Role of the Active Site Heme and Tyrosine in Styrene Oxide Isomerase’s Natural Isomerase and Unnatural Peroxidase and Peroxygenase Activity

Heme enzymes can perform a wide range of reactions in biological systems, often controlled by the heme surrounding amino acids or in conjunction with redox partners. Recently, we resolved the cryo-EM structure of styrene oxide isomerase, a transmembrane protein that catalyzes the isomerization of epoxides into carbonyl compounds. We discovered that heme acts as the cofactor and catalytic center, with tyrosine serving as the key residue in catalysis. While it is evident that tyrosine coordinates the substrate in the active site, the catalytic mechanism is not fully established yet. In this work, we advanced the investigation into a homologous enzyme to explore whether tyrosine plays a conserved role in catalysis. Site-directed mutagenesis of this tyrosine (Y131) to Ala, His, Ser, and Phe demonstrated a significant effect on the activity and kinetic parameters. The pH-dependent activity assay and inhibition of the heme site with carbon monoxide illustrated both ferric heme and tyrosine to be crucial for catalysis. The NMR-based enzyme kinetics suggested that heme b acts as a Lewis-acid in ring opening and Y131 facilitates the trans-methyl/hydride shift to perform the Meinwald-rearrangement reaction. Furthermore, our findings indicate that active heme can be utilized for various reactions, functioning as peroxidase with a turnover number (k_cat) of 2 s^–1 and as a peroxygenase with a total yield of 10% phenylacetaldehyde. Although its peroxidase activity is ca. 1000-fold less efficient compared to dye decolorizing peroxidase DyP, the multifunctionality of ZcSOI makes it an intriguing enzyme for application studies. This work deepens our understanding of the SOI mechanism and also reports on the less efficient peroxidase and peroxygenase activity. This, in turn makes SOI a promising candidate for protein engineering to apply in the field of biotechnology, such as improving the epoxidation and isomerization of styrene to produce phenylacetaldehyde by a single enzyme.