The N-terminal Domain of the Reticulocyte-type 15-Lipoxygenase Is Not Essential for Enzymatic Activity but Contains Determinants for Membrane Binding*

The rabbit reticulocyte-type 15-lipoxygenase is capable of oxygenating biomembranes and lipoproteins without the preceding action of ester lipid cleaving enzymes. This reaction requires an efficient membrane binding, and the N-terminal β-barrel domain of the enzyme has been implicated in this process. To obtain detailed information on the structural requirements for membrane oxygenation, we expressed the rabbit wild-type 15-lipoxygenase, its β-barrel deletion mutant (catalytic domain), and several lipoxygenase point mutations as His-tagged fusion proteins in Escherichia coli and tested their membrane binding characteristics. We found that: (i) the β-barrel deletion mutant was catalytically active and its enzymatic properties (K_M,V_max, pH optimum, substrate specificity) were similar to those of the wild-type enzyme; (ii) when compared with the wild-type lipoxygenase, the membrane binding properties of the N-terminal truncation mutant were impaired but not abolished, suggesting a role of the catalytic domain in membrane binding; and (iii) Phe-70 and Leu-71 (constituents of the β-barrel domain) but also Trp-181, which is located in the catalytic domain, were identified as sequence determinants for membrane binding. Mutation of these amino acids to more polar residues (F70H, L71K, W181E) impaired the membrane binding capacity of the recombinant enzyme. These data indicate that the C-terminal catalytic domain of the rabbit 15-lipoxygenase is enzymatically active and that the membrane binding properties of the enzyme are determined by a concerted action of the N-terminal β-barrel and the C-terminal catalytic domain.