Kinetics of manganese lipoxygenase with a catalytic mononuclear redox center

Manganese lipoxygenase was isolated from the take-all fungus, Gaeumannomyces graminis, and the oxygenation mechanism was investigated, A kinetic isotope effect, k(H)/k(D) = 21-24, was observed with [U-H-2]linoleic acid as a substrate, The relative biosynthesis of (13R)-hydroperoxylinoleate (11S-HPODE) and (13R)-hydroperoxylinoleate (13R-HPODE) was pH-dependent and changed by [U-H-2]linoleic acid. Stopped-flow kinetic traces of linoleic and alpha-linolenic acids indicated catalytic lag times of similar to 45 ms, which were followed by bursts of enzyme activity for similar to 60 ms and then by steady state (k(cat) similar to 26 and similar to 47 s(-1), respectively). 11S-HPODE was isomerized by manganese lipoxygenase to 13R-HPODE and formed from linoleic acid at the same rates (k(cat) 7-9 s(-1)). Catalysis was accompanied by collisional quenching of the long wavelength fluorescence (640-685 nm) by fatty acid substrates and 13R-HPODE, Electron paramagnetic resonance (EPR) of native manganese lipoxygenase showed weak 6-fold hyperfine splitting superimposed on a broad resonance indicating two populations of Mn-II bound to protein. The addition of linoleic acid decreased both components, and denaturation of the lipoxygenase liberated similar to 0.8 Mn2+ atoms/lipoxygenase molecule. These observations are consistent with a mononuclear Mn-II center in the native state, which is converted during catalysis to an EPR silent Mn-III state. We propose that manganese lipoxygenase has kinetic and redox properties similar to iron lipoxygenases.