Effect of exogenous iron on aerobic catalytic mechanism of soybean lipoxygenase

Effect of exogenous iron on the enzymatic activity of soybean lipoxygenase (LOX) in aerobic catalysis is investigated in this paper. It is found that a certain level of exogenous Fe3+, Fe2+ or linoleic acid hydroperoxide (LA-HPOD) could speed up the oxidation process of linoleic acid catalyzed by soybean LOX. Absorbency band between 300 and 400 nm in UV spectrum of soybean LOX was strengthened once upon adding of Fe3+ but only changed slightly with adding of Fe2+ or other ions. Therefore, Fe3+ is much more affinitive to the catalytic site of LOX than other ions, although the previous experimental results have indicated that the iron inside native enzyme mainly exists as a more stable Fe2+-LOX state. The potential function of exogenous Fe3+, Fe2+ or LA-HPOD in aerobic catalysis mechanism of soybean LOX is proposed as electron transfer from the intrinsic electron donor to the extrinsic electron acceptor, which speeds up the conversion from Fe2+ to Fe3+ in situ inside the active site of LOX. Since the above exogenous matters can accelerate the initial rate and thus the overall rate of aerobic catalysis of linoleic acid catalyzed by LOX, it becomes evident from our study that the conversion from Fe2+ to Fe3+ in situ is the control step in the above aerobic reaction.