Arachidonate 5-lipoxygenase

3O8Y, 3V92, 3V98, 3V9924011689ENSG00000275565ENSG00000012779ENSMUSG00000025701P09917P48999NM_000698NM_001256153NM_001256154NM_001320861NM_001320862NM_009662NP_000689NP_001243082NP_001243083NP_001307790NP_001307791NP_033792Arachidonate 5-lipoxygenase, also known as ALOX5, 5-lipoxygenase, 5-LOX, or 5-LO, is a non-heme iron-containing enzyme (EC 1.13.11.34) that in humans is encoded by the ALOX5 gene. Arachidonate 5-lipoxygenase is a member of the lipoxygenase family of enzymes. It transforms essential fatty acids (EFA) substrates into leukotrienes as well as a wide range of other biologically active products. ALOX5 is a current target for pharmaceutical intervention in a number of diseases. Arachidonate 5-lipoxygenase, also known as ALOX5, 5-lipoxygenase, 5-LOX, or 5-LO, is a non-heme iron-containing enzyme (EC 1.13.11.34) that in humans is encoded by the ALOX5 gene. Arachidonate 5-lipoxygenase is a member of the lipoxygenase family of enzymes. It transforms essential fatty acids (EFA) substrates into leukotrienes as well as a wide range of other biologically active products. ALOX5 is a current target for pharmaceutical intervention in a number of diseases. The ALOX5 gene, which occupies 71.9 kilobase pairs (kb) on chromosome 10 (all other human lipoxygenases are clustered together on chromosome 17), is composed of 14 exons divided by 13 introns encoding the mature 78 kilodalton (kD) ALOX5 protein consisting of 673 amino acids. The gene promoter region of ALOX5 contains 8 GC boxes but lacks TATA boxes or CAT boxes and thus resembles the gene promoters of typical housekeeping genes. Five of the 8 GC boxes are arranged in tandem and are recognized by the transcription factors Sp1 and Egr-1. A novel Sp1-binding site occurs close to the major transcription start site (position - 65); a GC-rich core region including the Sp1/Egr-1 sites may be critical for basal 5-LO promoter activity. Cells primarily involved in regulating inflammation, allergy, and other immune responses, e.g. neutrophils, eosinophils, basophils, monocytes, macrophages, mast cells, dendritic cells, and B-lymphocytes express ALOX5. Platelets, T cells, and erythrocytes are ALOX5-negative. In skin, Langerhans cells strongly express ALOX5. Fibroblasts, smooth muscle cells and endothelial cells express low levels of ALOX5. Up-regulation of ALOX5 may occur during the maturation of leukocytes and in human neutrophils treated with granulocyte macrophage colony-stimulating factor and then stimulated with physiological agents. Aberrant expression of LOX5 is seen in various types of human cancer tumors in vivo as well as in various types of human cancer cell lines in vitro; these tumors and cell lines include those of the pancreas, prostate and colon. ALOX5 products, particularly 5-hydroxyeicosatetraenoic acid and 5-oxo-eicosatetraenoic acid, promote the proliferation of these ALOX5 aberrantly expressing tumor cell lines suggesting that ALOX5 acts as a pro-malignancy factor for them and by extension their parent tumors. Studies with cultured human cells have found that there are a large number of ALOX5 mRNA splice variants due to Alternative splicing. The physiological and/or pathological consequences of this slicing has yet to be defined. In one study, however, human brain tumors were shown to express three mRNA splice variants (2.7, 3.1, and 6.4 kb) in addition to the full 8.6 lb species; the abundance of the variants correlated with the malignancy of these tumors suggesting that they may play a role in the development of these tumors. Human ALOX5 is a soluble, monomeric protein consisting of 673 amino acids with a molecular weight of ~78 kDa. Structurally, ALOX5 possesses: The enzyme possesses two catalytic activities as illustrated by its metabolism of arachidonic acid. ALOX5's dioxygenase activity adds a hydroperoxyl (i.e. HO2) residue to arachidonic acid (i.e. 5Z,8Z,11Z,14Z-eicosatetraenoic acid) at carbon 5 of its 1,4 diene group (i.e. its 5Z,8Z double bonds) to form 5(S)-hydroperoxy-6E,8Z,11Z,14Z-eicosatetraenoic acid (i.e. 5S-HpETE). The 5S-HpETE intermediate may then be released by the enzyme and rapidly reduced by cellular glutathione peroxidases to its corresponding alcohol, 5(S)-hydroxy-6E,8Z,11Z,14Z-eicosatetraenoic acid (i.e. 5-HETE), or, alternatively, further metabolized by ALOX5's epoxidase (also termed LTA4 synthase) activity which converts 5S-HpETE to its epoxide, 5S,6S-hydroxy-6E,8Z,11Z,14Z-eicosatetraenoic acid (i.e. LTA4). LTA4 is then acted on by a separate, soluble enzyme, Leukotriene-A4 hydrolase, to form the dihydroxyl product, Leukotriene B4 (LTB4, i.e. 5S,12R-dihydroxy-5S,6Z,8E,10E,12R,14Z-eicosatetraenoic acid) or by either LTC4 synthase or microsomal glutathione S-transferase 2 (MGST2), which bind the sulfur of cysteine's thio (i.e. SH) residue in the tripeptide glutamate-cysteine-glycine to carbon 6 of LTA4 thereby forming LTC4 (i.e. 5S-hydroxy,6R-(S-glutathionyl)-7E,9E,11Z,14Z-eicosatetraenoic acid). The Glu and Gly residues of LTC4 may be removed step-wise by gamma-glutamyltransferase and a dipeptidase to form sequentially LTD4 and LTE4. To varying extents, the other PUFA substrates of ALOX5 follow similar metabolic pathways to form analogous products. Sub-human mammalian Alox5 enzymes like those in rodents appear to have, at least in general, similar structures, distributions, activities, and functions as human ALOX5. Hence, model Alox5 studies in rodents appear to be valuable for defining the function of ALOX5 in humans (see lipoxygenase#Mouse lipoxygenases). ALOX5 exists primarily in the cytoplasm and nucleoplasm of cells. Upon cell stimulation, ALOX5: a) may be phosphorylated on serine 663, 523, and/or 271 by Mitogen-activated protein kinases, S6 kinase, protein kinase A (PKA), protein kinase C, Cdc2, and/or a Ca2+/calmodulin-dependent protein kinase; b) moves to bind with phospholipids in the nuclear membrane and, probably, endoplasmic reticulum membrane; c) is able to accept substrate fatty acids presented to it by the 5-lipoxygenase-activating protein (FLAP) which is embedded in these membranes; and d) thereby becomes suited for high metabolic activity. These events, along with rises in cytosolic Ca2+ levels, which promote the translocation of ALOX5 form the cytoplasm and nucleoplasm to the cited membranes, are induced by cell stimulation such as that caused by chemotactic factors on leukocytes. Rises in cytosolic Ca2+, ALOX5's movement to membranes, and ALOX5's interaction with FLAP are critical to the physiological activation of the enzylme. Serine 271 and 663 phosphorylations do not appear to alter ALOX5's activity. Serine 523 phosphorylation (which is conducted by PKA) totally inactivates the enzyme and prevents its nuclear localization; stimuli which cause cells to activate PKA can thereby block production of ALOX5 metaboites.