Styrene oxide Isomerase, a catalytic efficient enzyme acting as peroxidase, peroxygenase, and isomerase

Styrene oxide isomerase (SOI), an integral membrane protein in the upper styrene degradation pathway, which catalyses a rare chemistry of Meinwald rearrangement in converting epoxides into aldehyde by 1,2-hydride/methyl shift1. The stereo- and regio-selective conversion by SOI makes it appealing for biocatalysis. But the molecular mechanism of SOI was not studied so far. Our recent collaborative work on SOI, and the cryo-EM structure showed that the enzyme possesses heme as a cofactor and active site. Hence, the study aimed at understanding the mechanism as well as to uncover the potential of heme-based isomerase.

This study included screening of eight SOIs from different classes of bacteria and fungi, where all the enzymes were purified to its homogeneity and screened for specific activity of its natural substrate (R/S)-styrene oxide. Out of all the SOIs tested, the enzyme from Zavarzinia compransoris Z-1155 (ZcSOI) stood out in protein yield and activity which was further characterized for its stability and kinetics. The enzyme showed a turnover frequency of about 250 s^-1 with a catalytic efficiency of 2.5×10⁶ s^-1M^-1. The mutation and reduction study revealed that both tyrosine and heme are co-ordinating the substrate and required for effective catalysis. Owing to the fact that heme is the active site and reducible, the study was conducted to utilize ZcSOI as peroxygenase. The ABTS assay proved that ZcSOI can accepts H₂O₂ with kcat of 2.3 s^-1, hence the enzyme was used to achieve both epoxidation and isomerization of styrene to produce phenylacetaldehyde. The biotransformation of ZcSOI in presence of H₂O₂ showed about 10-15% conversion of styrene to phenylacetaldehyde after 2h, indicating that the enzyme functions as peroxygenases in addition to isomerase. This study opened up the new regime in the biocatalysis of industrially high valuable product production with single enzyme.

Xin, R. P., See, W. W. L., Yun, H., Li, X. R. & Li, Z. Enzyme-Catalyzed Meinwald Rearrangement with an Unusual Regioselective and Stereospecific 1,2-Methyl Shift. Angewandte Chemie-International Edition 61 (2022). https://doi.org/10.1002/anie.202204889