ANIONIC TOBACCO PEROXIDASE IS ACTIVE AT EXTREMELY LOW PH - VERATRYL ALCOHOL OXIDATION WITH A PH OPTIMUM OF 1.8

Tobacco peroxidase (36 kDa, pI 3.5) exhibits unique catalytic and spec tral properties that are modulated by pH, calcium and magnesium ions. It catalyses the oxidation of veratryl alcohol by hydrogen peroxide ov er a wide pH range (1.5-5.0) in the presence of these metal ions with a pH optimum of 1.8. This is the only example of a holoperoxidase desc ribed so far that is active and comparatively stable at such a low pH. The enhancement of tobacco peroxidase activity by magnesium ions is t o our knowledge the first example of a magnesium-induced peroxidase ac tivation. UV/visible spectra of tobacco peroxidase showed that the Sor et band shifted and its absorption coefficient increased upon the addi tion of calcium or magnesium ions and on lowering the pH. The tobacco peroxidase spectrum at pH 1.85, in the presence of calcium chloride (> 50 mM), is similar to that of lignin peroxidase at pH 6.0, with the S oret band shifting from 403 to 409 nm and the molar absorption coeffic ient increasing from 108000 to 148000+/-2000 M(-1). cm(-1) (results gi ven +/-S.E.M.; n = 3). The data provide evidence for a low-affinity si te for bivalent metal ion binding in addition to the two constitutive calcium sites that are present in all plant peroxidases. The presence of a glutamic acid residue (Glu-141) at the entrance to the haem-bindi ng pocket, analogous to Glu-146 in lignin peroxidase and not present i n other plant peroxidases, may account for these novel properties.