Kinetic Study of Lignol-Specific Peroxidase Isoenzymes from Populus Callus Cultures

We examined the kinetic characteristics of coniferyl alcohol (CA) and sinapyl alcohol (SA) oxidation with two peroxidase isozymes, SyPO (syringyl specific peroxidase) and GPO-2 (guaiacyl specific peroxidase), which were bound ionically to the cell walls of Populus callus. SyPO had a small K_m, value and a large K_cat, value for SA, whereas GPO-2 had large K_cat and K_m, values for CA as compared with horseradish peroxidase (HRP). The effects of substrate analogs on the enzymatic oxidation were investigated. Syringyl type analogs inhibited the SyPO-catalyzed oxidation of SA more effectively than did guaiacyl type analogs. On the other hand, GPO-2 reaction was inhibited by both the syringyl and guaiacyl type analogs to a similar extent. These results suggest that SyPO has a very great faculty to recognize syringyl substrates at its active site, whereas GPO-2 recognizes both syringyl and guaiacyl compounds as its substrates. As compared with HRP, SyPO readily oxidizes syringyl type dilignols while GPO-2 efficiently catalyzes the oxidation of both syringyl and guaiacyl substrates. These results show that SyPO and GPO-2 have different kinetic characteristics in catalyzing the polymerization of monolignols. These isoperoxidases may contribute to the stage of polymerization of oligolignols as well as monolignols during lignification.