title: The involvement of histidine at the active site of Harding-Passey mouse melanoma tyrosinase.
abstract: The nature of the essential residues at the active site of Harding-Passey mouse melanoma tyrosinase has been explored by kinetic and photochemical modification studies. Km for L-dopa depends strongly on pH, so that acidic pH prevents the formation of the enzyme-substrate complex because the protonation of an enzyme group with a pKa of 6.6. Halide ions inhibit competitively the enzyme activity, being F the more potent one. This inhibition is also pH-dependent, showing the involvement of a protonatable group of the enzyme with apparent pKa ranging from 5.9 to 7.0. Tyrosinase has also been modified with visible light using Rose Bengal as photosensitizer, yielding a pH-dependent photoinactivation, characteristic of histidyl residues. All these results strongly support that histidine plays an important role in the dopa-oxidase activity of the enzyme, very probably acting as the ligand of copper at the active site of the enzyme.
mesh_terms: D000818:Animals; D001665:Binding Sites; D004156:Catechol Oxidase / Q000378:metabolism*; D002460:Cell Line; D006639:Histidine; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D007980:Levodopa / Q000378:metabolism; D008545:Melanoma / Q000201:enzymology*; D051379:Mice; D008810:Mice, Inbred C57BL; D014442:Monophenol Monooxygenase / Q000302:isolation & purification / Q000378:metabolism*; D010084:Oxidation-Reduction; D010777:Photochemistry
chemical_list: D007980:Levodopa; D006639:Histidine; D004156:Catechol Oxidase; D014442:Monophenol Monooxygenase