Unraveling how enzymes can use bulky residues to drive site-selective C-H activation: the case of mammalian lipoxygenases catalyzing arachidonic acid oxidation

The regioselective activation of C-H bonds in complex molecules containing several of them is still an exciting challenge in chemistry. However, many enzymes catalyze these processes, and much can be learned from the way they do it. For example, each mammalian lipoxygenase isoenzyme abstracts a hydrogen atom from essentially a unique carbon position. This paper presents a comprehensive study at the B3LYP(QM)/CHARMM(MM) level of the hydrogen abstraction reaction from arachidonic acid (AA) catalyzed by rabbit 15-lipoxygenase (15-rLO). Most of the products of this reaction arise from the initial hydrogen abstraction from the carbon C₁₃ of AA. Nevertheless, we have shown that 15-rLO seems able to catalyze not only the abstraction of H₁₃ but also that of H₁₀. After having studied 20 of these hydrogen transfers initiated from different snapshots of an extensive MD sampling of the Michaelis complex, we have even concluded that the reaction mechanisms for both abstractions are identical (proton-coupled electron transfer processes), with transition state structures matching their geometries around the shifting hydrogen. Despite that similarity, the average potential energy barrier for the H₁₃ abstractions is 4.0 kcal/mol lower than for the H₁₀ abstractions, in good agreement with the experimental C₁₃:C₁₀ ratio of 97:3. We have found that a subtle steric hindrance by Leu597 and Ile663 is the main cause for that difference. Driving the strict regiospecificity exhibited by 15-rLO appears to be the essential function of the bulky side chains of those conserved residues, in this way making possible the vital physiological role of 15-rLO and, probably, of all the mammalian lipoxygenase isoenzymes. The understanding of how nature uses residues with the bulkiest aliphatic side chains to achieve the selective activation of C-H bonds can stimulate the design of efficient biocatalysts to that aim.