Characterization of a half-apo derivative of peptidylglycine monooxygenase. Insight into the reactivity of each active site copper

A derivative of peptidylglycine monooxygenase which lacks the Cu_H center has been prepared and characterized. This form of the enzyme is termed the half-apo protein. Copper-to-protein stoichiometric measurements establish that the protein binds only one of the two copper centers (Cu_M and Cu_H) found in the native enzyme. Confirmation that the methionine-containing CU_M has been retained has been obtained from EXAFS experiments which show that the characteristic signature of the Cu - S(Met) interaction is preserved. The half-apo derivative binds 1 equiv of CO per copper with an IR frequency of 2092 cm^-1, and this monocarbonyl also displays the Cu - S(Met) interaction in its EXAFS spectrum. These results allow unambiguous assignment of the 2092 cm^-1 band as a Cu_M - CO species. Binding of CO in the presence of peptide substrate was also investigated. In the native enzyme, substrate induced binding of a second CO molecule with an IR frequency of 2062 cm^-1, tentatively assigned to a CO complex of the histidine-containing Cu_H site. Unexpectedly, this reactivity is also observed in the half-apo derivative, although the intensity distribution of the CO stretches now indicates that the copper has been partially transferred to a second site, believed to be Cu_H. The implications of this observation are discussed in terms of a possible additional peptide binding site close to the Cu_H center.