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Carbenes created in a cell

Chemists coax Streptomyces albus into making the carbene precursor azaserine, which reacts with styrene also made biosynthetically

by Bethany Halford
May 3, 2023 | A version of this story appeared in Volume 101, Issue 15


A scheme showing how styrene and azaserine react in the presenence of an engineered P450 catalyst to form a molecule with a cyclopropyl group.

Synthetic chemists have long used carbenes to create cyclopropyl groups and to strategically insert carbon into certain bonds. But these highly reactive intermediates haven’t been available in the realm of synthetic biology—until now. Scientists have created Streptomyces albus cells that can biosynthesize azaserine, a natural product and carbene precursor, and they get it to react with styrene also made by the same cellular system. An engineered P450 enzyme catalyzes the cyclopropanation reaction (shown).

“The whole process end to end is happening in the cell. You basically are adding glucose on one side and getting a cyclopropanated bioactive compound on the other side,” says Aindrila Mukhopadhyay, an expert in microbial processes at Lawrence Berkeley National Laboratory, who was a leader on the project.

The synthetic biology system could help researchers biosynthetically create cyclopropanated versions of natural products, says John F. Hartwig, a chemist at the University of California, Berkeley, who also co-led the project along with colleagues Jay D. Keasling and Douglas S. Clark. The findings suggest synthetic biology could one day also be used to make drugs that contain cyclopropane groups.

The whole process end to end is happening in the cell. You basically are adding glucose on one side and getting a cyclopropanated bioactive compound on the other side.
Aindrila Mukhopadhyay, staff scientist, Lawrence Berkeley National Laboratory

Compared with synthetic chemistry, which can use toxic reagents and generate solvent waste, “biology is inherently more sustainable, scalable, and allows more benign processes to be used,” Mukhopadhyay says. The trick is getting this chemistry to take place in a biological system. She creditsBerkeley postdoctoral scholar Jing Huang with orchestrating the complex biochemical processes that build the azaserine and styrene in the cell and also incorporating an engineered P450 enzyme that could catalyze the cyclopropanation reaction (Nature 2023, DOI: 10.1038/s41586-023-06027-2).

The cyclopropanation reaction had excellent diastereoselectivity, preferably producing one of four possible diastereomers. The researchers also demonstrated that they could use the azaserine to insert a carbene into a carbon-hydrogen bond in phthalan. Next, Hartwig says, the chemists on the team would like to improve the C–H insertion capabilities and to do the cyclopropanation on substrates other than styrene.

Frances H. Arnold, an expert in enzyme evolution at the California Institute of Technology, says in an email that the work “is a tour-de-force integration of natural and ‘new-to-nature’ biocatalysis.” She also says the researchers show “that enzymes can do chemistry that once belonged only to humans—such as alkene cyclopropanation—and in many cases do it better than humans. Such enzymes can greatly expand the scope of chemicals and materials we can build using synthetic biology, as beautifully demonstrated here.”



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