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

Abstract

A protease (melain G) was isolated from the greenish fruits of the bead tree, Melia azedarach var. japonica Makino. Melain G shares 110 identical amino acid residues (50%) with papain, 112 (51%) with actinidain, and 91 (41%) with stem bromelain. From the sites cleaved in the oxidized insulin B-chain and 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 the large hydrophobic amino acid residues such as Phe, Val, and Leu at the P 2 position. Accordingly, it is presumed that the bottom of the S 2 pocket of melain G is shallow due to the presence of a Phe residue, and a bulky P 2 substrate (for example Phe residue) is not preferred by the enzyme. Negatively charged residues at the P 3 position of substrates well suited the S 3 site of melain G for making a salt bridge. It is likely that Arg61 is the S 3 position of melain G by analogy with papain.