Studies on the structure and function of plant protein inhibitors of trypsin and α-amylase

Abstract

Studies of the primary structure of protein inhibitors of proteolytic enzymes and α-amylase were undertaken. The complete amino acid sequence of an α-amylase inhibitor and a bifunctional trypsin/α - amylase inhibitor from seeds of ragi (Eleusine coracana Gaertn.) and of the proteinase inhibitor PI-I from the Tracy cultivar of soybean (Glycine max L. Merr) were determined. The bifunctional trypsin/ α-amylase inhibitor from ragi seeds was shown to be a single polypeptide of 122 amino acids with a molecular weight of 13400. The two reactive (trypsin inhibitory) sites were also determined. Sequence comparisons revealed that this inhibitor seems to be divergently related to other trypsin and α-amylase inhibitors and also to the reserve protein from castor bean. In addition it is proposed that a new inhibitor family should be added to the existing ones (Laskowski and Kato, 1980) to accomodate this bifunctional inhibitor and its related proteins. The secondary structure of this inhibitor was also predicted. The α-amylase inhibitor from ragi seeds was shown to be a single polypeptide of 95 amino acids with a molecular weight of 9300. The existence of two homologous regions in the amino acid sequence of this inhibitor seemed, to indicate that the inhibitor molecule has arisen by a process of gene duplication. Sequence comparisons revealed that this inhibitor has no homology to any other α-amylase inhibitors, proteolytic enzyme inhibitors or any other plant protein of known primary structure. In addition the secondary structure of the α-amylase inhibitor from ragi seeds was also predicted. The amino acid sequence of the proteinase inhibitor PI-I from the Tracy cultivar of soybean (Glycine max L. Merr) was shown to be identical to another proteinase inhibitor PI-II) from the same cultivar, which have been previously sequenced by Kashlan(1980).