Melain G, a cysteine protease from green fruits of the bead tree, Melia azedarach: a protease affected by specific amino acids at P-3 position

A protease (melain G) was isolated from the greenish fruits of the bead tre e, Melia azedarach var.japonica Makino. Melain G shares 110 identical amino acid residues (50%) with papain, 112(51%) with actinidain, and 91 (41%) wi th stem bromelain. From the sites cleaved in the oxidized insulin B-chain a nd synthetic oligopeptide substrates by melain G the enzyme preferred small amino acid residues such as Gly or Ser at the P-2 position and negatively charged residues such as glutamic or cysteic acid at the P-3 position. This is clearly different from the specificity of papain, which prefers rs the large hydrophobic amino acid residues such as Phe, Val, and Leu at the Pt p osition. Accordingly, it is presumed that the bottom of the S-2 pocket of m elain G is shallow due to the presence of a Phe residue. and a bulky P-2 su bstrate (for example Phe residue) is not preferred by the enzyme. Negativel y charged residues at the P-3 position of substrates well suited the S-3 Si te of melain G for making a salt bridge. It is likely that Arg61 is the S3 position of melain G by analogy with papain. (C) 1999 Elsevier Science B.V. All rights reserved.