Six of seven serpins detected in grains of rye (Secale cereale) were purified and characterized. The amino acid sequence close to the blocked N-terminus, the reactive center loop sequence and the second order association rate constant (k a′) for irreversible complex formation with chymotrypsin were determined for each serpin. Three of four serpins containing the unusual reactive center P2–P1′ QQ↓S and one with P2–P1′ PQ↓M were equally efficient inhibitors of chymotrypsin (k a′∼105 M−1 s−1). One serpin with P2–P1′ PY↓M was a faster inhibitor (k a′∼106 M−1 s−1). Similar but differently organized glutamine-rich reactive centers were recently found in grain serpins cloned from wheat [Østergaard et al. (2000) J. Biol. Chem. 275, 33272] but not from barley. The prolamin storage proteins of cereal grains contain similar sequences in their glutamine-rich repeats. A possible adaption of hypervariable serpin reactive centers late in Triticeae cereal evolution as defence against insects feeding on cereal grains is discussed.
