Catalytic iron-carbene intermediate revealed in a cytochrome c carbene transferase

Significance Here, we capture and study a reactive iron porphyrin carbene (IPC) intermediate in the heme binding pocket of an engineered cytochrome c protein. IPCs have never before been directly characterized in a protein, although they are thought to be the key catalytic intermediate common to an array of abiological but synthetically useful carbene transfer reactions catalyzed by wild-type and engineered heme proteins. Our work provides insight into how a “carbene transferase” acquired its new-to-nature function as well as how it facilitates efficient and selective transfer of the carbene to a second substrate. Knowledge gained by studying this versatile intermediate provides a foundation for studying the mechanisms of carbene transfer reactions and will facilitate the engineering of carbene transfer enzymes.