The introduction to the chapter on heme enzyme structure and function provides a comprehensive overview of the role of heme in biological processes, particularly in enzymatic reactions. Heme, an iron protoporphyrin IX, is widely used in the biosphere for shuttling electrons and transporting oxygen. The chapter highlights the discovery of heme-containing oxygenases, such as P450, which catalyze the oxidation of organic substrates using molecular oxygen. The text also discusses the challenges posed by the spin barrier of molecular oxygen and the role of transition metals in overcoming this barrier. The chapter then focuses on peroxidases, which use hydrogen peroxide as an oxidant, and their catalytic cycle, including the formation of Compound I and the heterolytic cleavage of the O-O bond. The text delves into the structural and functional details of peroxidases, including the role of amino acid radicals and the stability of the Compound I ferryl center. It also explores the substrate binding mechanisms and the importance of nonpolar interactions in complex formation. The chapter concludes with a discussion of cytochrome P450, a large enzyme family involved in xenobiotic detoxification and steroid biosynthesis, and its unique structure and electron transfer mechanisms.The introduction to the chapter on heme enzyme structure and function provides a comprehensive overview of the role of heme in biological processes, particularly in enzymatic reactions. Heme, an iron protoporphyrin IX, is widely used in the biosphere for shuttling electrons and transporting oxygen. The chapter highlights the discovery of heme-containing oxygenases, such as P450, which catalyze the oxidation of organic substrates using molecular oxygen. The text also discusses the challenges posed by the spin barrier of molecular oxygen and the role of transition metals in overcoming this barrier. The chapter then focuses on peroxidases, which use hydrogen peroxide as an oxidant, and their catalytic cycle, including the formation of Compound I and the heterolytic cleavage of the O-O bond. The text delves into the structural and functional details of peroxidases, including the role of amino acid radicals and the stability of the Compound I ferryl center. It also explores the substrate binding mechanisms and the importance of nonpolar interactions in complex formation. The chapter concludes with a discussion of cytochrome P450, a large enzyme family involved in xenobiotic detoxification and steroid biosynthesis, and its unique structure and electron transfer mechanisms.