Inhibitory serpins from rye grain with glutamine as P 1 and P 2 residues in the reactive center

Abstract

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 P 2–P 1′ QQ↓S and one with P 2–P 1′ PQ↓M were equally efficient inhibitors of chymotrypsin ( k a′∼10 5 M −1 s −1). One serpin with P 2–P 1′ PY↓M was a faster inhibitor ( k a′∼10 6 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.