Genetically programmed chiral organoborane synthesis

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Recent advances in enzyme engineering and design have expanded nature’s catalytic repertoire to functions that are new to biology1,2,3. However, only a subset of these engineered enzymes can function in living systems4,5,6,7. Finding enzymatic pathways that form chemical bonds that are not found in biology is particularly difficult in the cellular environment, as this depends on the discovery not only of new enzyme activities, but also of reagents that are both sufficiently reactive for the desired transformation and stable in vivo. Here we report the discovery, evolution and generalization of a fully genetically encoded platform for producing chiral organoboranes in bacteria. Escherichia coli cells harbouring wild-type cytochrome c from Rhodothermus marinus8 (Rma cyt c) were found to form carbon–boron bonds in the presence of borane–Lewis base complexes, through carbene insertion into boron–hydrogen bonds. Directed evolution of Rma cyt c in the bacterial catalyst provided access to 16 novel chiral organoboranes. The catalyst is suitable for gram-scale biosynthesis, providing up to 15,300 turnovers, a turnover frequency of 6,100 h–1, a 99:1 enantiomeric ratio and 100% chemoselectivity. The enantiopreference of the biocatalyst could also be tuned to provide either enantiomer of the organoborane products. Evolved in the context of whole-cell catalysts, the proteins were more active in the whole-cell system than in purified forms. This study establishes a DNA-encoded and readily engineered bacterial platform for borylation; engineering can be accomplished at a pace that rivals the development of chemical synthetic methods, with the ability to achieve turnovers that are two orders of magnitude (over 400-fold) greater than those of known chiral catalysts for the same class of transformation9,10,11. This tunable method for manipulating boron in cells could expand the scope of boron chemistry in living systems.

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This work was supported in part by the National Science Foundation, Office of Chemical, Bioengineering, Environmental and Transport Systems SusChEM Initiative (grant CBET-1403077), the Gordon and Betty Moore Foundation through grant GBMF2809 to the Caltech Programmable Molecular Technology Initiative, and the Jacobs Institute for Molecular Engineering for Medicine at Caltech. X.H. is supported by a Ruth L. Kirschstein National Institutes of Health Postdoctoral Fellowship (F32GM125231). We thank O. F. Brandenberg, S. Brinkmann-Chen, T. Hashimoto, R. D. Lewis, and D. K. Romney for discussions and/or comments on the manuscript, and N. W. Goldberg and A. Zutshi for experimental assistance. We are grateful to S. Virgil, N. Torian, M. K. Takase and L. Henling for analytical support, and H. Gray for providing the pEC86 plasmid.

Author information

Author notes

    • S. B. Jennifer Kan
    •  & Xiongyi Huang

    These authors contributed equally to this work.


  1. Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 East California Boulevard, MC 210-41, Pasadena, California 91125, USA

    • S. B. Jennifer Kan
    • , Xiongyi Huang
    • , Yosephine Gumulya
    • , Kai Chen
    •  & Frances H. Arnold


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S.B.J.K. and X.H. designed the research with guidance from F.H.A. S.B.J.K., X.H., Y.G. and K.C. performed the experiments and analysed the data. S.B.J.K., X.H. and F.H.A. wrote the manuscript with input from all authors.

Competing interests

A provisional patent application has been filed through the California Institute of Technology based on the results presented here.

Corresponding author

Correspondence to Frances H. Arnold.

Reviewer Information Nature thanks M. Fischbach and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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    Supplementary Information

    This file contains information on borylation with additional figures.

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