Abstract
The crystal structure of the complex of lactoperoxidase (LPO) with its physiological substrate thiocyanate (SCN(-)) has been determined at 2.4A resolution. It revealed that the SCN(-) ion is bound to LPO in the distal heme cavity. The observed orientation of the SCN(-) ion shows that the sulfur atom is closer to the heme iron than the nitrogen atom. The nitrogen atom of SCN(-) forms a hydrogen bond with a water (Wat) molecule at position 6'. This water molecule is stabilized by two hydrogen bonds with Gln(423) N(epsilon2) and Phe(422) oxygen. In contrast, the placement of the SCN(-) ion in the structure of myeloperoxidase (MPO) occurs with an opposite orientation, in which the nitrogen atom is closer to the heme iron than the sulfur atom. The site corresponding to the positions of Gln(423), Phe(422) oxygen, and Wat(6)' in LPO is occupied primarily by the side chain of Phe(407) in MPO due to an entirely different conformation of the loop corresponding to the segment Arg(418)-Phe(431) of LPO. This arrangement in MPO does not favor a similar orientation of the SCN(-) ion. The orientation of the catalytic product OSCN(-) as reported in the structure of LPO.OSCN(-) is similar to the orientation of SCN(-) in the structure of LPO.SCN(-). Similarly, in the structure of LPO.SCN(-).CN(-), in which CN(-) binds at Wat(1), the position and orientation of the SCN(-) ion are also identical to that observed in the structure of LPO.SCN.
Highlights
Lactoperoxidase (LPO4; EC 1.11.1.7) is a Fe3ϩ heme enzyme that belongs to the mammalian peroxidase family [1]
In the structure of LPO1⁄7SCNϪ, the sulfur atom is closer to the heme iron than the nitrogen atom, whereas in that of MPO1⁄7SCNϪ, the nitrogen atom is closer to the heme iron than the sulfur atom
The substrate-binding channel in mammalian peroxidases is extended from the distal heme cavity to the surface of the protein
Summary
STRUCTURE OF THE THIOCYANATE COMPLEX WITH LACTOPEROXIDASE AND ITS INTERACTIONS AT 2.4 Å RESOLUTION*□S. The crystal structure of the complex of lactoperoxidase (LPO) with its physiological substrate thiocyanate (SCN؊) has been determined at 2.4 Aresolution It revealed that the SCN؊ ion is bound to LPO in the distal heme cavity. Crystal Structures of Lactoperoxidase Complexes with Thiocyanate specificity of SCNϪ, a comparison of the structures of LPO1⁄7SCNϪ and MPO1⁄7SCNϪ has been made, revealing many valuable differences pertaining to the observed orientations of the common substrate, SCNϪ ion, when bound at the substrate-binding site in the distal heme cavity of the two structures. Gln423, a conserved water (Wat) molecule at position 6Ј, and a well aligned carbonyl oxygen of Phe422 in the proximity of the substrate-binding site in LPO against a protruding Phe407 in MPO seem to play the key roles in inducing the observed orientations of SCNϪ ions in LPO and MPO. The structure of LPO1⁄7SCNϪ has been compared with the structure of its ternary complex with SCNϪ and CNϪ ions
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