Structure of soybean seed coat peroxidase: A plant peroxidase with unusualstability and haem-apoprotein interactions

Soybean seed coat peroxidase (SBP) is a peroxidase with extraordinary stabi lity and catalytic properties, It belongs to the family of class III plant peroxidases that can oxidize a wide variety of organic and inorganic substr ates using hydrogen peroxide. Because the plant enzyme is a heterogeneous g lycoprotein, SEP was produced recombinant in Escherichia coli for the prese nt crystallographic study. The three-dimensional structure of SEP shows a b ound tris(hydroxymethyl)aminomethane molecule (TRIS). This TRIS molecule ha s hydrogen bonds to active site residues corresponding to the residues that interact with the small phenolic substrate ferulic acid in the horseradish peroxidase C (HRPC):ferulic acid complex. TRIS is positioned in what has b een described as a secondary substrate-binding site in HRPC, and the struct ure of the SBP:TRIS complex indicates that this secondary substrate-binding site could be of functional importance. SEP has one of the most solvent ac cessible delta -meso haem edge (the site of electron transfer from reducing substrates to the enzymatic intermediates compound I and II) so far descri bed for a plant peroxidase and structural alignment suggests that the volum e of Ile74 is a factor that influences the solvent accessibility of this im portant site. A contact between haem C8 vinyl and the sulphur atom of Met37 is observed in the SEP structure. This interaction might affect the stabil ity of the haem group by stabilisation/delocalisation of the porphyrin pi - cation of compound I.