Abstract
The activity of cathepsin E (M(r) approximately 80K), a dimeric aspartic proteinase with two active sites per molecule, toward a protein substrate (reduced and carboxymethylated ribonuclease A) was shown to be completely inhibited by alpha 2-macroglobulin (alpha 2M) at pH 5.5. On the other hand, the activity toward a peptide substrate (oxidized insulin B chain) was scarcely inhibited. Under these conditions, cathepsin E cleaved alpha 2M at the Phe684-Tyr685 bond in the bait region sequence, resulting in a drastic conformational change (from a doughnut to an H shape) in the inhibitor as revealed by electron microscopy, and was non-covalently trapped by alpha 2M in an approximate molar ratio (enzyme: alpha 2M) of 2:1.
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