Legumain

Legumain is a cysteine protease with a strict specificity for cleaving after asparagine and, to a lesser extent, aspartic acid residues. Therefore, it is synonymously named the asparaginyl endopepti-dase or AEP. Its very strict substrate specificity makes it an ideal enzyme for example for digestion proteomics. Human legumain is synthesized as an inactive proenzyme (56 kDa) composed of the caspase-like catalytic AEP domain (36 kDa) and a C-terminal death-domain-like prodomain. Activation to the active AEP proceeds via the pH-dependent autocatalytic release of the C-terminal prodomain at acidic pH. While prolegumain is stable at neutral pH, AEP is stable at acidic pH (≤ 6.0) but will irreversibly unfold at near neutral pH conditions. The pH-optimum of its endopeptidase activity is at pH 5.5 with a strong preference for cleaving after asparagine residues. Hydrolysis of aspartyl peptide bonds is preferentially catalyzed at more acidic pH (pH 4.0). In addition to its well established protease activity, legumain harbors a pH-dependent ligase activity at near neutral pH.