The catalytic role of the copper ligand H172 of peptidylglycine α-hydroxylating monooxygenase (PHM):: A spectroscopic study of the H172A mutan

The spectroscopic characterization of the H172A mutant of peptidylglycine a-hydroxylating monooxygenase (PHM) was undertaken to determine the importance of this CUB ligand in the catalytic mechanism of PHM. Mutation of this histidine reduced the activity of the enzyme over 300-fold with little effect on the structure of the oxidized form. However, the reduced enzyme showed a decrease in the average Cu-N(His) distances from 1.96 Angstrom in wild-type PHM to 1.89 Angstrom in H172A associated with a change in the structure Of Cu-H from distorted T-shaped planar in the wild type to 2-coordinate in the mutant. Binding of CO was retained at the Cum site (similar to wild type), and peptide substrate binding continued to activate a second site for CO binding. Confirmation of this substrate-induced CO binding site at Cu-H was obtained through the observation that loss of the H172 Cu-H ligand caused a 3 cm(-1) blue shift in the v(CO) for this copper carbonyl. Possible mechanistic roles for the H172 ligand are discussed.