Melain G, a cysteine protease from green fruits of the bead tree, Melia azedarach: a protease affected by specific amino acids at P3 position.

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 P2 position and negatively charged residues such as glutamic or cysteic acid at the P3 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 P2 position. Accordingly, it is presumed that the bottom of the S2 pocket of melain G is shallow due to the presence of a Phe residue, and a bulky P2 substrate (for example Phe residue) is not preferred by the enzyme. Negatively charged residues at the P3 position of substrates well suited the S3 site of melain G for making a salt bridge. It is likely that Arg61 is the S3 position of melain G by analogy with papain.