With the Arnold group at Caltech, Houk’s group characterizes an unusual reactive carbene intermediate in an artificial metalloenzyme that functionalizes Si-H bonds.
The team’s findings were reported in a paper published in the June 26, 2018, issue of the Proceedings of the National Academy of Sciences (PNAS). The group of Prof. Frances Arnold, Department of Chemical Engineering, California Institute of Technology (Caltech), previously engineered heme-containing proteins to catalyze chemical transformations that are biochemically unprecedented, to create new carbon–silicon bonds or carbon–boron bonds in a highly selective manner. Many of these non-natural enzyme-catalyzed reactions are assumed to proceed through a catalytic iron porphyrin carbene intermediate, but this intermediate was never before observed in a protein.
Now, a catalytic iron porphyrin carbene intermediate in the active site of an enzyme derived from thermostable Rhodothermus marinus (Rma) cytochrome c has been captured and studied by Arnold postdoctoral researcher Dr. S. B. Jennifer Kan, and graduate student Rusty Lewis, and other researchers from the Arnold lab. The computational modeling was carried out by Dr. Marc Garcia-Borràs (pictured right), a postdoctoral fellow in the group of Professor Ken Houk, Saul Winstein Chair in Organic Chemistry at UCLA. These revealed how directed evolution created an active site for stabilizing the carbene intermediate in an electron transfer protein without prior catalytic activity, and how the laboratory-evolved enzyme achieves perfect carbene transfer stereoselectivity towards silicon-hydrogen insertions. This information provides useful understanding about this new abiological reactivity and selectivity of carbene transfer enzymes, offering important insights that will guide and inspire future engineering efforts.
Figure 1: QM/MM optimized iron porphyrin carbene intermediate starting from the new X-ray structure, where solvent accessible surface is shown in orange; and DFT optimized transition state (TS) for Si-H carbene insertion, using a truncated computational model, to create a new silicon-carbon bond in a stereo-selective manner.