In this study, we have applied ab initio quantum mechanics together with molecular mechanics and the known crystal structure of cytochrome P-450cam to assess the relative importance of electronic and steric factors in determining the suicide substrate activity of terminal alkenes. A current hypothesis focuses on competition between active oxygen addition to the terminal versus internal alkene carbon as the major determinant of N-alkylation of the heme. To test this hypothesis, we have calculated the preferential addition of small models for the active oxygen of P-450 to one or the other carbon atoms of the alkene bond in three prototypical terminal olefins, ethylene and propene, both of which are known suicide substrates, and 2-methylpropene, a model for a class of olefins known to be inactive as suicide substrates. In these studies four models for the active oxygen species in cytochrome P-450 with varying radical and anionic character, HO, LiO, O-, and OH-, were used. Ab initio studies were performed by optimization with a 3-21G basis set and MP2/6-31G⋆ single-point calculations. To investigate the possible role of steric factors, empirical energy methods were used to calculate the interaction energy between an extended binding site, constructed from the crystal structure of P-450cam, and the three alkenes in a geometry poised for covalent bond formation with each of the four pyrrole nitrogens. Taken together, the results suggest that steric rather than electronic factors determine suicide substrate activity for terminal alkenes. Specifically, the amino acids in the vicinity of the heme group, Gly 248 and Thr 252, play a major role in determining the regiospecificity of heme alkylation. © 1990, American Chemical Society. All rights reserved.