Structural evidence for the order of preference of inorganic substrates in mammalian heme peroxidases: crystal structure of the complex of lactoperoxidase with four inorganic substrates,SCN-, I-, Br– and Cl-

Lactoperoxidase (LPO) is a member of the family of mammalian heme peroxidases. It catalyzes the oxidation of halides and pseudohalides in presence of hydrogen peroxide. LPO has been co-crystallized with inorganic substrates, SCN^-, I^-, Br^- and Cl^-. The structure determination of the complex of LPO with above four substrates showed that all of them occupied distinct positions in the substrate binding site on the distal heme side. The bound substrate ions were separated from each other by one or more water molecules. The heme iron is coordinated to His-351 N^ϵ2 on the proximal side while it is coordinated to conserved water molecule W-1 on the distal heme side. W-1 is hydrogen bonded to Br^- ion which is followed by Cl^- ion with a hydrogen bonded water molecule W-5′ between them. Next to Cl^- ion is a hydrogen bonded water molecule W-7′ which in turn is hydrogen bonded to W-8′ and N atom of SCN^-. W-80 is hydrogen bonded to W-9′ which is hydrogen bonded to I^-. SCN^- ion also interacts directly with Asn-230 and through water molecules with Ser-235 and Phe-254. Therefore, according to the locations of four substrate anions, the order of preference for binding to lactoperoxidase is observed as Br^- > Cl^- > SCN^- > I^-. The positions of anions are further defined in terms of subsites where Br^- is located in subsite 1, Cl^- in subsite 2, SCN^- in subsite 3 and I^- in subsite 4.