Designed metalloenzymes made by modifying a native protein scaffold have shown unusually high levels of efficiency and longevity and have revealed new details about the way oxidases, the natural enzymes they mimic, may work. The study is an important step in efforts to design customized enzymes for a variety of potential biological and alternative-energy applications.
Most designed enzymes have relatively simple active-site structures, low activity, and limited turnovers (duration of activity). Designing artificial enzymes with complicated metal-based active sites, respectable activity, and high turnover numbers remains a major challenge.
Now, metalloprotein specialist Yi Lu of the University of Illinois, Urbana-Champaign, and coworkers, in collaboration with Jiangyun Wang’s group at the Institute of Biophysics of the Chinese Academy of Sciences, in Beijing, report the rational design of metalloenzymes with heme-copper centers, unusual efficiency, and more than 1,000 turnovers in catalyzing the reduction of oxygen to water (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201201981 and DOI: 10.1002/anie.201108756).
The catalytic activities of the designed enzymes are about 0.7% those of comparable native oxidases. This is about the same level of mimicry as a synthetic hydrolase developed last year by chemistry professor Vincent L. Pecoraro’s group at the University of Michigan, Ann Arbor (Nat. Chem., DOI: 10.1038/nchem.1201; C&EN, Dec. 5, 2011, page 7), an artificial enzyme that was considered to work unusually well.
Turnover numbers achieved by the synthetic oxidases are, likewise, relatively high compared with those of previously designed enzymes. Natural enzymes typically have thousands to millions of turnovers.
In addition, the new catalysts generate only limited quantities of reactive oxygen species.
As products of incomplete reduction, reactive oxygen species such as peroxide and superoxide are damaging not only to biomolecules but also to fuel-cell components. Preventing their formation is a challenge to scientists trying to improve the efficiencies of biological and industrial catalysts.
“Cleanly reducing oxygen to water with minimal formation of reactive oxygen species has been a major challenge in protein design, and it is a significant accomplishment” for the designed enzyme to have achieved that goal, comments protein design expert William F. DeGrado of the University of California, San Francisco.
Despite long-standing efforts, details of how some oxidases work—such as the roles of copper and a tyrosine-histidine cross-link in the active site—are not well understood. In the new catalysts, copper is not essential in triggering catalytic activity, but tyrosine is, and tyrosine-histidine positioning and cross-linking have major effects on catalytic power. These findings could prove applicable to natural oxidases.
The studies demonstrate “the power of protein redesign as it addresses decades-old questions in an elegant way,” Pecoraro says.
“The work looks like a significant step forward in the design of metalloproteins,” DeGrado adds.