We demonstrated earlier that the heme in cytochrome P450 enzymes of the CYP4A family is covalently attached to the protein through an I-helix glutamic acid residue [Hoch, U., and Ortiz de Montellano, P. R. (2001) J. Biol. Chem. 276, 11339-11346]. As the critical glutamic acid residue is conserved in many members of the CYP4F class of cytochrome P450 enzymes, we investigated covalent heme binding in this family of enzymes. Chromatographic analysis indicates that the heme is covalently bound in CYP4F1 and CYP4F4, which have the required glutamic acid residue, but not in CYP4F5 and CYP4F6, which do not. Catalytic turnover of CYP4F4 with NADPH-cytochrome P450 reductase shows that the heme is covalently bound through an autocatalytic process. Analysis of the prosthetic group in the CYP4F5 G330E mutant, into which the glutamic acid has been reintroduced, shows that the heme is partially covalently bound and partially converted to noncovalently bound 5-hydroxymethylheme. The modified heme presumably arises by trapping of a 5-methyl carbocation intermediate by a water molecule. CYP4F proteins thus autocatalytically bind their heme groups covalently in a process that requires a glutamic acid both to generate a reactive (cationic) form of the heme methyl and to trap it to give the ester bond.