Bowman-Birk Inhibitor from lentil bean
M.W.: 7.6 kDa
BBI from lentil bean is a monomer comprised of 68 amino acids and forms a 1:1 stoichiometric complex with the protease active site. The complex of the inhibitor with either trypsin or chymotrypsin has no further inhibitory effect toward more of the same enzyme, but has full activity towards the other enzyme (forming 1:1:1 complex). This implies that there is one binding site for trypsin and another for chymotrypsin. Trypsin binds to a Lys-Ser site, while chymotrypsin binds to a Leu-Ser site. Inhibition is both reversible and pH dependent. Dissociation of this complex may yield a modified or native form of the inhibitor. The optimal pH for trypsin binding is 8.0 with an association constant of greater than 109 at pH 8.0, and an association constant of 0.15-2.6 x 104 at pH 3.6-4.4.