The Thr–His Connection on the Distal Heme of Catalase‐Related Hemoproteins: A Hallmark of Reaction with Fatty Acid Hydroperoxides

This review focuses on a group of heme peroxidases that retain the catalase fold in structure, yet show little or no reaction with hydrogen peroxide. Instead of having a role in oxidative defense, these enzymes are involved in secondary metabolite biosynthesis. The prototypical enzyme is catalase‐related allene oxide synthase, an enzyme that converts a specific fatty acid hydroperoxide to the corresponding allene oxide (epoxide). Other catalase‐related enzymes form allylic epoxides, aldehydes, or a bicyclobutane fatty acid. In all catalases (including these relatives), a His residue on the distal face of the heme is absolutely required for activity. Its immediate neighbor in sequence as well as in 3 D space is conserved as Val in true catalases and Thr in the fatty acid hydroperoxide‐metabolizing enzymes. Thr–His on the distal face of the heme is critical in switching the substrate specificity from H₂O₂ to fatty acid hydroperoxide.