The anthrax lethal factor (LF) is a Zn2+ endopeptidase specific for mitogen-activated protein kinase kinases (MAPKKs), which are cleaved within their N termini. Here, the proteolytic activity of LF has been investigated using novel chromogenic MAPKK-derived peptide substrates, which allowed us to determine the kinetic parameters of the reaction. LF displayed maximal proteolytic activity at the pH and temperature values of the cell cytosol, which is its site of action. LF undergoes substrate inhibition, in keeping with the non-productive binding geometry of the MAPPK-2 N terminus to LF.
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