Structural insights into the bioengineering of heme-containing enzymes.

Heme-containing enzymes represent one of nature's most versatile catalytic platforms, capable of mediating a broad spectrum of redox and group-transfer reactions. This review provides a comprehensive overview of the structural and mechanistic principles that underpin the bioengineering of heme proteins for both natural and abiological transformations. Here, we discuss that the core of reactivity of such metalloenzymes is the iron-porphyrin cofactor, whose tunable oxidation and spin states enable the formation of diverse high-energy intermediates, including iron-oxo, carbene, and nitrene species. Using recent bioengineering applications, we also show that the catalytic behavior of these intermediates is not solely determined by intrinsic cofactor chemistry but is profoundly influenced by the surrounding protein scaffold. Key factors such as axial ligation, second coordination sphere interactions, hydrogen-bonding networks, and local electric fields collectively govern substrate binding, intermediate stabilization, and reaction selectivity. The review further discusses a few applications of de novo protein design and artificial metalloenzymes that provide unprecedented control over active-site architecture, allowing the creation of highly robust and tunable catalysts.