Mutagenesis of some conserved residues in human 5-lipoxygenase: effects on enzyme activity

Investigating anti-inflammatory actions of marine algal compound against lipoxygenase concentrating on therapeutic applications through computational approach

Inflammation is the preliminary response given to any possible harmful stimuli including infections, injury or stress by immune system where neutrophils and macrophages gets activated and produces mediators, such as nitric oxide and cytokines that serves as biomarkers of inflammation. Lipoxygenases are enzymes that peroxidises lipids and are involved in the pathogenesis of several diseases including inflammatory diseases. These are oxidative enzymes comprising a non-heme iron atom in active site and are convoluted in inflammatory reactions. Fucoidan is sulphated polysaccharide that has numerous pharmacological implications. Implications of fucoidan on inflammatory diseases are still an objective of rigorous research. Therefore, this study focusses on investigating lipoxygenase inhibitory activities of fucoidan. The mechanism of lipoxygenase inhibitory activities of fucoidan was studied via molecular docking and molecular dynamics simulations. The docking score produced by the binding of the fucoidan to the lipoxygenase was - 6.69 kcal/mol whereas, the docking score in case of Aspirin and Zileuton were -5.8 kcal/mol and -7.0 kcal/mol and it was found that fucoidan makes hydrogen bonds with lipoxygenase protein through polar amino acid glutamine at GLN 514. The results obtained from molecular dynamics simulations proposed the development of a stable complex between fucoidan and lipoxygenase due to the establishment of favourable interactions with amino acid residues and indicated efficient results when compared with Aspirin and Zileuton. This study suggested that fucoidan had anti-inflammatory potentials and thus can be used as a promising drug candidate against inflammation.Communicated by Ramaswamy H. Sarma.

Design, synthesis and identification of novel coumaperine derivatives for inhibition of human 5-LOX: Antioxidant, pseudoperoxidase and docking studies

5-Lipoxygenase (5-LOX) is a key enzyme involved in the biosynthesis of pro-inflammatory leukotrienes, leading to asthma. Developing potent 5-LOX inhibitors especially, natural product based ones, are highly attractive. Coumaperine, a natural product found in white pepper and its derivatives were herein developed as 5-LOX inhibitors. We have synthesized twenty four derivatives, characterized and evaluated their 5-LOX inhibition potential. Coumaperine derivatives substituted with multiple hydroxy and multiple methoxy groups exhibited best 5-LOX inhibition. CP-209, a catechol type dihydroxyl derivative and CP-262-F2, a vicinal trihydroxyl derivative exhibited, 82.7% and 82.5% inhibition of 5-LOX respectively at 20 µM. Their IC₅₀ values are 2.1 ± 0.2 µM and 2.3 ± 0.2 µM respectively, and are comparable to zileuton, IC₅₀ = 1.4 ± 0.2 µM. CP-155, a methylenedioxy derivative (a natural product) and CP-194, a 2,4,6-trimethoxy derivative showed 76.0% and 77.1% inhibition of 5-LOX respectively at 20 µM. Antioxidant study revealed that CP-209 and 262-F2 (at 20 µM) scavenged DPPH radical by 76.8% and 71.3% respectively. On the other hand, CP-155 and 194 showed very poor DPPH radical scavenging activity. Pseudo peroxidase assay confirmed that the mode of action of CP-209 and 262-F2 were by redox process, similar to zileuton, affecting the oxidation state of the metal ion in the enzyme. On the contrary, CP-155 and 194 probably act through some other mechanism which does not involve the disruption of the oxidation state of the metal in the enzyme. Molecular docking of CP-155 and 194 to the active site of 5-LOX and binding energy calculation suggested that they are non-competitive inhibitors. The In-Silico ADME/TOX analysis shows the active compounds (CP-155, 194, 209 and 262-F2) are with good drug likeliness and reduced toxicity compared to existing drug. These studies indicate that there is a great potential for coumaperine derivatives to be developed as anti-inflammatory drug.

A novel class of tyrosine derivatives as dual 5-LOX and COX-2/mPGES1 inhibitors with PGE2 mediated anticancer properties

Leukotriene and prostaglandin pathways are controlled by the enzymes, LOX and COX/mPGES1 respectively and are responsible for inflammatory responses.

Synthesis of diarylidenecyclohexanone derivatives as potential anti-inflammatory leads against COX-2/mPGES1 and 5-LOX

This study establishes the diarylidenecyclohexanones as good anti-inflammatory pharmacophores with selective high potency against PGE₂and 5-LOX without toxicity towards healthy human cells.

Design, synthesis and identification of novel substituted 2-amino thiazole analogues as potential anti-inflammatory agents targeting 5-lipoxygenase

Human 5-Lipoxygenase (5-LOX) is a key enzyme targeted for asthma and inflammation. Zileuton, the only drug against 5-LOX, was withdrawn from the market due to several problems. In the present study, the performance of rationally designed conjugates of thiazole (2) and thiourea (3) scaffolds from our previously reported 2-amino-4-aryl thiazole (1) is reported. They are synthesized (total 31 derivatives), characterized, and tested against the 5-LOX enzyme in vitro and the mode of action of the most active ones are determined. Compound 2m exhibited an IC₅₀ of 0.9 ± 0.1 μM acting through competitive (non-redox) mechanism, unlike Zileuton, and found to be devoid of radical scavenging properties. Computational studies are in good agreement with the experimental data supporting its mechanism of action. Another lead molecule from the thiourea series (3), 3f, exhibited an IC₅₀ of 1.4 ± 0.1 μM against 5-LOX whose mode of action is redox type (non-competitive). It is promising to note that the activities displayed by both the lead inhibitors, 2m and 3f, are better than the commercial drug, Zileuton (IC₅₀ = 1.5 ± 0.3 μM). These inhibitors could be further developed as drugs against inflammation.

Interfacial Enzymes: Membrane Binding, Orientation, Membrane Insertion, and Activity

Most interfacial enzymes undergo activation upon membrane binding. Interfacial activation is determined not only by the binding strength but also by the specific mode of protein-membrane interactions, including the angular orientation and membrane insertion of the enzymes. This chapter describes biophysical techniques to quantitatively evaluate membrane binding, orientation, membrane insertion, and activity of secreted phospholipase A2 (PLA2) and lipoxygenase (LO) enzymes. Procedures for recombinant production and purification of human pancreatic PLA2 and human 5-lipoxygenase (5-LO) are also presented. Several methods for measurements of membrane binding of peripheral proteins are described, i.e., fluorescence resonance energy transfer (FRET) from tryptophan or tyrosine residues of the protein to a fluorescent lipid in vesicles, changes in fluorescence of an environment-sensitive fluorescent lipid upon binding of proteins to membranes, and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. These methods produce the apparent binding constant, the protein-to-lipid binding stoichiometry, and the Hill cooperativity coefficient. Experimental procedures for segmental isotope labeling of proteins and determination of the orientation of membrane-bound proteins by polarized ATR-FTIR spectroscopy are described. Furthermore, evaluation of membrane insertion of peripheral proteins by a fluorescence quenching technique is outlined. Combination of the orientation and membrane insertion provides a unique configuration of the protein-membrane complex and hence elucidates certain details of the enzyme function, such as the modes of acquisition of a membrane-residing substrate and product release. Finally, assays for determination of the activities of secreted PLA2, soybean LO, and human 5-LO are described.

2-Amino-4-aryl thiazole: a promising scaffold identified as a potent 5-LOX inhibitor

Human 5-lipoxygenase (5-LOX) is a target for asthma and allergy treatment. Zileuton is the only marketed drug targeting this enzyme (IC₅₀ ∼ 1 μM). The current study identifies a promising lead molecule which could be improved to match the activity of zileuton.

Molecular pharmacological profile of a novel thiazolinone-based direct and selective 5-lipoxygenase inhibitor

BACKGROUND AND PURPOSE

The potency of many 5-lipoxygenase (5-LOX) inhibitors depends on the cellular peroxide tone and the mechanism of 5-LOX enzyme activation. Therefore, new inhibitors that act regardless of the mode of enzyme activation need to be developed. Recently, we identified a novel class of thiazolinone-based compounds as potent 5-LOX inhibitors. Here, we present the molecular pharmacological profile of (Z)-5-(4-methoxybenzylidene)-2-(p-tolyl)-5H-thiazol-4-one, compound C06.

EXPERIMENTAL APPROACH

Inhibition of 5-LOX product formation was determined in intact cells [polymorphonuclear leukocytes (PMNL), rat basophilic leukaemia-1, RAW264.7] and in cell-free assays [homogenates, 100, 000×g supernatant (S100), partially purified 5-LOX] applying different stimuli for 5-LOX activation. Inhibition of peroxisome proliferator-activated receptor (PPAR), cytosolic phospholipase A(2) (cPLA(2) ), 12-LOX, 15-LOX-1 and 15-LOX-2 as well as cyclooxygenase-2 (COX-2) were measured in vitro.

KEY RESULTS

C06 induced non-cytotoxic, direct 5-LOX inhibition with IC(50) values about 0.66 µM (intact PMNL, PMNL homogenates) and approximately 0.3 µM (cell-free PMNL S100, partially purified 5-LOX). Action of C06 was independent of the stimulus used for 5-LOX activation and cellular redox tone and was selective for 5-LOX compared with other arachidonic acid binding proteins (PPAR, cPLA(2) , 12-LOX, 15-LOX-1, 15-LOX-2, COX-2). Experimental results suggest an allosteric binding distinct from the active site and the C2-like domain of 5-LOX.

CONCLUSIONS AND IMPLICATIONS

C06 was identified as a potent selective direct 5-LOX inhibitor exhibiting a novel and unique mode of action, different from other established 5-LOX inhibitors. This thiazolinone may possess potential for intervention with inflammatory and allergic diseases and certain types of cancer.

LOX genes in blast fungus (Magnaporthe grisea) resistance in rice

Plant Lipoxygenases (LOX) are known to play major role in plant immunity by providing front-line defense against pathogen-induced injury. To verify this, we isolated a full-length OsLOX3 gene and also 12 OsLOX cDNA clones from Oryza sativa indica (cultivar Pusa Basmati 1). We have examined the role played by LOXs in plant development and during attack by blast pathogen Magnaporthe grisea. Gene expression, promoter region analysis, and biochemical and protein structure analysis of isolated OsLOX3 revealed significant homology with LOX super family. Protein sequence comparison of OsLOXs revealed high levels of homology when compared with japonica rice (up to100%) and Arabidopsis (up to 64%). Isolated LOX3 gene and 12 OsLOX cDNAs contained the catalytic LOX domains much required for oxygen binding and synthesis of oxylipins. Amino acid composition, protein secondary structure, and promoter region analysis (with abundance of motifs CGTCA and TGACG) support the role of OsLOX3 gene in providing resistance to diseases in rice plants. OsLOX3 gene expression analysis of root, shoot, flag leaf, and developing and mature seed revealed organ specific patterns during rice plant development and gave evidence to association between tissue location and physiological roles played by individual OsLOXs. Increased defense activity of oxylipins was observed as demonstrated by PCR amplification of OsLOX3 gene and upon inoculation with virulent strains of M. grisea and ectopic application of methyl jasmonate in the injured leaf tissue in adult rice plants.

The coding sequence mediates induction of 5-lipoxygenase expression by Smads3/4

5-Lipoxygenase (5-LO) expression is strongly induced by transforming growth factor-beta (TGFbeta) and 1alpha,25-dihydroxyvitamin D(3) in Mono Mac 6 cells. Since Smads have been described as downstream effectors of TGFbeta, we have investigated the role of the TGFbeta/Smad signalling system in the regulation of 5-LO gene expression. The rapid induction of 5-LO mRNA, determined with real-time quantitative RT-PCR, suggests that 5-LO is a primary TGFbeta target gene. In reporter gene assays with plasmids containing the 5-LO promoter plus different parts of the gene, Smads3/4 mediate a prominent upregulation of reporter activity that strongly depends on the coding sequence and to a lesser extent on the 3'-UTR and introns J-M. Deletion studies revealed the most profound decrease of inducibility by Smads3/4 when exons 10-14 are deleted. Sequence analysis and deletion studies indicate the existence of up to four Smad binding elements and at least one TGFbeta responsive element far downstream of the transcriptional start site.

Binding investigation of human 5-lipoxygenase with its inhibitors by SPR technology correlating with molecular docking simulation

The binding features of a series of 5-lipoxygenase (5-LOX) inhibitors (caffeic acid, NDGA, AA-861, CDC, esculetin, gossypol and phenidone) to human 5-LOX have been studied by using surface plasmon resonance biosensor (SPR) technology based Biacore 3000 and molecular docking simulation analyses. The SPR results showed that the equilibrium dissociation constant (KD) values evaluated by Biacore 3000 for the inhibitors showed a good correlation with its reported IC50, suggesting that SPR technology might be applicable as a direct assay method in screening new 5-LOX inhibitors at an early stage. In addition, the 3D structural model of 5-LOX was generated according to the crystal structure of rabbit reticulocyte 15-lipoxygenase, and the molecular docking simulation analyses revealed that the predicted binding free energies for the inhibitors correlated well with the KD values measured by SPR assay, which implies the correctness of the constructed 3D structural model of 5-LOX. This current work has potential for application in structure-based 5-LOX inhibitor discovery.

Mutation analysis of the human 5-lipoxygenase C-terminus: support for a stabilizing C-terminal loop

Lipoxygenases contain prosthetic iron, in human 5-lipoxygenase (5LO) the C-terminal isoleucine carboxylate constitutes one of five identified ligands. ATP is one of several factors determining 5LO activity. We compared properties of a series of 5LO C-terminal deletion mutants (one to six amino acid residues deleted). All mutants were enzymatically inactive (expected due to loss of iron), but expression yield (in E. coli) and affinity to ATP-agarose was markedly different. Deletion of up to four C-terminal residues was compatible with good expression and retained affinity to the ATP-column, as for wild-type 5LO. However when also the fifth residue was deleted (Asn-669) expression yield decreased and the affinity to ATP was markedly diminished. This was interpreted as a result of deranged structure and stability, due to loss of a hydrogen bond between Asn-669 and His-399. Mutagenesis of these residues supported this conclusion. In the structure of soybean lipoxygenase-1, a C-terminal loop was pointed out as important for correct orientation of the C-terminus. Accordingly, a hydrogen bond appears to stabilize such a C-terminal loop also in 5LO.

Membrane fluidity is a key modulator of membrane binding, insertion, and activity of 5-lipoxygenase

Mammalian 5-lipoxygenase (5-LO) catalyzes conversion of arachidonic acid to leukotrienes, potent mediators of inflammation and allergy. Upon cell stimulation, 5-LO selectively binds to nuclear membranes and becomes activated, yet the mechanism of recruitment of 5-LO to nuclear membranes and the mode of 5-LO-membrane interactions are poorly understood. Here we show that membrane fluidity is an important determinant of membrane binding strength of 5-LO, penetration into the membrane hydrophobic core, and activity of the enzyme. The membrane binding strength and activity of 5-LO increase with the degree of lipid acyl chain cis-unsaturation and reach a plateau with 1-palmitoyl-2-arachidonolyl-sn-glycero-3-phosphocholine (PAPC). A fraction of tryptophans of 5-LO penetrate into the hydrocarbon region of fluid PAPC membranes, but not into solid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine membranes. Our data lead to a novel concept of membrane binding and activation of 5-LO, suggesting that arachidonic-acid-containing lipids, which are present in nuclear membranes at higher fractions than in other cellular membranes, may facilitate preferential membrane binding and insertion of 5-LO through increased membrane fluidity and may thereby modulate the activity of the enzyme. The data presented in this article and earlier data allow construction of a model for membrane-bound 5-LO, including the angular orientation and membrane insertion of the protein.

Characterization of calcium and magnesium binding domains of human 5-lipoxygenase

Two calcium binding sites, separated by about 9.3A, present in the loops that connect the beta-sheets of N-terminal domain contain the ligating residues F14, A15, G16, D79, and D18, D19, L76, respectively. Magnesium is found to bind in regions, which are marginally different owing to the disparity in the ionic radii of Ca2+ and Mg2+. The entropy analysis on the loops of 5-lipoxygenase, implementing the wormlike chain model, explains that the N-terminal beta-barrel is well suited to accommodate calcium binding sites. The large buried side chain area of W102 (compared to W13 and W75) and comparatively smaller fraction of side chain exposed to polar atoms corroborate the calcium induced higher affinity to phosphatidylcholine (PC). However, W80 lying in close proximity of the calcium binding sites is expected to have considerable PC affinity but negligible calcium induced effect on PC binding.

Arachidonate 5-lipoxygenase

In this article, it has been attempted to review data primarily on the activation of human 5-lipoxygenase, in vitro and in the cell. First, structural properties and enzyme activities are described. This is followed by the activating factors: Ca2+, membranes, ATP, and lipid hydroperoxide. Also, studies on phosphorylation of 5-lipoxygenase, interaction with other proteins, and the intracellullar mobility of 5-lipoxygenase, are reviewed.

Flavonoids of cocoa inhibit recombinant human 5-lipoxygenase

(-)-Epicatechin and its related oligomers, the procyanidins, are present in sizable amounts in some cocoas and chocolates. Intake of flavonoid-rich chocolate in humans has been reported to increase the plasma level of (-)-epicatechin and concomitantly to significantly decrease the plasma level of proinflammatory cysteinyl leukotrienes. Because leukotrienes are formed via the 5-lipoxygenase pathway of arachidonic acid metabolism, we examined whether 5-lipoxygenase is a possible target for the flavonoids of cocoa. Recombinant human 5-lipoxygenase was reacted with arachidonic acid and yielded a mixture of mainly 5-hydroperoxy-6E,8Z, 11Z,14Z-eicosatetraenoic acid (5-HpETE) and hydrolysis products of 5,6-leukotriene A(4) (LTA(4)). The formation of these products was significantly inhibited by (-)-epicatechin in a dose-dependent manner with 50% inhibitory concentrations (IC(50)) of 22 and 50 micromol/L, respectively. Among the procyanidin fractions isolated from the seeds of Theobroma cacao, only the dimer fraction and, to a lesser extent, the trimer through pentamer fractions exhibited comparable effects, whereas the larger procyanidins (hexamer through nonamer) were almost inactive. We conclude that (-)-epicatechin and its low-molecular procyanidins inhibit both dioxygenase and LTA(4) synthase activities of human 5-lipoxygenase and that this action may contribute to a putative anti-inflammatory effect of cocoa products.

Expression of 5-lipoxygenase in pulmonary artery endothelial cells

Increased expression of 5-lipoxygenase (5LO) in pulmonary artery endothelial cells (PAECs) has been observed in disease states such as pulmonary hypertension and allergen challenge. To understand the function of endothelial 5LO, we examined the expression of this enzyme in normally cultured human PAECs and its characteristics when overexpressed. A small amount of 5LO message and protein was detected by reverse-transcriptase-mediated PCR (RT-PCR) and Western blotting in PAECs. Sequencing of the RT-PCR products that overlapped the entire coding region of 5LO mRNA indicated that the sequence of PAEC 5LO was identical with that of leucocyte 5LO. Incubation of the PAECs with A23187 and arachidonic acid led to a small production of 5-hydroxyeicosatetraenoic acid (5-HETE) (46-98 pmol/4x10(6) cells) but no leukotrienes. Overexpression of 5LO in PAECs by adenovirus-mediated gene transfer revealed that the enzyme was localized in the nucleus. Incubation of the transduced cells with A23187 (5 microM) caused the production of both 5LO products and downstream leukotrienes. The proportions of the produced leukotriene A(4) (LTA(4)) hydrolates (sum of 6-trans-LTB(4) and 12-epi-6-trans-LTB(4)), LTB(4) and cysteinyl leukotriene were approx. 17:14:10. cGMP production in the 5LO-transduced PAECs was decreased by 33+/-14% on stimulation with A23187. These results show that cultured PAECs express a minimal amount of 5LO, which can generate some 5-HETE, but not leukotrienes. However, increased expression of 5LO in PAECs can lead to the production of all downstream leukotrienes, which could potentially cause endothelial dysfunction in the pulmonary vasculature.

5-Lipoxygenase interacts with coactosin-like protein

We have recently identified coactosin-like protein (CLP) in a yeast two-hybrid screen using 5-lipoxygenase (5LO) as a bait. In this report, we demonstrate a direct interaction between 5LO and CLP. 5LO associated with CLP, which was expressed as a glutathione S-transferase fusion protein, in a dose-dependent manner. Coimmunoprecipitation experiments using epitope-tagged 5LO and CLP proteins transiently expressed in human embryonic kidney 293 cells revealed the presence of CLP in 5LO immunoprecipitates. In reciprocal experiments, 5LO was detected in CLP immunoprecipitates. Non-denaturing polyacrylamide gel electrophoresis and cross-linking experiments showed that 5LO binds CLP in a 1:1 molar stoichiometry in a Ca(2+)-independent manner. Site-directed mutagenesis suggested an important role for lysine 131 of CLP in mediating 5LO binding. In view of the ability of CLP to bind 5LO and filamentous actin (F-actin), we determined whether CLP could physically link 5LO to actin filaments. However, no F-actin-CLP.5LO ternary complex was observed. In contrast, 5LO appeared to compete with F-actin for the binding of CLP. Moreover, 5LO was found to interfere with actin polymerization. Our results indicate that the 5LO-CLP and CLP-F-actin interactions are mutually exclusive and suggest a modulatory role for 5LO in actin dynamics.

Mutagenesis and modelling of linoleate-binding to pea seed lipoxygenase

We have produced a model to define the linoleate-binding pocket of pea 9/13-lipoxygenase and have validated it by the construction and characterization of eight point mutants. Three of the mutations reduced, to varying degrees, the catalytic centre activity (kcat) of the enzyme with linoleate. In two of the mutants, reductions in turnover were associated with changes in iron-coordination. Multiple sequence alignments of recombinant plant and mammalian lipoxygenases of known positional specificity, and the results from numerous other mutagenesis and modelling studies, have been combined to discuss the possible role of the mutated residues in pea 9/13-lipoxygenase catalysis. A new nomenclature for recombinant plant lipoxygenases based on positional specificity has subsequently been proposed. The null-effect of mutating pea 9/13-lipoxygenase at the equivalent residue to that which controlled dual positional specificity in cucumber 13/9-lipoxygenase, strongly suggests that the mechanisms controlling dual positional specificity in pea 9/13-lipoxygenase and cucumber 13/9-lipoxygenase are different. This was supported from modelling of another isoform of pea lipoxygenase, pea 13/9-lipoxygenase. Dual positional specificity in pea lipoxygenases is more likely to be determined by the degree of penetration of the methyl terminus of linoleate and the volume of the linoleate-binding pocket rather than substrate orientation. A single model for positional specificity, that has proved to be inappropriate for arachidonate-binding to mammalian 5-, 12- and 15-lipoxygenases, would appear to be true also for linoleate-binding to plant 9- and 13-lipoxygenases.

The N-terminal domain of 5-lipoxygenase binds calcium and mediates calcium stimulation of enzyme activity

Human 5-lipoxygenase (5-LO) is a key enzyme in the conversion of arachidonic acid into leukotrienes and lipoxins, mediators and modulators of inflammation. In this study, we localized a stimulatory Ca(2+)-binding site to the N-terminal region of the enzyme. Thus, in a (45)Ca(2+) overlay assay, the N-terminal 128 amino acids of recombinant human 5-LO (fused to glutathione S-transferase) bound radioactive calcium to about the same extent as intact 5-LO. The glutathione S-transferase fusion protein of the C-terminal part of 5-LO (amino acids 120-673) showed much weaker binding. A model of a putative 5-LO N-terminal domain was calculated based on the structure of rabbit reticulocyte 15-LO. This model resembles beta-sandwich C2 domains of other Ca(2+)-binding proteins. Comparison of our model with the C2 domain of cytosolic phospholipase A(2) suggested a number of amino acids, located in the loops that connect the beta-strands, as potential Ca(2+) ligands. Indeed, mutations particularly in loop 2 (N43A, D44A, and E46A) led to decreased Ca(2+) binding and a requirement for higher Ca(2+) concentrations to stimulate enzyme activity. Our data indicate that an N-terminal beta-sandwich of 5-LO functions as a C2 domain in the calcium regulation of enzyme activity.

Analysis of a nucleotide-binding site of 5-lipoxygenase by affinity labelling: binding characteristics and amino acid sequences

5-Lipoxygenase (5LO) catalyses the first two steps in the biosynthesis of leukotrienes, which are inflammatory mediators derived from arachidonic acid. 5LO activity is stimulated by ATP; however, a consensus ATP-binding site or nucleotide-binding site has not been found in its protein sequence. In the present study, affinity and photoaffinity labelling of 5LO with 5'-p-fluorosulphonylbenzoyladenosine (FSBA) and 2-azido-ATP showed that 5LO bound to the ATP analogues quantitatively and specifically and that the incorporation of either analogue inhibited ATP stimulation of 5LO activity. The stoichiometry of the labelling was 1.4 mol of FSBA/mol of 5LO (of which ATP competed with 1 mol/mol) or 0.94 mol of 2-azido-ATP/mol of 5LO (of which ATP competed with 0.77 mol/mol). Labelling with FSBA prevented further labelling with 2-azido-ATP, indicating that the same binding site was occupied by both analogues. Other nucleotides (ADP, AMP, GTP, CTP and UTP) also competed with 2-azido-ATP labelling, suggesting that the site was a general nucleotide-binding site rather than a strict ATP-binding site. Ca(2+), which also stimulates 5LO activity, had no effect on the labelling of the nucleotide-binding site. Digestion with trypsin and peptide sequencing showed that two fragments of 5LO were labelled by 2-azido-ATP. These fragments correspond to residues 73-83 (KYWLNDDWYLK, in single-letter amino acid code) and 193-209 (FMHMFQSSWNDFADFEK) in the 5LO sequence. Trp-75 and Trp-201 in these peptides were modified by the labelling, suggesting that they were immediately adjacent to the C-2 position of the adenine ring of ATP. Given the stoichiometry of the labelling, the two peptide sequences of 5LO were probably near each other in the enzyme's tertiary structure, composing or surrounding the ATP-binding site of 5LO.

5-Lipoxygenase upregulation by dexamethasone in human mast cells

In spite of intensive research, our understanding of the regulation of expression of 5-LO (the key enzyme in the leukotriene metabolism) remains fragmentary. We investigated the effects of dexamethasone on the expression of this gene in a binary model consisting of two clones of the human mast cell line HMC-1, one with a 5-LO-negative and the other with a 5-LO-positive phenotype, respectively. When dexamethasone was included in the culture medium at a physiologically relevant concentration, biosynthesis of 5-LO derivatives increased considerably not only in the 5-LO-negative HMC-1 cells (approx 10-fold) but also in the 5-LO-positive cells, characterized by an already substantial enzyme activity. Consistently, Northern blot analysis revealed that a dramatic increase in the abundance of 5-LO mRNA occurred when the cells were exposed to dexamethasone. Likewise, a significant increase in the immunoreactive 5-LO protein was detected by Western blotting. In contrast, dexamethasone seemed to have no effect on the expression of two other genes of pivotal importance in leukotriene biosynthesis, viz. FLAP and LTC(4) synthase. We conclude that in human mast cells glucocorticoids effectively and selectively upregulate the expression of 5-LO.

Interaction of 5-lipoxygenase with cellular proteins

5-Lipoxygenase (5LO) plays a pivotal role in cellular leukotriene synthesis. To identify proteins interacting with human 5LO, we used a two-hybrid approach to screen a human lung cDNA library. From a total of 1.5 x 10(7) yeast transformants, nine independent clones representing three different proteins were isolated and found to specifically interact with 5LO. Four 1.7- to 1.8-kb clones represented a 16-kDa protein named coactosin-like protein for its significant homology with coactosin, a protein found to be associated with actin in Dictyostelium discoideum. Coactosin-like protein thus may provide a link between 5LO and the cytoskeleton. Two other yeast clones of 1.5 kb encoded transforming growth factor (TGF) type beta receptor-I-associated protein 1 partial cDNA. TGF type beta receptor-I-associated protein 1 recently has been reported to associate with the activated form of the TGF beta receptor I and may be involved in the TGF beta-induced up-regulation of 5LO expression and activity observed in HL-60 and Mono Mac 6 cells. Finally, three identical 2.1-kb clones contained the partial cDNA of a human protein with high homology to a hypothetical helicase K12H4. 8 from Caenorhabditis elegans and consequently was named DeltaK12H4. 8 homologue. Analysis of the predicted amino acid sequence revealed the presence of a RNase III motif and a double-stranded RNA binding domain, indicative of a protein of nuclear origin. The identification of these 5LO-interacting proteins provides additional approaches to studies of the cellular functions of 5LO.

Determinants of 5-lipoxygenase nuclear localization using green fluorescent protein/5-lipoxygenase fusion proteins

5-Lipoxygenase catalyzes the first two steps in the biosynthesis of leukotrienes, potent extracellular mediators of inflammation and allergic disorders. The unanticipated observation of 5-lipoxygenase in the nucleus of some cell types including bone marrow-derived mast cells (Chen, X. S., Naumann, T. A., Kurre, U., Jenkins, N. A., Copeland, N. G., and Funk, C. D. (1995) J. Biol. Chem. 270, 17993-17999) has raised speculation about intranuclear actions of leukotrienes or the enzyme itself. To explore the entry of 5-lipoxygenase into the nucleus we have transfected various cell types with expression vectors encoding native 5-lipoxygenase and green fluorescent protein/5-lipoxygenase (GFP-5LO) fusion proteins. 5-Lipoxygenase and green fluorescent protein/5-lipoxygenase co-localized with the nuclear DNA stain Hoechst 33258 in each cell type. The three main basic regions of 5-lipoxygenase were incapable of acting as "classical" nuclear localization signal sequences. Mutations that abolished enzyme activity/non-heme iron resulted in proteins that would no longer enter the nucleus. An NH2-terminal 5-lipoxygenase fragment of 80 residues was sufficient for directing nuclear localization of green fluorescent protein but not cytosolic pyruvate kinase. The combined data suggest that 5-lipoxygenase enters the nucleus not by a classical nuclear localization signal but by a non-conventional signal located in the predicted beta-barrel domain that may be masked by structural alterations.

Nonredox 5-lipoxygenase inhibitors require glutathione peroxidase for efficient inhibition of 5-lipoxygenase activity

Nonredox type 5-lipoxygenase (5-LO) inhibitors, such as ZM 230487, its methyl analogue ZD 2138, or the Merck compound L-739,010, suppress cellular leukotriene synthesis of ionophore stimulated granulocytes with IC50 values of about 50 nM. However, in cell homogenates or in preparations of purified enzyme, up to 150-fold higher concentrations are required for similar inhibition of 5-LO activity. This loss of 5-LO inhibition in cell homogenates was reversed by addition of glutathione or dithiothreitol, which increased the inhibitory potency of ZM 230487 or L-739,010 by about 100 to 150-fold so that 5-LO inhibition was comparable with that of intact cells. In the presence of thiols, addition of hydroperoxide [13(S)-HpODE], glutathione-peroxidase inhibition by iodacetate or selenium-deficiency lead to impaired 5-LO inhibition by ZM 230487 in cell homogenates. Moreover, addition of glutathione peroxidase was required for efficient inhibition of purified human 5-LO by ZM 230487. The data suggest that low hydroperoxide concentrations are important for efficient 5-LO inhibition by ZM 230487. The kinetic analysis revealed a noncompetitive inhibition of 5-LO by ZM 230487 at low hydroperoxide levels, whereas it acted as a competitive inhibitor with low affinity under nonreducing conditions in granulocyte homogenates. No such redox-dependent effects were observed with the 5-LO inhibitor BWA4C, the 5-LO activating protein-inhibitor MK-886 or the pentacyclic triterpene acetyl-11-keto-beta-boswellic acid. These data suggest that physiological conditions associated with oxidative stress and increased peroxide levels lead to impaired efficacy of nonredox type 5-LO inhibitors like ZM 230487 or L-739,010. This could explain the reported lack of activity of this class of 5-LO inhibitors in chronic inflammatory processes.

Molecular characterization of L2 lipoxygenase from maize embryos

We investigated the expression and accumulation pattern of lipoxygenase isoforms throughout the maize plant life. Two forms of lipoxygenase L1 and L2 have been identified as acidic proteins of 100 kDa (pI 6.4) and 90 kDa (pI 5.5-5.7) which accumulate in dry embryos and in various organs of maize seedlings. In young embryos, only the L2 form was detected and accumulation of L2 mRNA decreased during embryo development. Identification of lipoxygenases from in vivo and in vitro synthesized proteins indicates that similar levels of both L1 and L2 forms accumulated during treatment with abscisic acid, (ABA) gibberellic acid (GA3) and jasmonic acid (JA). However, differences in the activity of both enzymes were detected. By using an antiserum directed against purified L2 we isolated and characterized a partial cDNA clone of maize embryos encoding a lipoxygenase. The deduced amino acid sequence of L2 cDNA shares 78% identity with the rice L2 protein, and 51-56% identity with lipoxygenases from the dicotyledonous plants soybean and Arabidopsis. DNA blot analysis indicated that maize contains a family of lipoxygenase genes which are presently being characterized.

Lipid body lipoxygenase characterized by protein fragmentation, cDNA sequence and very early expression of the enzyme during germination of cucumber seeds

Lipid bodies are cellular compartments containing triacylglycerols. They are encompassed by a phospholipid monolayer and decorated with characteristic proteins. In plants, lipid bodies are synthesized during seed formation but acquire new proteins during seed germination. In germinating cucumber (Cucumis sativus) seeds, the set of newly synthesized proteins appearing in the lipid bodies at the early stage of triacylglycerol mobilization comprises a special form of lipoxygenase. We isolated the lipid body lipoxygenase and characterized fragments prepared by limited proteolysis and cleavage with cyanogen bromide. A very early expression of lipid body lipoxygenase was found by studying the rate of de novo synthesis of lipoxygenase forms during germination. This allowed a clear distinction of this enzyme from other lipoxygenase isoforms. Hence, for determining the molecular structure of lipid body lipoxygenase we analyzed a cDNA prepared from mRNA of cotyledons at day 1 of germination. From the cDNA sequence, oligonucleotides were derived that specifically detected lipid body lipoxygenase mRNA on northern blots. The very early expression of lipid body lipoxygenase was corroborated by this approach. Good agreement was observed between the amino acid sequence deduced from the cDNA sequence and the peptide structures analyzed biochemically. In particular, the cleavage products of cyanogen bromide treatment indicated that we had isolated the lipid body lipoxygenase cDNA. The sequence data show a lipoxygenase form characterized by a molecular mass of 99655 Da, which is significantly higher than the molecular masses of the cytosolic forms. Compared to the cytosolic forms that exhibit a molecular mass of 95 kDa, the lipid body form has an N-terminal extension of 34 amino acid residues. No evidence for a cotranslational or post-translational proteolytic processing was obtained by the size comparison of the in vitro-translated lipoxygenase and the lipid body form.

Leukotriene A4 hydrolase: protection from mechanism-based inactivation by mutation of tyrosine-378

Leukotriene A4 (LTA4) hydrolase [(7E,9E,11Z,14Z)-(5S,6S)-5,6-epoxyicosa-7, 9,11,14-tetraenoate hydrolase; EC 3.3.2.6] is a bifunctional zinc metalloenzyme that catalyzes the final step in the biosynthesis of the potent chemotactic agent leukotriene B4 (LTB4). LTA4 hydrolase/aminopeptidase is suicide inactivated during catalysis via an apparently mechanism-based irreversible binding of LTA4 to the protein in a 1:1 stoichiometry. Previously, we have identified a henicosapeptide, encompassing residues Leu-365 to Lys-385 in human LTA4 hydrolase, which contains a site involved in the covalent binding of LTA4 to the native enzyme. To investigate the role of Tyr-378, a potential candidate for this binding site, we exchanged Tyr for Phe or Gln in two separate mutants. In addition, each of two adjacent and potentially reactive residues, Ser-379 and Ser-380, were exchanged for Ala. The mutated enzymes were expressed as (His)6-tagged fusion proteins in Escherichia coli, purified to apparent homogeneity, and characterized. Enzyme activity determinations and differential peptide mapping, before and after repeated exposure to LTA4, revealed that wild-type enzyme and the mutants [S379A] and [S380A]LTA4hydrolase were equally susceptible to suicide inactivation whereas the mutants in position 378 were no longer inactivated or covalently modified by LTA4. Furthermore, in [Y378F]LTA4 hydrolase, the value of kcat for epoxide hydrolysis was increased 2.5-fold over that of the wild-type enzyme. Thus, by a single-point mutation in LTA4 hydrolase, catalysis and covalent modification/inactivation have been dissociated, yielding an enzyme with increased turnover and resistance to mechanism-based inactivation.

Structure conservation in lipoxygenases: structural analysis of soybean lipoxygenase-1 and modeling of human lipoxygenases

Lipoxygenases are a class of non-heme iron dioxygenases which catalyze the hydroperoxidation of fatty acids for the biosynthesis of leukotrienes and lipoxins. The structure of the 839-residue soybean lipoxygenase-1 was used as a template to model human 5-, 12-, and 15-lipoxygenases. A distance-based algorithm for placing side chains in a low homology environment (only the four iron ligands were fixed during side chain placement) was devised. Twenty-six of the 56 conserved lipoxygenase residues were grouped in four distinct regions of the enzyme. These regions were analyzed to discern whether the side chain interactions could be duplicated in the models or whether alternate conformers should be considered. The effects of site directed mutagenesis variants were rationalized using the models of the human lipoxygenases. In particular, variants which shifted positional specificity between 12- and 15-lipoxygenase activity were analyzed. Analysis of active site residues produced a model which accounts for observed lipoxygenase positional specificity and stereospecificity.

Functional analysis of conserved histidine residues in Cephalosporium acremonium isopenicillin N synthase by site-directed mutagenesis

The isopenicillin N synthase of Cephalosporium acremonium (cIPNS) involves a catalytically important non-heme iron which is coordinated credibly to histidine residues. A comparison of the IPNS genes from various microbial sources indicated that there are seven conserved histidine residues. These were individually replaced by leucine residues through site-directed mutagenesis, and the sites of mutation were confirmed by DNA sequencing. The seven mutant genes were cloned separately into the vector pET24d for expression in Escherichia coli BL21 (DE3), and the proteins were expressed as soluble enzymes. All the resulting mutant enzymes obtained have mobilities of approximately 38 kDa, identical with the wild-type enzyme on SDS-polyacrylamide gel electrophoresis, and were also reactive to cIPNS antibodies. The enzymes were purified by ammonium sulfate precipitation and DEAE-Sephadex A-50 ion exchange chromatography, and these were analyzed for enzyme activity. A group of mutant enzymes, H49L, H64L, H116L, H126L, and H137L, were found to be enzymatically active with reduced activities of 16-93.7%, indicating that they are not essential for catalysis. Two of the mutant enzymes, H216L and H272L, were found to have lost their enzymatic activity completely, indicating that both His-216 and His-272 are crucial for catalysis. It is suggested that these histidines are likely to serve as ligands for binding to the non-heme iron in the IPNS active site. Alignment of the amino acid sequence of IPNS to related non-heme Fe(2+)-requiring enzymes indicated that the two essential histidine residues correspond to two invariant residues located in highly homologous regions. The conservation of the two closely located histidine residues indicates the possible conservation of similar iron-binding sites in these enzymes.

cDNA cloning, expression, mutagenesis, intracellular localization, and gene chromosomal assignment of mouse 5-lipoxygenase

5-Lipoxygenase of mouse macrophages and bone marrow-derived mast cells (BMMC) was investigated. Indirect immunocytofluorescence combined with confocal microscopy provided evidence for distinct intracellular expression patterns and trafficking of 5-lipoxygenase upon cellular activation. In resting BMMC, 5-lipoxygenase was found within the nucleus co-localizing with the nuclear stain Yo-Pro-1. When BMMC were IgE/antigen-activated the 5-lipoxygenase immunofluorescence pattern was changed from nuclear to perinuclear. The absence of divalent cations in the incubation medium, or calcium ionophore A23187 challenge, altered the predominantly nuclear expression pattern to new sites both cytosolic and intranuclear. The cDNA for murine macrophage 5-lipoxygenase was cloned by the polymerase chain reaction and would predict a 674 amino acid protein. Using control cells obtained from 5-lipoxygenase-deficient mice it was determined that a single isoform accounts for both soluble and membrane-bound and nuclear and cytosolic-localized enzyme in macrophages and BMMC. A mutation at amino acid 672 (Val-->Met) introduced serendipitously during the cloning process was found to completely abolish 5-lipoxygenase enzyme activity when the enzyme was expressed in human embryonic kidney 293 cells. This subtle change is proposed to affect the ability of the COOH-terminal isoleucine to coordinate the essential non-heme iron atom. In macrophages and BMMC obtained from 5-lipoxygenase-deficient mice, compensatory changes in expression of genes involved in the biosynthesis of leukotriene B4 were investigated. 5-Lipoxygenase-activating protein expression was reduced by 50%, while leukotriene A4 hydrolase expression was unaltered. The 5-lipoxygenase gene was mapped to the central region of mouse chromosome 6 in a region that shares homology with human chromosome 10 by interspecific backcross analysis. These studies provide a global picture of the murine 5-lipoxygenase system and raise questions about the role of 5-lipoxygenase and leukotrienes within the nucleus.

Mutations at the C-terminal isoleucine and other potential iron ligands of 5-lipoxygenase

The non-heme iron centre in human 5-lipoxygenase was studied. Recombinant enzyme was expressed in Escherichia coli, purified and assayed for iron content and enzyme activity. For non-mutated enzyme, the iron content was 1.01 +/- 0.19 mol/mol. Deletion of the C-terminal Ile673 resulted in an iron content of 0.03 +/- 0.07 mol/mol and undetectable lipoxygenase activity. Mutations at His367, Glu376 and Asn554 led to drastically decreased enzyme activity (< 2% of non-mutated control) but iron was still present. In addition to Glu376, eight other conserved acidic residues (Asp/Glu) in 5-lipoxygenase were replaced, none of which was crucial for enzyme activity. We conclude that Ile673 is an iron ligand in 5-lipoxygenase, while our results do not support that Glu376 or Asn554 have this function. The possible role of His367 as a replaceable iron ligand is discussed.

12-lipoxygenases and 12(S)-HETE: role in cancer metastasis

Arachidonic acid metabolites have been implicated in multiple steps of carcinogenesis. Their role in tumor cell metastasis, the ultimate challenge for the treatment of cancer patients, are however not well-documented. Arachidonic acid is primarily metabolized through three pathways, i.e., cyclooxygenase, lipoxygenase, and P450-dependent monooxygenase. In this review we focus our attention on one specific lipoxygenase, i.e., 12-lipoxygenase, and its potential role in modulating the metastatic process. In mammalian cells there exist three types of 12-lipoxygenases which differ in tissue distribution, preferential substrates, and profile of their metabolites. Most of these 12-lipoxygenases have been cloned and sequenced, and the molecular and biochemical determinants responsible for catalysis of specific substrates characterized. Solid tumor cells express 12-lipoxygenase mRNA, possess 12-lipoxygenase protein, and biosynthesize 12(S)-HETE [12(S)-hydroxyeicosatetraenoic acid], as revealed by numerous experimental approaches. The ability of tumor cells to generate 12(S)-HETE is positively correlated to their metastatic potential. A large collection of experimental data suggest that 12(S)-HETE is a crucial intracellular signaling molecule that activates protein kinase C and mediates the biological functions of many growth factors and cytokines such as bFGF, PDGF, EGF, and AMF. 12(S)-HETE plays a pivotal role in multiple steps of the metastatic 'cascade' encompassing tumor cell-vasculature interactions, tumor cell motility, proteolysis, invasion, and angiogenesis. The fact that 12-lipoxygenase is expressed in a wide diversity of tumor cell lines and 12(S)-HETE is a key modulatory molecule in metastasis provides the rationale for targeting these molecules in anti-cancer and anti-metastasis therapeutic protocols.

Role of leukotrienes revealed by targeted disruption of the 5-lipoxygenase gene

Leukotrienes constitute a class of potent biological mediators of inflammation and anaphylaxis (for reviews see refs 1 and 2). Their biosynthesis derives from 5-lipoxygenase-catalysed oxygenation of arachidonic acid in granulocytes, macrophages and mast cells. To examine the physiological importance of leukotrienes, we have disrupted the 5-lipoxygenase gene by homologous recombination in embryonic stem cells. 5-Lipoxygenase-deficient (5LX-/-) mice develop normally and are healthy. They show a selective opposition to certain inflammatory insults. Although there is no difference in their reaction to endotoxin shock, the 5LX-/- animals resist the lethal effects of shock induced by platelet-activating factor. Reaction to ear inflammation induced by phorbol ester is normal, whereas inflammation induced by arachidonic acid is markedly reduced. Contrasts were also found in two models of leukocyte chemotaxis in vivo. The phenotype of 5LX-/- mice under injurious insult identifies the role for leukotrienes in the pathophysiology of select inflammatory states.

Molecular basis of a null mutation in soybean lipoxygenase 2: substitution of glutamine for an iron-ligand histidine

We have investigated the nucleotide sequence and expression of lox2, a mutant form of the gene encoding lipoxygenase 2, an enzyme responsible for unpleasant flavors in soybean [Glycine max (L) Merr.] seeds. Although lox2 transcripts accumulate normally, there are no detectable transcripts for lipoxygenase 1 or 3 in mutant lines that display similar phenotypes characterized by the lack of corresponding lipoxygenase activity and protein in mature seeds. The enzymatically inactive lox2 gene product is readily detectable in mid-maturation-stage seeds but is apparently unstable, since it is absent from mature seed. The protein sequence deduced from the cDNA and genomic DNA sequences of lox2 differs from that of the wild-type gene, Lox2, in the substitution of glutamine for His-532. It is known that His-504 in soybean lipoxygenase 1, which corresponds to His-532 in lipoxygenase 2, is one of the iron-binding ligands essential for lipoxygenase activity. Here we present evidence that the missense mutation substituting Gln for His-532 results in the loss of lipoxygenase 2 from mature soybean seeds.

Characterization of an Arabidopsis lipoxygenase gene responsive to methyl jasmonate and wounding

A cDNA corresponding to the gene AtLox2 was isolated from an Arabidopsis thaliana library using a lipoxygenase (LOX) probe from soybean. AtLox2 encodes a 102-kD protein, AtLOX2, which has 42 to 45% amino acid sequence identity with other plant LOX sequences. The AtLOX2 sequence is more than 30 amino acids longer at the amino terminus than other plant LOX sequences, and this extension has features reminiscent of chloroplast transit peptides, suggesting that AtLOX2 may be chloroplast localized. AtLox2 mRNA levels are high in leaves and inflorescences but very low in seeds, roots, and stems. AtLox2 mRNA accumulation is rapidly induced in leaves in response to methyl jasmonate. Leaves that have been wounded and adjacent leaves on the same plant also accumulate AtLox2 mRNA.

Characterization of the non-heme iron center of human 5-lipoxygenase by electron paramagnetic resonance, fluorescence, and ultraviolet-visible spectroscopy: redox cycling between ferrous and ferric states

Purified human 5-lipoxygenase, a non-heme iron containing enzyme, has been characterized by electron paramagnetic resonance, (EPR), ultraviolet (UV)-visible and fluorescence spectroscopy. As isolated, the enzyme is largely in the ferrous state and shows a weak X-band EPR signal extending from 0 to 700 G at 15 K, tentatively ascribed to integer spin Fe(II). Titration of the protein with 13-HPOD (13-hydroperoxyoctadecadienoic acid) generates a strong multicomponent EPR signal in the g' approximately 6 region, a yellow color associated with an increased absorption between 310 and 450 nm (epsilon 330nm = 2400 M-1 cm-1), and a 17% decrease in the intrinsic protein fluorescence. The multiple component nature of the g' approximately 6 signal indicates that the metal center in its oxidized state exists in more than one but related forms. The g' approximately 6 EPR signal and the yellow color reach a maximum when approximately 1 mol of 13-HPOD is added/mol of iron; the resultant EPR spectrum accounts quantitatively for all of the iron in the protein with a signal at g' = 4.3 representing less than 3% of the total iron in the majority of samples. Addition of a hydroxyurea reducing agent abolished the g' approximately 6 signal and yellow color of the protein and also reversed the decrease in fluorescence caused by the oxidant 13-HPOD. The results indicate that the g' approximately 6 EPR signal, the yellow color, and the decreased fluorescence are associated with the formation of the Fe(III) form of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Crystallographic determination of the active site iron and its ligands in soybean lipoxygenase L-1

Five ligands of the active site iron atom in soybean lipoxygenase L-1 have been identified from the electron density map of the crystallized enzyme. The position of the iron atom can be readily and independently located from an anomalous difference electron density map. The ligands identified are His-499, His-504, His-690, Asn-694, and Ile-839, the carboxy-terminal residue. Our previous view that these three histidines are essential for activity and binding of iron, based on site-specific mutation studies, is confirmed. A sixth protein ligand is not present, and the sixth coordination site opens into a wide cleft. The structure of the soybean lipoxygenase was solved by multiple anomalous isomorphous replacements.

The three-dimensional structure of an arachidonic acid 15-lipoxygenase

In mammals, the hydroperoxidation of arachidonic acid by lipoxygenases leads to the formation of leukotrienes and lipoxins, compounds that mediate inflammatory responses. Lipoxygenases are dioxygenases that contain a nonheme iron and are present in many animal cells. Soybean lipoxygenase-1 is a single-chain, 839-residue protein closely related to mammalian lipoxygenases. The structure of soybean lipoxygenase-1 solved to 2.6 angstrom resolution shows that the enzyme has two domains: a 146-residue beta barrel and a 693-residue helical bundle. The iron atom is in the center of the larger domain and is coordinated by three histidines and the COO- of the carboxyl terminus. The coordination geometry is nonregular and appears to be a distorted octahedron in which two adjacent positions are not occupied by ligands. Two cavities, in the shapes of a bent cylinder and a frustum, connect the unoccupied positions to the surface of the enzyme. The iron, with two adjacent and unoccupied positions, is poised to interact with the 1,4-diene system of the substrate and with molecular oxygen during catalysis.