Less is more: miniaturized heme enzymes with broad reactivity

Heme enzymes are among the most efficient and versatile biocatalysts known, exploiting the rich redox chemistry of iron porphyrins to perform a wide range of transformations. Replicating and expanding this catalytic diversity within artificial systems remains a major challenge in bioinorganic chemistry. Among the strategies developed to design and engineer artificial heme enzymes, miniaturization occupies a distinctive position. Rather than reproducing the complexity of natural protein scaffolds, it builds on the structural knowledge of natural enzymes and works backward to identify the minimal peptide fragment that retains the essential information required for metal cofactor binding and catalytic function. Mimochromes (MCs) are among the most illustrative examples of this approach: a compact, synthetically accessible peptide scaffold surrounds a metalloporphyrin cofactor, providing a well-defined yet adaptable active site. This perspective outlines the design and development of MCs, from the early prototypes to the current benchmark catalyst MC6*a. Attention is devoted to the role of the MC scaffold in accommodating a broad range of substrates and modulating the reactivity of different metal ions. These features have enabled the development of first-in-class catalysts among both natural enzymes and artificial analogues, capable of promoting a remarkable variety of highly efficient and selective catalytic transformations spanning peroxidation and peroxygenation chemistry, electro- and photocatalytic hydrogen evolution, and electrocatalytic CO₂ reduction. After discussing the catalytic features of MCs in comparison with selected natural enzymes and small-molecule catalysts, we identify key future directions for this research area. In this context, the integration of MCs into functional nanomaterials highlights emerging opportunities for the development of hybrid catalytic platforms. Future efforts will also focus on broadening the scope of accessible transformations, unlocking their potential in chemical synthesis, diagnostics, energy conversion, and environmental remediation.