The presence of a nonheme metal, such as copper and iron, in the heme-copper oxidase (HCO) superfamily is critical to the enzymatic activity of reducing O2 to H2O, but the exact mechanism the nonheme metal ion uses to confer and fine-tune the activity remains to be understood. We herein report that manganese and cobalt can bind to the same nonheme site and confer HCO activity in a heme-nonheme biosynthetic model in myoglobin. While the initial rates of O2 reduction by the Mn, Fe, and Co derivatives are similar, the percentages of reactive oxygen species (ROS) formation are 7%, 4%, and 1% and the total turnovers are 5.1 ± 1.1, 13.4 ± 0.7, and 82.5 ± 2.5, respectively. These results correlate with the trends of nonheme-metal-binding dissociation constants (35, 22, and 9 μM) closely, suggesting that tighter metal binding can prevent ROS release from the active site, lessen damage to the protein, and produce higher total turnover numbers. Detailed spectroscopic, electrochemical, and computational studies found no evidence of redox cycling of manganese or cobalt in the enzymatic reactions and suggest that structural and electronic effects related to the presence of different nonheme metals lead to the observed differences in reactivity. This study of the roles of nonheme metal ions beyond the Cu and Fe found in native enzymes has provided deeper insights into nature's choice of metal ion and reaction mechanism and allows for finer control of the enzymatic activity, which is a basis for the design of efficient catalysts for the oxygen reduction reaction in fuel cells.
Bibliographical noteFunding Information:
The authors thank Daniel G. Nocera for helpful discussions and comments. We also thank P. Hosseinzadeh for experimental assistance involving data collection and analysis. J.R. was supported by predoctoral training fellowship 5T32-GM070421 from the U.S. National Institutes of Health (NIH). E.M. was supported by predoctoral training fellowship 5T32-GM8276-24 from the NIH. This material was supported in part by the U.S. NIH under Awards NIH R01GM06211 (to Y.L) and NIH R01GM074785 (to P.M.-L.) and by a U.S. National Science Foundation (NSF) Award (NSF CHE-1300912 to Y.Z.). Use of the Stanford Synchrotron Radiation Lightsource (SSRL), SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-76SF00515. The SSRL Structural Molecular Biology Program is supported by the Department of Energy Office of Biological and Environmental Research and by the NIH National Institute of General Medical Sciences (NIHMS).
© 2017 American Chemical Society.