Kinetic Characterization of Lysine-Specific Metalloendopeptidases from Grifola frondosa and Pleurotus ostreatus Fruiting Bodies

Two zinc-metalloendopeptidases, GFMEP (accession number P81054) and POMEP (accession number P81055), from the fruiting bodies of two edible mushrooms, Grifola frondosa and Pleurotus ostreatus, respectively, specifically hydrolyze peptidyl-lysine bonds (-X-Lys-) in polypeptides. To understand detailed substrate specificities and kinetic characters of these enzymes, we have synthesized various intramolecularly quenched fluorescent peptide substrates and determined their kinetic constants with these substrates. Each synthesized fluorogenic peptide has a fluorescent residue, tryptophan, at its carboxyl terminus and a quenching group, dinitrophenyl (Dnp), at its amino terminus. Quenching of the Trp fluorescence in an intact substrate is relieved on hydrolysis of the -X-Lys- bond, giving rise to a continuous increase in fluorescence. The octapeptide substrate, Dnp-Ser-Thr-Ala-Thr-Lys-Leu-Ser-Trp, was an efficient substrate for both enzymes, the kcat/Km values being 9.8×106 and 7.0×105 M−1.s−1 for GF- and POMEP, respectively. Peptides with aspartic acid adjacent to the Lys residue were found to be poor substrates for both enzymes. Neither the shortest peptide, Dnp-Thr-Lys-Trp, nor peptides with substitution of L-Arg, L-ornithine, or D-Lys for Lys were hydrolyzed by either enzyme. These results confirmed the strict specificities of GF- and POMEP toward the peptide bond, -X-Lys-. Substitution of Co2+ for Zn2+ enhanced the activity, while the Km values were comparable. All peptides not hydrolyzed by either enzyme had inhibitory effects on GFMEP activity. The active site structure is discussed in relation to these observations.