Financial support for this work was provided by the NIH through GM043214 and a postdoctoral fellowship to A.E.W. We thank S. McCann and C. Fry for assistance with NMR experiments, E. E. Kwan for discussions regarding the KIE studies, and S.-L. Zheng for X-ray structure determination.

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Nature thanks R. Gilmour and the other anonymous reviewer(s) for their contribution to the peer review of this work.