Lipoxygenase


Lipoxygenases are a family of iron-containing enzymes, more specifically oxidative enzymes, most of which catalyze the dioxygenation of polyunsaturated fatty acids in lipids containing a cis,cis-1,4-pentadiene into cell signaling agents that serve diverse roles as autocrine signals that regulate the function of their parent cells, paracrine signals that regulate the function of nearby cells, and endocrine signals that regulate the function of distant cells.
The lipoxygenases are related to each other based upon their similar genetic structure and dioxygenation activity. However, one lipoxygenase, ALOXE3, while having a lipoxygenase genetic structure, possesses relatively little dioxygenation activity; rather its primary activity appears to be as an isomerase that catalyzes the conversion of hydroperoxy unsaturated fatty acids to their 1,5-epoxide, hydroxyl derivatives.
Lipoxygenases are found in eukaryotes ; while the third domain of terrestrial life, the archaea, possesses proteins with a slight amino acid sequence similarity to lipoxygenases, these proteins lack iron-binding residues and therefore are not projected to possess lipoxygenase activity.

Biochemistry

Based on detailed analyses of 15-lipoxygenase 1 and stabilized 5-lipoxygenase, lipoxygenase structures consist of a 15 kilodalton N-terminal beta barrel domain, a small linker inter-domain, and a relatively large C-terminal catalytic domain which contains the non-heme iron critical for the enzymes' catalytic activity. Most of the lipoxygenases catalyze the reaction Polyunsaturated fatty acid + O2 → fatty acid hydroperoxide in four steps:
  • the rate-limiting step of hydrogen abstraction from a bisallylic methylene carbon to form a fatty acid radical at that carbon
  • rearrangement of the radical to another carbon center
  • addition of molecular oxygen to the rearranged carbon radical center thereby forming a peroxy radical bond to that carbon
  • reduction of the peroxy radical to its corresponding anion
The residue may then be protonated to form a hydroperoxide group and further metabolized by the lipoxygenase to e.g. leukotrienes, hepoxilins, and various specialized pro-resolving mediators, or reduced by ubiquitous cellular glutathione peroxidases to a hydroxy group thereby forming hydroxylated polyunsaturated fatty acids such as the hydroxyeicosatetraenoic acids and HODEs.
Polyunsaturated fatty acids that serve as substrates for one or more of the lipoxygenases include the omega 6 fatty acids, arachidonic acid, linoleic acid, dihomo-γ-linolenic acid, and adrenic acid; the omega-3 fatty acids, eicosapentaenoic acid, docosahexaenoic acid, and alpha-linolenic acid; and the omega-9 fatty acid, mead acid. Certain types of the lipoxygenases, e.g. human and murine 15-lipoxygenase 1, 12-lipoxygenase B, and ALOXE3, are capable of metabolizing fatty acid substrates that are constituents of phospholipids, cholesterol esters, or complex lipids of the skin. Most lipoxygenases catalyze the formation of initially formed hydroperoxy products that have S chirality. Exceptions to this rule include the 12R-lipoxygenases of humans and other mammals.
Lipoxygenases depend on the availability of their polyunsaturated fatty acid substrates which, particularly in mammalian cells, is normally maintained at extremely low levels. In general, various phospholipase A2s and diacylglycerol lipases are activated during cell stimulation, proceed to release these fatty acids from their storage sites, and thereby are key regulators in the formation of lipoxygenase-dependent metabolites. In addition, cells, when so activated, may transfer their released polyunsaturated fatty acids to adjacent or nearby cells which then metabolize them through their lipoxygenase pathways in a process termed transcellular metabolism or transcellular biosynthesis.

Biological function and classification

These enzymes are most common in plants where they may be involved in a number of diverse aspects of plant physiology including growth and development, pest resistance, and senescence or responses to wounding. In mammals a number of lipoxygenases isozymes are involved in the metabolism of eicosanoids. Sequence data is available for the following lipoxygenases:

Plant lipoxygenases

Plants express a variety of cytosolic lipoxygenases as well as what seems to be a chloroplast isozyme. Plant lipoxygenase in conjunction with hydroperoxide lyases are responsible for many fragrances and other signalling compounds. One example is cis-3-hexenal, the odor of freshly cut grass.

Human lipoxygenases

With the exception of the gene encoding 5-LOX, which is located on chromosome 10q11.2, all six human LOX genes are located on chromosome 17.p13 and code for a single chain protein of 75–81 kilodaltons that consists of 662–711 amino acids. Mammalian LOX genes contain 14 or 15 exons with exon/intron boundaries at highly conserved positions. The 6 human lipoxygenases along with some of the major products that they make, as well as some of their associations with genetic diseases, are as follows:
  • Arachidonate 5-lipoxygenase , also termed 5-lipoxygenase, 5-LOX, and 5-LO. Major products: it metabolizes arachidonic acid to 5-hydroperoxy-eicostetraeoic acid 5-hydroxyicosatetraenoic acid and then to 5-oxo-eicosatetraenoic acid leukotriene A4 which may then be converted to leukotriene B4 or leukotriene C4 acting in series with ALOX15, to the specialized pro-resolving mediators, lipoxins A4 and B4. ALOX5 also metabolizes eicosapentaenoic acid to a set of metabolites that contain 5 double bounds as opposed to the 4 double bond-containing arachidonic acid metabolites. The enzyme, when acting in series with other lipoxygenase, cyclooxygenase, or cytochrome P450 enzymes, contributes to the metabolism of eicosapentaenoic acid to E series resolvins and of docosahexaenoic acid to D series resolvins. These resolvins are also classified as specialized pro-resolving mediators.
  • Arachidonate 12-lipoxygenase , also termed 12-lipoxygenase, platelet type platelet lipoxygenase 12-LOX, and 12-LO. It metabolizes arachidonic acid to 12-hydroperoxyeiocsatetraeoic acid which is further metabolized to 12-hydroxyeicosatetraenoic acid or to various hepoxilins.
  • Arachidonate 15-lipoxygenase-1 , also termed 15-lipoxygenase-1, erythrocyte type 15-lipoxygenase, reticulocyte type 15-lipoxygenase 15-hydroperoxyeiocatetraenoic acid which is further metabolized to 15-hydroxyicosatetraenoic acid ' 12-hydroperoxyeicosatetraenoic acid which is further metabolized to 12-hydroxyeicosatetraenoic acid and possibly the hepoxilins. ALOX15 actually prefers linoleic acid over arachidonic acid, metabolizing linoleic acid to 12-hydroperoxyoctadecaenoic acid which is further metabolized to 13-hydroxyoctadecadienoic acid. ALOX15 can metabolize polyunsaturated fatty acids that are esterified to phospholipids and/or to the cholesterol, i.e. cholesterol esters, in lipoproteins. This property along with its dual specificity in metabolizing arachidonic acid to 12-HpETE and 15-HpETE are similar to those of mouse Alox15 and has led to both enzymes being termed 12/15-lipoxygenases.
  • Arachidonate 15-lipoxygenase type II, also termed 15-lipoxygenase-2, 15-LOX-2, and 15-LOX-2. It metabolizes arachidonic acid to 15-hydroperoxyeicosatetraenoic which is further metabolized to 15-hydroxyicosatetraenoic acid. ALOX15B has little or no ability to metabolize arachidonic acid to 12-hydroperoxeiocosatetraenoic acid and only minimal ability to metabolize linoleic acid to 13-hydroperoxyoctadecaenoic acid.
  • Arachidonate 12-lipoxygenase, 12R type, also termed 12R''-lipoxygenase, 12R-LOX, and 12R-LO. It metabolizes arachidonic acid to 12R-hydroxyeicosatetraenoic acid but does so only with low catalytic activity; its most physiologically important substrate is thought to be a sphingosine which contains a very long chain omega-hydroxyl fatty acid that is in amide linkage to the sn-2 nitrogen of sphingosine at its carboxy end and esterified to linoleic acid at its omega hydroxyl end. In skin epidermal cells, ALOX12B metabolizes the linoleate in this esterified omega-hydroxyacyl-sphingosine to its 9R-hydroperoxy analog. Inactivating mutations of ALOX12B are associated with the human skin disease, autosomal recessive congenital ichthyosiform erythroderma.
  • Epidermis-type lipoxygenase, also termed eLOX3 and lipoxygenase, epidermis type. Unlike other lipoxygenases, ALOXE3 exhibits only a latent dioxygenase activity. Rather, its primary activity is as a hydroperoxide isomerase that metabolizes certain unsaturated hydroperoxy fatty acids to their corresponding epoxy alcohol and epoxy keto derivatives and thereby is also classified as a hepoxilin synthase. While it can metabolize 12S-hydroperoxyeicosatetraenoic acid to the R stereoisomers of hepoxilins A3 and B3, ALOXE3 favors metabolizing R hydroperoxy unsaturated fatty acids and efficiently converts the 9-hydroperoxy analog of EOS made by ALOX15B to its 9R,13R-trans-epoxy-11E,13R and 9-keto-10E,12Z EOS analogs. ALOXE3 is thought to act with ALOX12B in skin epidermis to form the latter two EOS analogs; inactivation mutations of ALOX3 are, similar to inactivating mutations in ALOX12B, associated with autosomal recessive congenital ichthyosiform erythroderma in humans. Inactivating mutations in ALOX3 are also associated with the human disease lamellar ichthyosis.
Two lipoxygenases may act in series to make di-hydroxy or tri-hydroxy products that have activities quite different than either lipoxyenases' products. This serial metabolism may occur in different cell types that express only one of the two lipoxygenases in a process termed transcellular metabolism. For example, ALOX5 and ALOX15 or, alternatively, ALOX5 and ALOX12 can act serially to metabolize arachidonic acid into lipoxins -HpETE, 15-HETE, 15-HpETE, 15 while ALOX15 and possibly ALOX15B can act with ALOX5 to metabolize eicosapentaenoic acid to resolvin D's.