Major changes in copper coordination accompany reduction of peptidylglycine monooxygenase: implications for electron transfer and the catalytic mechanism

X-ray absorption spectroscopy has been used to probe the local coordination of the copper centers in the oxidized and reduced states of the peptidylglycine monooxygenase catalytic core (PHMcc) in both the resting and substrate-bound forms of the enzyme. The results indicate that reduction causes significant changes in coordination number and geometry of both Cu centers (Cu-H and Cu-M). The Cu-H center changes from 4- or 5-coordinate tetragonal to a 2-coordinate configuration, with one of the three histidine ligands becoming undetectable by EXAFS (suggesting that it has moved away from the Cu-H by at least 0.3 Angstrom). The Cu-M center changes from 4- or 5-coordinate tetragonal to a trigonal or tetrahedral configuration, with an estimated 0.3-0.5 Angstrom movement of the M314 S ligand. Reduction also leads to loss of coordinated water from both of the coppers. Substrate binding has little or no effect on the local environment of the Cu centers in either oxidation state. These findings bring into question whether direct electron transfer between Cu-H and Cu-M via a tunneling mechanism can be fast enough to support the observed catalytic rate, and suggest that some other mechanism for electron transfer, such as superoxide channeling, should be considered.