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What are Lipoxygenases?

definition
distribution
nomenclature and classification
protein structure
LOX enzyme reaction
gene structure
LOX function



definition:

Lipoxygenases (LOX; EC 1.13.11._) consist of a structurally related family of non-heme iron-containing dioxygenases that catalyse the addition of molecular oxygen to polyunsaturated fatty acids with a (Z,Z)-1,4-pentadiene structural unit to give an unsaturated fatty acid hydroperoxide. The reaction is stereo- and regiospecific [1,2]

distribution:

LOX are widely distributed in plants, fungi, invertebrates and mammals [1-3]. Only recently LOX have also been found in bacteria [4].

nomenclature and classification:

A LOX definition according to enzyme classification is linoleate: oxygen oxidoreductase (for plant LOX) and arachidonate: oxygen oxidoreductase (for mammalian LOX). Historically LOX are classified according to their positional specificity of the dioxygenation of their most common substrates linoleate (C-18) in plants, and arachidonic acid (C-20) in mammals. Accordingly, a LOX reacting at the ω-6 position of the substrate (as counted from the tail end of the fatty acid) in plants is referred to as a 13-LOX (alternate: linoleate ω6-LOX) and in mammals as a 15-LOX (alternate: arachidonate ω6-LOX). In plants 9-, and 13-LOX and in mammals, 5-, 8-, 12- and 15-LOX are known. When necessary, the stereoconfiguration is specified (e.g. 12S-LOX and 12R-LOX). In mammals, isoenzymes that exhibit the same position-specificity are named additionally after the prototypical tissue of their occurrence (e.g. platelet-type, leukocyte-type and epidermis-type 12S-LOX) [1].
Based upon the structural features of the proteins plant LOX can be grouped into LOX1 gene family and LOX2 gene family, the latter of which is characterized by the presence of an additional amino-terminal putative transit peptide sequence [2] Based on the phylogenetic relatedness an alternate classification for mammalian LOX is proposed [5,6], comprising i) 5-LOX, ii) platelet-type 12-LOX, iii) 15/12-LOX (reticulocyte-type 15-LOX-1 and leukocyte-type 12-LOX, both exhibiting a dual positional specificity) and iv) epidermis-type LOX (12R-LOX, 15-LOX-2, 8-LOX, epidermis-type LOX-3).


Figure 1. Phylogenetic tree of mammalian lipoxygenases. GenBank accession numbers of the sequences used for multiple sequence alignments are give in parentheses. (Click on the lipoxygenases' names to access the related lox-db entries, on the group names at the right to access the related alignments.)

protein structure:

LOX proteins have a single polypeptide chain with a molecular mass of 75-81 kDa (≈662-711 amino acids) in mammals and 94-103 kDa (≈838-923 amino acids) in plants [1,2]. LOX are members of a multigene family exhibiting an overall sequence identity of ≈ 25-40%, while close functional homologues across species share 70-95% identity. LOX proteins contain highly conserved domains and sequence motifs which are important for the distinct structure and the binding of the catalytic iron ("LOX motif": His-X4-His-X4-His-X17-His-X8-His). The tertiary structure which is the same in plant [7] and mammalian LOX [8] reveals two domains: The N-terminal beta-barrel (=PLAT (Polycystin-1, lipoxygenase, Alpha-Toxin) domain or LH2 (Lipoxygenase homology) domain) and a catalytic C-terminal part. The catalytic iron is ligated in an octahedral arrangement by 3 conserved histidines, one His/Asn/Ser, and the C-terminal isoleucine [9]. (Get further information from the pfam database.)

LOX enzyme reaction:

LOX catalysis includes three steps (Fig.2): i) the stereospecific hydrogen removal from a doubly allylic methylene group ii) radical rearrangement involving position (n+2) or (n-2) and accompanied by a Z,E diene conjugation depending on LOX specificity iii) stereospecific antarafacial insertion of molecular oxygen and reduction of the hydroperoxy radical intermediate to the corresponding anion. The one-electron transitions are catalyzed by the non-heme iron in the catalytic center of the enzyme. Stage i) is considered to be rate limiting in LOX catalysis. An important property of LOX reaction is sterospecificity with S-hydroperoxides being the predominant products of plant and mammalian LOX, while R-epimers are formed predominantly by invertebrate LOX. Most LOX also exhibit high regiospecificity.


Figure 2. Model of the catalytic cycle of LOX. LOX are usually in the inactive ferrous state. Oxidation to the active ferric state is required for catalysis. The homolytic scission of hydrogen from a doubly allylic methylene leads to the formation of a pentadienyl radical and a proton. The remaining electron reduces iron to the ferrous state (A). Antarafacial insertion of molecular oxygen generates a hydroperoxide radical which is reduced to the hydroperoxide anion by the simultaneous oxidation of iron to the ferric state (B). A proton is accepted to form the hydroperoxide (C). The ferric enzyme is able to initiate a new reaction cycle.

Gene structure

The known LOX genes show a conserved structure (plant LOX 8 or 9 exons, mammalian LOX 14 or 15 exons) with exon/intron boundaries at highly conserved positions. Mammalian LOX map close together (with the exception of 5-LOX all human LOX at Chr 17.p13.1) indicating their origin from a common ancestor gene.

LOX function:

In plants, the most common LOX substrates linoleic acid and linolenic acids are converted into a variety of bioactive mediators involved in plant defence, senescence, seed germination, plant growth and development [10]. In mammals, the predominant LOX substrate is arachidonic acid. Its products, the eicosanoids comprise a variety of bioregulators including leukotrienes, hepoxilins and HETEs, that play important roles in the maintenance of the homeostasis of the animal cell and also have been shown to be implicated in a variety of human diseases such as inflammation, fever, arthritis and cancer [11-15]. LOX are also able to oxidize complex lipids and modify membrane structures leading to structural changes that play a role in the maturation of various cell types including erythrocytes, lens epithelial cells, and keratinocytes [16].


references:

  1. Brash AR. Lipoxygenases: occurrence, functions, catalysis, and acquisition of substrate.
    J Biol Chem. 1999; 274(34):23679-82. (
    more)

  2. Shibata D, Axelrod B. Plant lipoxygenases.
    J Lipid Mediat Cell Signal. 12 ; 1995: 213-28. (
    more)

  3. Gerwick WH. Structure and biosynthesis of marine algal oxylipins.
    Biochim Biophys Acta. 1994; 1211(3):243-55. (
    more)

  4. Porta H, Rocha-Sosa M. Lipoxygenase in bacteria: a horizontal transfer event?
    Microbiology. 2001;147: 3199-200. (
    more)

  5. Kühn H, Thiele BJ.The diversity of the lipoxygenase family. Many sequence data but little information on biological significance.
    FEBS Lett. 1999; 449(1):7-11. (
    more)

  6. Heidt M, Furstenberger G, Vogel S, Marks F, Krieg P. Diversity of mouse lipoxygenases: identification of a subfamily of epidermal isozymes exhibiting a differentiation-dependent mRNA expression pattern.
    Lipids. 2000; 35(7):701-7. (
    more)

  7. Boyington JC, Gaffney BJ, Amzel LM. The three-dimensional structure of an arachidonic acid 15-lipoxygenase.
    Science. 1993; 260(5113):1482-6. (
    more)

  8. Gillmor SA, Villasenor A, Fletterick R, Sigal E, Browner MF. The structure of mammalian 15-lipoxygenase reveals similarity to the lipases and the determinants of substrate specificity.
    Nat Struct Biol. 1997; 4(12):1003-9. (
    more)

  9. Minor W, Steczko J, Stec B, Otwinowski Z, Bolin JT, Walter R, Axelrod B. Crystal structure of soybean lipoxygenase L-1 at 1.4 A resolution.
    Biochemistry. 1996; 35(33):10687-701. (
    more)

  10. Grechkin A. Recent developments in biochemistry of the plant lipoxygenase pathway.
    Prog Lipid Res. 1998; 37(5):317-52. (
    more)

  11. Radmark O. Arachidonate 5-lipoxygenase.
    J Lipid Mediat Cell Signal. 1995;12(2-3):171-84. (
    more)

  12. Yamamoto S, Suzuki H, Ueda N. Arachidonate 12-lipoxygenases.
    Prog Lipid Res. 1997; 36(1):23-41. (
    more)

  13. Kühn H, Thiele BJ. Arachidonate 15-lipoxygenase.
    J Lipid Mediat Cell Signal. 1995; 12(2-3):157-70. (
    more)

  14. Funk CD. The molecular biology of mammalian lipoxygenases and the quest for eicosanoid functions using lipoxygenase-deficient mice.
    Biochim Biophys Acta. 1996;1 304(1):65-84. (
    more)

  15. Shureiqi I., and Lippman, SM. Lipoxygenase Modulation to Reverse Carcinogenesis.
    Cancer Res., 2001; 61: 6307- 6312. (
    more)

  16. Schewe T, Kühn H. Do 15-lipoxygenases have a common biological role?
    Trends Biochem Sci. 1991; 16(10):369-73. (
    more)


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