Factors influencing the rearrangement of bis-allylic hydroperoxides by manganese lipoxygenase

Diversity of the manganese lipoxygenase gene family - A mini-review

Analyses of fungal genomes of escalate from biological and evolutionary investigations. The biochemical analyses of putative enzymes will inevitably lag behind and only a selection will be characterized. Plant-pathogenic fungi secrete manganese-lipoxygenases (MnLOX), which oxidize unsaturated fatty acids to hydroperoxides to support infection. Six MnLOX have been characterized so far including the 3D structures of these enzymes of the Rice blast and the Take-all fungi. The goal was to use this information to evaluate MnLOX-related gene transcripts to find informative specimens for further studies. Phylogenetic analysis, determinants of catalytic activities, and the C-terminal amino acid sequences divided 54 transcripts into three major subfamilies. The six MnLOX belonged to the same "prototype" subfamily with conserved residues in catalytic determinants and C-terminal sequences. The second subfamily retained the secretion mechanism, presumably necessary for uptake of Mn²⁺, but differed in catalytic determinants and by cysteine replacement of an invariant Leu residue for positioning ("clamping") of fatty acids. The third subfamily contrasted with alanine in the Gly/Ala switch for regiospecific oxidation and a minority contained unprecedented C-terminal sequences or lacked secretion signals. With these exceptions, biochemical analyses of transcripts of the three subfamilies appear to have reasonable prospects to find active enzymes.

Iron and manganese lipoxygenases of plant pathogenic fungi and their role in biosynthesis of jasmonates

Lipoxygenases (LOX) contain catalytic iron (FeLOX), but fungi also produce LOX with catalytic manganese (MnLOX). In this review, the 3D structures and properties of fungal LOX are compared and contrasted along with their associations with pathogenicity. The 3D structures and properties of two MnLOX (Magnaporthe oryzae, Geaumannomyces graminis) and the catalysis of five additional MnLOX have provided information on the metal center, substrate binding, oxygenation, tentative O₂ channels, and biosynthesis of exclusive hydroperoxides. In addition, the genomes of other plant pathogens also code for putative MnLOX. Crystals of the 13S-FeLOX of Fusarium graminearum revealed an unusual altered geometry of the Fe ligands between mono- and dimeric structures, influenced by a wrapping sequence extension near the C-terminal of the dimers. In plants, the enzymes involved in jasmonate synthesis are well documented whereas the fungal pathway is yet to be fully elucidated. Conversion of deuterium-labeled 18:3n-3, 18:2n-6, and their 13S-hydroperoxides to jasmonates established 13S-FeLOX of F. oxysporum in the biosynthesis, while subsequent enzymes lacked sequence homologues in plants. The Rice-blast (M. oryzae) and the Take-all (G. graminis) fungi secrete MnLOX to support infection, invasive hyphal growth, and cell membrane oxidation, contributing to their devastating impact on world production of rice and wheat.

Concerted proton-electron transfer reactions of manganese-hydroxo and manganese-oxo complexes

The enzymes manganese superoxide dismutase and manganese lipoxygenase use MnIII-hydroxo centres to mediate proton-coupled electron transfer (PCET) reactions with substrate. As manganese is earth-abundant and inexpensive, manganese catalysts are of interest for synthetic applications. Recent years have seen exciting reports of enantioselective C-H bond oxidation by Mn catalysts supported by aminopyridyl ligands. Such catalysts offer economic and environmentally-friendly alternatives to conventional reagents and catalysts. Mechanistic studies of synthetic catalysts highlight the role of Mn-oxo motifs in attacking substrate C-H bonds, presumably by a concerted proton-electron transfer (CPET) step. (CPET is a sub-class of PCET, where the proton and electron are transferred in the same step.) Knowledge of geometric and electronic influences for CPET reactions of Mn-hydroxo and Mn-oxo adducts enhances our understanding of biological and synthetic manganese centers and informs the design of new catalysts. In this Feature article, we describe kinetic, spectroscopic, and computational studies of MnIII-hydroxo and MnIV-oxo complexes that provide insight into the basis for the CPET reactivity of these species. Systematic perturbations of the ligand environment around MnIII-hydroxo and MnIV-oxo motifs permit elucidation of structure-activity relationships. For MnIII-hydroxo centers, electron-deficient ligands enhance oxidative reactivity. However, ligand perturbations have competing consequences, as changes in the MnIII/II potential, which represents the electron-transfer component for CPET, is offset by compensating changes in the pKa of the MnII-aqua product, which represents the proton-transfer component for CPET. For MnIV-oxo systems, a multi-state reactivity model inspired the development of significantly more reactive complexes. Weakened equatorial donation to the MnIV-oxo unit results in large rate enhancements for C-H bond oxidation and oxygen-atom transfer reactions. These results demonstrate that the local coordination environment can be rationally changed to enhance reactivity of MnIII-hydroxo and MnIV-oxo adducts.

Crystal structure of linoleate 13R-manganese lipoxygenase in complex with an adhesion protein

The crystal structure of 13R-manganese lipoxygenase (MnLOX) of Gaeumannomyces graminis (Gg) in complex with zonadhesin of Pichia pastoris was solved by molecular replacement. Zonadhesin contains β-strands in two subdomains. A comparison of Gg-MnLOX with the 9S-MnLOX of Magnaporthe oryzae (Mo) shows that the protein fold and the geometry of the metal ligands are conserved. The U-shaped active sites differ mainly due to hydrophobic residues of the substrate channel. The volumes and two hydrophobic side pockets near the catalytic base may sanction oxygenation at C-13 and C-9, respectively. Gly-332 of Gg-MnLOX is positioned in the substrate channel between the entrance and the metal center. Replacements with larger residues could restrict oxygen and substrate to reach the active site. C18 fatty acids are likely positioned with C-11 between Mn(2+)OH2 and Leu-336 for hydrogen abstraction and with one side of the 12Z double bond shielded by Phe-337 to prevent antarafacial oxygenation at C-13 and C-11. Phe-347 is positioned at the end of the substrate channel and replacement with smaller residues can position C18 fatty acids for oxygenation at C-9. Gg-MnLOX does not catalyze the sequential lipoxygenation of n-3 fatty acids in contrast to Mo-MnLOX, which illustrates the different configurations of their substrate channels.

Crystal Structure of Manganese Lipoxygenase of the Rice Blast Fungus Magnaporthe oryzae

Lipoxygenases (LOX) are non-heme metal enzymes, which oxidize polyunsaturated fatty acids to hydroperoxides. All LOX belong to the same gene family, and they are widely distributed. LOX of animals, plants, and prokaryotes contain iron as the catalytic metal, whereas fungi express LOX with iron or with manganese. Little is known about metal selection by LOX and the adjustment of the redox potentials of their protein-bound catalytic metals. Thirteen three-dimensional structures of animal, plant, and prokaryotic FeLOX are available, but none of MnLOX. The MnLOX of the most important plant pathogen, the rice blast fungusMagnaporthe oryzae(Mo), was expressed inPichia pastoris.Mo-MnLOX was deglycosylated, purified to homogeneity, and subjected to crystal screening and x-ray diffraction. The structure was solved by sulfur and manganese single wavelength anomalous dispersion to a resolution of 2.0 Å. The manganese coordinating sphere is similar to iron ligands of coral 8R-LOX and soybean LOX-1 but is not overlapping. The Asn-473 is positioned on a short loop (Asn-Gln-Gly-Glu-Pro) instead of an α-helix and forms hydrogen bonds with Gln-281. Comparison with FeLOX suggests that Phe-332 and Phe-525 might contribute to the unique suprafacial hydrogen abstraction and oxygenation mechanism of Mo-MnLOX by controlling oxygen access to the pentadiene radical. Modeling suggests that Arg-525 is positioned close to Arg-182 of 8R-LOX, and both residues likely tether the carboxylate group of the substrate. An oxygen channel could not be identified. We conclude that Mo-MnLOX illustrates a partly unique variation of the structural theme of FeLOX.

Kinetics of Bis-Allylic Hydroperoxide Synthesis in the Iron-Containing Lipoxygenase 2 from Cyanothece and the Effects of Manganese Substitution

Lipoxygenases (LOX) catalyze the regio- and stereospecific insertion of dioxygen into polyunsaturated fatty acids. While the catalytic metal of LOX is typically a non-heme iron, some fungal LOX contain manganese as catalytic metal (MnLOX). In general, LOX insert dioxygen at C9 or C13 of linoleic acid leading to the formation of conjugated hydroperoxides. MnLOX (EC 1.13.11.45), however, catalyze the oxygen insertion also at C11, resulting in bis-allylic hydroperoxides. Interestingly, the iron-containing CspLOX2 (EC 1.13.11.B6) from Cyanothece PCC8801 also produces bis-allylic hydroperoxides. What role the catalytic metal plays and how this unusual reaction is catalyzed by either MnLOX or CspLOX2 is not understood. Our findings suggest that only iron is the catalytically active metal in CspLOX2. The enzyme loses its catalytic activity almost completely when iron is substituted with manganese, suggesting that the catalytic metal is not interchangeable. Using kinetic and spectroscopic approaches, we further found that first a mixture of bis-allylic and conjugated hydroperoxy products is formed. This is followed by the isomerization of the bis-allylic product to conjugated products at a slower rate. These results suggest that MnLOX and CspLOX2 share a very similar reaction mechanism and that LOX with a Fe or Mn cofactor have the potential to form bis-allylic products. Therefore, steric factors are probably responsible for this unusual specificity. As CspLOX2 is the LOX with the highest proportion of the bis-allylic product known so far, it will be an ideal candidate for further structural analysis to understand the molecular basis of the formation of bis-allylic hydroperoxides.

Manganese lipoxygenase of F. oxysporum and the structural basis for biosynthesis of distinct 11-hydroperoxy stereoisomers

The biosynthesis of jasmonates in plants is initiated by 13S-lipoxygenase (LOX), but details of jasmonate biosynthesis by fungi, including Fusarium oxysporum, are unknown. The genome of F. oxysporum codes for linoleate 13S-LOX (FoxLOX) and for F. oxysporum manganese LOX (Fo-MnLOX), an uncharacterized homolog of 13R-MnLOX of Gaeumannomyces graminis. We expressed Fo-MnLOX and compared its properties to Cg-MnLOX from Colletotrichum gloeosporioides. Electron paramagnetic resonance and metal analysis showed that Fo-MnLOX contained catalytic Mn. Fo-MnLOX oxidized 18:2n-6 mainly to 11R-hydroperoxyoctadecadienoic acid (HPODE), 13S-HPODE, and 9(S/R)-HPODE, whereas Cg-MnLOX produced 9S-, 11S-, and 13R-HPODE with high stereoselectivity. The 11-hydroperoxides did not undergo the rapid β-fragmentation earlier observed with 13R-MnLOX. Oxidation of [11S-(2)H]18:2n-6 by Cg-MnLOX was accompanied by loss of deuterium and a large kinetic isotope effect (>30). The Fo-MnLOX-catalyzed oxidation occurred with retention of the (2)H-label. Fo-MnLOX also oxidized 1-lineoyl-2-hydroxy-glycero-3-phosphatidylcholine. The predicted active site of all MnLOXs contains Phe except for Ser(348) in this position of Fo-MnLOX. The Ser348Phe mutant of Fo-MnLOX oxidized 18:2n-6 to the same major products as Cg-MnLOX. Our results suggest that Fo-MnLOX, with support of Ser(348), binds 18:2n-6 so that the proR rather than the proS hydrogen at C-11 interacts with the metal center, but retains the suprafacial oxygenation mechanism observed in other MnLOXs.

Kinetic investigation of the rate-limiting step of manganese- and iron-lipoxygenases

Lipoxygenases (LOX) oxidize polyunsaturated fatty acids to hydroperoxides, which are generated by proton coupled electron transfer to the metal center with FeIIIOH- or MnIIIOH-. Hydrogen abstraction by FeIIIOH- of soybean LOX-1 (sLOX-1) is associated with a large deuterium kinetic isotope effect (D-KIE). Our goal was to compare the D-KIE and other kinetic parameters at different temperatures of sLOX-1 with 13R-LOX with catalytic manganese (13R-MnLOX). The reaction rate and the D-KIE of sLOX-1 with unlabeled and [11-2H2]18:2n-6 were almost temperature independent with an apparent D-KIE of ∼56 at 30°C, which is in agreement with previous studies. In contrast, the reaction rate of 13R-MnLOX increased 7-fold with temperature (8-50°C), and the apparent D-KIE decreased linearly from ∼38 at 8°C to ∼20 at 50°C. The kinetic lag phase of 13R-MnLOX was consistently extended at low temperatures. The Phe337Ile mutant of 13R-MnLOX, which catalyzes antarafacial hydrogen abstraction and oxygenation in analogy with sLOX-1, retained the large D-KIE and its temperature-dependent reaction rate. The kinetic differences between 13R-MnLOX and sLOX-1 may be due to protein dynamics, hydrogen donor-acceptor distances, and to the metal ligands, which may not equalize the 0.7V-gap between the redox potentials of the free metals.

An iron 13S-lipoxygenase with an α-linolenic acid specific hydroperoxidase activity from Fusarium oxysporum

Jasmonates constitute a family of lipid-derived signaling molecules that are abundant in higher plants. The biosynthetic pathway leading to plant jasmonates is initiated by 13-lipoxygenase-catalyzed oxygenation of α-linolenic acid into its 13-hydroperoxide derivative. A number of plant pathogenic fungi (e.g. Fusarium oxysporum) are also capable of producing jasmonates, however, by a yet unknown biosynthetic pathway. In a search for lipoxygenase in F. oxysporum, a reverse genetic approach was used and one of two from the genome predicted lipoxygenases (FoxLOX) was cloned. The enzyme was heterologously expressed in E. coli, purified via affinity chromatography, and its reaction mechanism characterized. FoxLOX was found to be a non-heme iron lipoxygenase, which oxidizes C₁₈-polyunsaturated fatty acids to 13S-hydroperoxy derivatives by an antarafacial reaction mechanism where the bis-allylic hydrogen abstraction is the rate-limiting step. With α-linolenic acid as substrate FoxLOX was found to exhibit a multifunctional activity, because the hydroperoxy derivatives formed are further converted to dihydroxy-, keto-, and epoxy alcohol derivatives.

Secretion of two novel enzymes, manganese 9S-lipoxygenase and epoxy alcohol synthase, by the rice pathogen Magnaporthe salvinii

The mycelium of the rice stem pathogen, Magnaporthe salvinii, secreted linoleate 9S-lipoxygenase (9S-LOX) and epoxy alcohol synthase (EAS). The EAS rapidly transformed 9S-hydroperoxy-octadeca-10E,12Z-dienoic acid (9S-HPODE) to threo 10 (11)-epoxy-9S-hydroxy-12Z-octadecenoic acid, but other hydroperoxy FAs were poor substrates. 9S-LOX was expressed in Pichia pastoris. Recombinant 9S-LOX oxidized 18:2n-6 directly to 9S-HPODE, the end product, and also to two intermediates, 11S-hydroperoxy-9Z,12Z-octadecenoic acid (11S-HPODE; ∼5%) and 13R-hydroperoxy-9Z,11E-octadecadienoic acid (13R-HPODE; ∼1%). 11S- and 13R-HPODE were isomerized to 9S-HPODE, probably after oxidation to peroxyl radicals, β-fragmentation, and oxygen insertion at C-9. The 18:3n-3 was oxidized at C-9, C-11, and C-13, and to 9,16-dihydroxy-10E,12,14E-octadecatrienoic acid. 9S-LOX contained catalytic manganese (Mn:protein ∼0.2:1; Mn/Fe, 1:0.05), and its sequence could be aligned with 77% identity to 13R-LOX with catalytic manganese lipoxygenase (13R-MnLOX) of the Take-all fungus. The Leu350Met mutant of 9S-LOX shifted oxidation of 18:2n-6 from C-9 to C-13, and the Phe347Leu, Phe347Val, and Phe347Ala mutants of 13R-MnLOX from C-13 to C-9. In conclusion, M. salvinii secretes 9S-LOX with catalytic manganese along with a specific EAS. Alterations in the Sloane determinant of 9S-LOX and 13R-MnLOX with larger and smaller hydrophobic residues interconverted the regiospecific oxidation of 18:2n-6, presumably by altering the substrate position in relation to oxygen insertion.

Catalytic convergence of manganese and iron lipoxygenases by replacement of a single amino acid

Lipoxygenases (LOXs) contain a hydrophobic substrate channel with the conserved Gly/Ala determinant of regio- and stereospecificity and a conserved Leu residue near the catalytic non-heme iron. Our goal was to study the importance of this region (Gly(332), Leu(336), and Phe(337)) of a lipoxygenase with catalytic manganese (13R-MnLOX). Recombinant 13R-MnLOX oxidizes 18:2n-6 and 18:3n-3 to 13R-, 11(S or R)-, and 9S-hydroperoxy metabolites (∼80-85, 15-20, and 2-3%, respectively) by suprafacial hydrogen abstraction and oxygenation. Replacement of Phe(337) with Ile changed the stereochemistry of the 13-hydroperoxy metabolites of 18:2n-6 and 18:3n-3 (from ∼100% R to 69-74% S) with little effect on regiospecificity. The abstraction of the pro-S hydrogen of 18:2n-6 was retained, suggesting antarafacial hydrogen abstraction and oxygenation. Replacement of Leu(336) with smaller hydrophobic residues (Val, Ala, and Gly) shifted the oxygenation from C-13 toward C-9 with formation of 9S- and 9R-hydroperoxy metabolites of 18:2n-6 and 18:3n-3. Replacement of Gly(332) and Leu(336) with larger hydrophobic residues (G332A and L336F) selectively augmented dehydration of 13R-hydroperoxyoctadeca-9Z,11E,15Z-trienoic acid and increased the oxidation at C-13 of 18:1n-6. We conclude that hydrophobic replacements of Leu(336) can modify the hydroperoxide configurations at C-9 with little effect on the R configuration at C-13 of the 18:2n-6 and 18:3n-3 metabolites. Replacement of Phe(337) with Ile changed the stereospecific oxidation of 18:2n-6 and 18:3n-3 with formation of 13S-hydroperoxides by hydrogen abstraction and oxygenation in analogy with soybean LOX-1.

Stereoselective oxidation of regioisomeric octadecenoic acids by fatty acid dioxygenases

Seven Z-octadecenoic acids having the double bond located in positions 6Z to 13Z were photooxidized. The resulting hydroperoxy-E-octadecenoic acids [HpOME(E)] were resolved by chiral phase-HPLC-MS, and the absolute configurations of the enantiomers were determined by gas chromatographic analysis of diastereoisomeric derivatives. The MS/MS/MS spectra showed characteristic fragments, which were influenced by the distance between the hydroperoxide and carboxyl groups. These fatty acids were then investigated as substrates of cyclooxygenase-1 (COX-1), manganese lipoxygenase (MnLOX), and the (8R)-dioxygenase (8R-DOX) activities of two linoleate diol synthases (LDS) and 10R-DOX. COX-1 and MnLOX abstracted hydrogen at C-11 of (12Z)-18:1 and C-12 of (13Z)-18:1. (11Z)-18:1 was subject to hydrogen abstraction at C-10 by MnLOX and at both allylic positions by COX-1. Both allylic hydrogens of (8Z)-18:1 were also abstracted by 8R-DOX activities of LDS and 10R-DOX, but only the allylic hydrogens close to the carboxyl groups of (11Z)-18:1 and (12Z)-18:1. 8R-DOX also oxidized monoenoic C(14)-C(20) fatty acids with double bonds at the (9Z) position, suggesting that the length of the omega end has little influence on positioning for oxygenation. We conclude that COX-1 and MnLOX can readily abstract allylic hydrogens of octadecenoic fatty acids from C-10 to C-12 and 8R-DOX from C-7 and C-12.

Oxylipins in fungi

In nearly every living organism, metabolites derived from lipid peroxidation, the so-called oxylipins, are involved in regulating developmental processes as well as environmental responses. Among these bioactive lipids, the mammalian and plant oxylipins are the best characterized, and much information about their physiological role and biosynthetic pathways has accumulated during recent years. Although the occurrence of oxylipins and enzymes involved in their biosynthesis has been studied for nearly three decades, knowledge about fungal oxylipins is still scarce as compared with the situation in plants and mammals. However, the research performed so far has shown that the structural diversity of oxylipins produced by fungi is high and, furthermore, that the enzymes involved in oxylipin metabolism are diverse and often exhibit unusual catalytic activities. The aim of this review is to present a synopsis of the oxylipins identified so far in fungi and the enzymes involved in their biosynthesis.

A bisallylic mini-lipoxygenase from cyanobacterium Cyanothece sp. that has an iron as cofactor

Lipoxygenases are enzymes that are found ubiquitously in higher animals and plants, but have only recently been identified in a number of bacteria. The genome of the diazotrophic unicellular cyanobacterium Cyanothece sp. harbors two genes with homology to lipoxygenases. Here we describe the isolation of one gene, formerly named csplox2. It was cloned, and the protein was expressed in Escherichia coli and purified. The purified enzyme belongs to the group of prokaryotic mini lipoxygenases, because it had a molecular mass of 65 kDa. Interestingly, it catalyzed the conversion of linoleic acid, the only endogenously found polyunsaturated fatty acid, primarily to the bisallylic hydroperoxide 11R-hydroperoxyoctadecadienoic acid. This product had previously only been described for the manganese lipoxygenase from the take all fungus, Gaeumannomyces graminis. By contrast, CspLOX2 was shown to be an iron lipoxygenase. In addition, CspLOX2 formed a mixture of typical conjugated lipoxygenase products, e.g. 9R- and 13S-hydroperoxide. The conversion of linoleic acid took place with a maximum reaction rate of 31 s(-1). Incubation of the enzyme with [(11S)-(2)H]linoleic acid led to the formation of hydroperoxides that had lost the deuterium label, thus suggesting that CspLOX2 catalyzes antarafacial oxygenation as opposed to the mechanism of manganese lipoxygenase. CspLOX2 could also oxidize diarachidonylglycerophosphatidylcholine with similar specificity as the free fatty acid, indicating that binding of the substrate takes place with a "tail-first" orientation. We conclude that CspLOX2 is a novel iron mini-lipoxygenase that catalyzes the formation of bisallylic hydroperoxide as the major product.

An update on products and mechanisms of lipid peroxidation

The free radical reaction of polyunsaturated fatty acids with molecular oxygen leads to hydroperoxides as the first stable products. From linoleic acid the two conjugated diene hydroperoxides at carbons 9 and 13 were considered the only primary products until the recent discovery of the bis-allylic 11-hydroperoxide. The 11-carbon is the site of the initial hydrogen abstraction, and the 11-hydroperoxide is formed without isomerization of the 9,10 and 12,13 cis double bonds. In the autoxidation reaction, bis-allylic hydroperoxides are obtained only in the presence of an efficient antioxidant, for example, alpha-tocopherol. The antioxidant functions as a hydrogen atom donor, necessary to trap the fleeting bis-allylic peroxyl radical intermediate as the hydroperoxide. Understanding of the mechanism of formation of bis-allylic hydroperoxides has led to increased appreciation of the central role of the intermediate peroxyl radical in determining the outcome of lipid peroxidation.