Introducing mutations to modify the C13/C9 ratio in linoleic acid oxygenations catalyzed by rabbit 15-lipoxygenase: a QM/MM and MD study

Flipped regiospecificity in L434F mutant of 8-lipoxygenase

Conformational change of Phe434 controls regio- and stereospecificity of L434F lipoxygenase catalysis.

Mutations of Triad Determinants Changes the Substrate Alignment at the Catalytic Center of Human ALOX5

For the specificity of ALOX15 orthologs of different mammals, the geometry of the amino acids Phe353, Ile418, Met419, and Ile593 ("triad determinants") is important, and mutagenesis of these residues altered the reaction specificity of these enzymes. Here we expressed wild-type human ALOX5 and its F359W/A424I/N425M/A603I mutant in Sf9 insect cells and characterized the catalytic differences of the two enzyme variants. We found that wild-type ALOX5 converted arachidonic acid mainly to 5(S)-hydroperoxyeicosatetraenoic acid (HpETE). In contrast, 15(S)- and 8(S)-H(p)ETE were formed by the mutant enzyme. In addition to arachidonic acid, wild-type ALOX5 accepted eicosapentaenoic acid (EPA) as substrate, but C18 fatty acids were not oxygenated. The quadruple mutant also accepted linoleic acid and α- and γ-linolenic acid as substrate. Structural analysis of the oxygenation products and kinetic studies with stereospecifically labeled 11(S)- and 11(R)-deutero-linoleic acid suggested alternative ways of substrate orientation at the active site. In silico docking studies, molecular dynamics simulations, and quantum mechanics/molecular mechanics (QM/MM) calculations confirmed this hypothesis. These data indicate that "triad determinant" mutagenesis alters the catalytic properties of ALOX5 abolishing its leukotriene synthase activity but improving its biosynthetic capacity for pro-resolving lipoxins.

Understanding the Mechanism of the Hydrogen Abstraction from Arachidonic Acid Catalyzed by the Human Enzyme 15-Lipoxygenase-2. A Quantum Mechanics/Molecular Mechanics Free Energy Simulation

Lipoxygenases (LOXs) are a family of enzymes involved in the biosynthesis of several lipid mediators. In the case of human 15-LOX, the 15-LOX-1 and 15-LOX-2 isoforms show slightly different reaction regiospecificity and substrate specificity, indicating that substrate binding and recognition may be different, a fact that could be related to their different biological role. Here, we have used long molecular dynamics simulations, QM(DFT)/MM potential energy and free energy calculations (using the newly developed DHAM method), to investigate the binding mode of the arachidonic acid (AA) substrate into 15-LOX-2 and the rate-limiting hydrogen-abstraction reaction 15-LOX-2 catalyzes. Our results strongly indicate that hydrogen abstraction from C13 in 15-LOX-2 is only consistent with the "tail-first" orientation of AA, with its carboxylate group interacting with Arg429, and that only the pro-S H13 hydrogen will be abstracted (being the pro-R H13 and H10 too far from the acceptor oxygen atom). At the B3LYP/6-31G(d) level the potential and free energy barriers for the pro-S H13 abstraction of AA by 15-LOX-2 are 18.0 and 18.6 kcal/mol, respectively. To analyze the kinetics of the hydrogen abstraction process, we determined a Markov model corresponding to the unbiased simulations along the state-discretized reaction coordinate. The calculated rates based on the second largest eigenvalue of the Markov matrices agree well with experimental measurements, and also provide the means to directly determine the pre-exponential factor for the reaction by comparing with the free energy barrier height. Our calculated pre-exponential factor is close to the value of kBT/h. On the other hand, our results suggest that the spin inversion of the complete system (including the O2 molecule) that is required to happen at some point along the full process to lead to the final hydroperoxide product, is likely to take place during the hydrogen transfer, which is a proton coupled electron transfer. Overall, a different binding mode from the one accepted for 15-LOX-1 is proposed, which provides a molecular basis for 15-LOX-2 exclusive 15-HPETE production in front of the double (although highly 15-) 12/15 regiospecificity of 15-LOX-1. Understanding how these different isoenzymes achieve their regiospecificity is expected to help in specific inhibitor design.

Structural and functional biology of arachidonic acid 15-lipoxygenase-1 (ALOX15)

Lipoxygenases (LOX) form a family of lipid peroxidizing enzymes, which have been implicated in a number of physiological processes and in the pathogenesis of inflammatory, hyperproliferative and neurodegenerative diseases. They occur in two of the three domains of terrestrial life (bacteria, eucarya) and the human genome involves six functional LOX genes, which encode for six different LOX isoforms. One of these isoforms is ALOX15, which has first been described in rabbits in 1974 as enzyme capable of oxidizing membrane phospholipids during the maturational breakdown of mitochondria in immature red blood cells. During the following decades ALOX15 has extensively been characterized and its biological functions have been studied in a number of cellular in vitro systems as well as in various whole animal disease models. This review is aimed at summarizing the current knowledge on the protein-chemical, molecular biological and enzymatic properties of ALOX15 in various species (human, mouse, rabbit, rat) as well as its implication in cellular physiology and in the pathogenesis of various diseases.