Heme enzymes are crucial in biological processes, including electron transfer and oxygenation. Heme, a metalloporphyrin, is central to enzymes like peroxidases and P450s. Peroxidases use hydrogen peroxide for oxidation, while P450s use molecular oxygen. Heme enzymes catalyze both reductive and oxidative reactions, with a focus on oxidation. Heme oxygenases bind oxygen and store oxidizing equivalents for substrate oxidation, while peroxidases use hydrogen peroxide. The heme iron's redox potential is modulated by ligands, with peroxidases having a unique mechanism involving heterolytic cleavage of the O-O bond. Crystal structures reveal the ferryl center's stability and the role of amino acid radicals in catalysis. The structure of Compound I in peroxidases involves a porphyrin cation radical and a ferryl oxygen atom. The Trp radical in CCP and similar enzymes is essential for catalysis, with variations in radical stability affecting activity. The peroxidase mechanism involves electron transfer steps, with the distal His and Arg residues playing critical roles. The structure of peroxidases, including CCP and LmP, shows conserved active sites and differences in cation binding. Substrate binding sites vary among peroxidases, with some using the heme propionate for substrate interaction. Prostaglandin synthases, like PGHS, also use heme for oxidation, with unique structural features and catalytic mechanisms. Cytochrome P450s are a large family of enzymes involved in xenobiotic metabolism and steroid biosynthesis, with diverse structures and electron transfer systems. The study of heme enzymes provides insights into their structural and functional diversity, highlighting their importance in biological processes.Heme enzymes are crucial in biological processes, including electron transfer and oxygenation. Heme, a metalloporphyrin, is central to enzymes like peroxidases and P450s. Peroxidases use hydrogen peroxide for oxidation, while P450s use molecular oxygen. Heme enzymes catalyze both reductive and oxidative reactions, with a focus on oxidation. Heme oxygenases bind oxygen and store oxidizing equivalents for substrate oxidation, while peroxidases use hydrogen peroxide. The heme iron's redox potential is modulated by ligands, with peroxidases having a unique mechanism involving heterolytic cleavage of the O-O bond. Crystal structures reveal the ferryl center's stability and the role of amino acid radicals in catalysis. The structure of Compound I in peroxidases involves a porphyrin cation radical and a ferryl oxygen atom. The Trp radical in CCP and similar enzymes is essential for catalysis, with variations in radical stability affecting activity. The peroxidase mechanism involves electron transfer steps, with the distal His and Arg residues playing critical roles. The structure of peroxidases, including CCP and LmP, shows conserved active sites and differences in cation binding. Substrate binding sites vary among peroxidases, with some using the heme propionate for substrate interaction. Prostaglandin synthases, like PGHS, also use heme for oxidation, with unique structural features and catalytic mechanisms. Cytochrome P450s are a large family of enzymes involved in xenobiotic metabolism and steroid biosynthesis, with diverse structures and electron transfer systems. The study of heme enzymes provides insights into their structural and functional diversity, highlighting their importance in biological processes.