Proteolysis of Phaseolin in Relation to Its Structure

Abstract

Phaseolin differs from other native legume proteins in that its hydrolysis by trypsin and pepsin stops after a limited number of peptides have been cleaved off. Concomitantly, trypsin splits each phaseolin subunit approximately into halves. The N-terminal sequencing of these final hydrolysis products of high molecular mass showed loss of a tetrapeptide from the N terminus of each subunit, with a second cleavage in the interdomain linker. Other probable sites cleaved by trypsin, which account for the quantity of degraded protein, may be deduced from the tertiary structure of phaseolin and from the specificity of trypsin. Proteolysis by pepsin is limited to cleavage of seven amino acids from the N terminus, and of two to three peptides probably from the disordered C-terminal segment of phaseolin subunits. The cleavage site in the N-terminal sequence has been identified. The peculiarities of the phaseolin structure that may cause its resistance to the proteolytic attack are discussed. Keywords: Phaseolin; proteolysis; pepsin; trypsin