys‐10I‐22I and Cys‐16I‐28I. According to the polypeptide fold and disulfide connectivity its structure resembles that of the carboxypeptidase A inhibitor from potatoes. Thirteen of the 29 inhibitor residues are in direct contact with trypsin; most of them are in the primary binding segment Val‐2I (P4) — Glu‐9I (P4′) which contains the reactive site bond Arg‐5I — Ile‐6I and is in a conformation observed also for other serine proteinase inhibitors.

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