Naturally occurring metalloenzymes such as hydrogenase and carbon monoxide dehydrogenase are exemplary catalysts, using earth-abundant transition metals such as nickel and iron to reversibly generate and oxidize small-molecule fuels such as hydrogen and carbon monoxide with high rates, efficiencies, and under mild reaction conditions. While the native enzymes are poorly suited for large-scale application, they can offer design guidelines for anthropogenic catalysts. Recognizing the importance of the entire protein scaffold for the high levels of catalytic activity seen in native metalloenzymes, we are designing model metalloenzymes that recapitulate the activity of hydrogenase and carbon monoxide dehydrogenase within robust protein scaffolds. Rubredoxin and ferredoxin are robust, well-studied proteins within the bioinorganic community. By introducing non-native metals and redesigning the primary and secondary coordination spheres, we have been able to install novel activity into these simple electron transfer proteins, including catalytic hydrogen evolution and carbon dioxide fixation. Adding key elements from the secondary and tertiary coordination spheres of hydrogenase and CODH enhances both activity and selectivity, pointing to functional roles of specific residues within the natural enzymes. Optical, vibrational, and magnetic resonance spectroscopic techniques have been used in conjunction with density functional theory calculations and molecular dynamics simulations to probe the active-site structures across different states in order to determine the catalytic mechanisms. These findings will be discussed in the context of identifying the fundamental principles underlying highly active native enzymes and applying those principles towards engineering effective model metalloproteins for energy conversion reactions.

Designing model metalloenzymes that imitate hydrogenase and carbon monoxide fhydrogenase

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