NEWS OF THE W EEK
ENZYMES PLAY NICELY TOGETHER CATALYSIS: Natural enzymes and
artificial metalloenzymes work together in reaction cascades
cal catalysts don’t always play nicely. In fact, like playground bullies, they can be downright mean, shutting each other down. But now, an international team of chemists has found a way to make them friends. Making organometallic catalysts and enzymes compatible with one another could expand the range of reactions available for industrial biotechnology, such as the production of new types of biofuels. The team—Thomas R. Ward of the University of Basel, in Switzerland; Nicholas J. Turner of the University of Manchester, in England; and Frank Hollmann of Delft University of Technology, in the Netherlands; and coworkers—has developed an artificial metalloenzyme that works in reaction cascades with multiple natural enzymes (Nat. Chem., DOI: 10.1038/nchem.1498). This combination has been tough to achieve. “For many years, biocatalysts and organometallic catalysts have been developed separately,” says Yi Lu, a chemistry professor at the University of Illinois, UrbanaChampaign. “Although much effort has been devoted to combining the two fields, the incompatibility between the two types of catalysts has made it difficult to realize the promise, particularly in carrying out cascade reactions like those observed in biology.” The new work shows that an artificial metalloenzyme containing an organometallic center is “not only compatible with native enzymes but can also work in harmony with them to perform several cascade reactions,” Lu says. The researchers made an artificial transfer hydrogenase by embedding an iridium catalyst in a streptavidin
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RANSITION-METAL CATALYSTS and biologi-
scaffold. The streptavidin pocket protects the catalyst and prevents it from inactivating enzymes. “We’re keeping systems that are ‘allergic’ to each other apart,” Ward says. The researchers used genetic engineering to improve the metalloenzyme. “The organometallic catalyst alone is already active in the absence of any enzyme that inhibits it,” Ward says. “When inserted in streptavidin, the artificial metalloenzyme becomes compatible with a variety of natural enzymes. Thanks to genetic engiWORKING neering coupled with a TOGETHER colorimetric assay, we also In this reaction significantly improved the cascade, an performance of the artifienzyme turns cial metalloenzyme.” substrates into The researchers used the intermediates artificial transfer hydrogthat are enase in reaction cascades then used by with several other enzymes. an artificial For instance, they coupled metalloenzyme to it with a monoamine oxiform products. dase to catalyze the double stereoselective deracemization of amines. In addition, they made a four-enzyme cascade to synthesize l-pipecolic acid from l-lysine. “The authors have shown how molecular compartmentalization of organometallic catalysts can increase the efficiency of biocatalytic cascades and enable new cascades,” says Claudia SchmidtDannert, a biologist at the University of Minnesota, Twin Cities. “This work is just at the beginning of how protein scaffolds can be engineered and fitted with completely unnatural cofactors designed in the chemistry lab to perform novel types of enzyme reactions.”—CELIA ARNAUD
CLIMATE Thawing permafrost throws off global warming forecasts, report says Predictions of climate change—key to many policy-making efforts on global warming—likely lowball anticipated warming because they fail to account for emissions from thawing permafrost, says a new report. For instance, scientific assessments by the Intergovernmental Panel on Climate Change (IPCC) don’t factor in permafrost emissions. The report, released last week by the United Nations Environment Programme (UNEP), points out that human-caused climate change is expected to cause sig-
nificant amounts of permafrost to thaw. If that happens, organic material in this soil, frozen for millennia, will rot and release carbon dioxide and methane. Those greenhouse gas emissions from permafrost will add to global warming, the report says. This, in turn, will further accelerate thawing of the material, which is found beneath 24% of exposed land in the Northern Hemisphere. It lies under tundra, boreal forests, and alpine regions. International negotiators hammering out a treaty to limit climate change to 2 °C
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over preindustrial levels by 2100 need to take into account how emissions from permafrost will amplify global warming, says the report’s lead author, Kevin Schaefer, a researcher at the University of Colorado’s National Snow & Ice Data Center. He adds, “The release of CO2 and methane from warming permafrost is irreversible.” The UNEP report recommends that IPCC conduct a special assessment of how emissions from warming permafrost would influence Earth’s climate.— CHERYL HOGUE
