Styrene-oxide isomerase () is an enzyme that catalyzes the chemical reaction
The enzyme converts its substrate, styrene oxide, to phenylacetaldehyde. It belongs to the family of isomerases, specifically a class of other intramolecular oxidoreductases. The systematic name of this enzyme class is styrene-oxide isomerase (epoxide-cleaving). This enzyme is also called SOI and participates in styrene degradation: the reaction is the second step of the pathway after the epoxidation of styrene by styrene monooxygenase.
SOI is an integral membrane protein consisting of four transmembrane helices. It forms a novel homo-trimeric assembly, displaying a structural fold reminiscent of ion channels, as determined by cryogenic electron microscopy.
The trimeric organization of SOI is essential for its function and is guided by the ferric heme b prosthetic group positioned at the interface of its subunits. This ferric heme b acts as a Lewis acid, interacting with the epoxide oxygen atom to facilitate epoxide ring-opening of substrates.