We report a biocatalytic platform of engineered cytochrome P450 enzymes to carry out efficient cyclopropene synthesis via carbene transfer to internal alkynes. Directed evolution of a serine-ligated P450 variant, P411-C10, yielded a lineage of engineered P411 enzymes that together accommodate a variety of internal aromatic alkynes as substrates for cyclopropenation with unprecedented efficiencies and stereoselectivities (up to 5760 TTN, and all with >99.9% ee). Using an internal aliphatic alkyne bearing a propargylic ether group, different P411 variants can selectively catalyze cyclopropene formation, carbene insertion into a propargylic C-H bond or [3 + 2]-cycloaddition. This tunable reaction selectivity further highlights the benefit of using genetically encoded catalysts to address chemoselectivity challenges.