De novo design of four-helix bundle proteins to bind metalloporphyrin cofactors

The versatility of heme proteins in nature stems from the intricate control exerted by their protein scaffolds. De novo protein design offers a powerful means to dissect and recreate these structure-function relationships, enabling construction of novel metalloproteins with tailored functionalities. Here, we describe the computational design and characterization MPP1, a four-helix bundle protein designed to bind an abiological Mn-diphenylporphyrin (MnDPP) cofactor. Using parameterized coiled-coil backbones, flexible backbone sequence design in Rosetta, and structure-guided loop building, MPP1 was designed to accommodate the cofactor with precise positioning of axial ligands and second-shell interactions, as well as purposeful accessibility for oxidants and substrates. The resulting protein was the first crystallographically characterized de novo designed porphyrin-binding protein. MPP1 demonstrated the ability to stabilize a high-valent Mn(V)-oxo species and mediate thioether oxidation. This chapter details the computational strategies, cofactor incorporation, and solution characterization necessary to design and evaluate four-helix bundle proteins capable of binding porphyrin and porphyrin-like cofactors with atomic-level precision. Keywords: de novo design, protein design, bioinorganic chemistry, metalloporphyrins, heme proteins.