Bell pepper fruit fatty acid hydroperoxide lyase is a cytochrome P450 (CYP74B)

CYP74B34 Enzyme from Carrot (Daucus carota) with a Double Hydroperoxide Lyase/Epoxyalcohol Synthase Activity: Identification and Biochemical Properties

The lipoxygenase cascade in plants is a source of oxylipins (oxidized fatty acid derivatives), which play an important role in regulatory processes and formation of plant response to stress factors. Some of the most common enzymes of the lipoxygenase cascade are 13-specific hydroperoxide lyases (HPLs, also called hemiacetal synthases) of the CYP74B subfamily. In this work, we identified and cloned the CYP74B34 gene from carrot (Daucus carota L.) and described the biochemical properties of the corresponding recombinant enzyme. The CYP74B34 enzyme was active towards 9- and 13-hydroperoxides of linoleic (9-HPOD and 13-HPOD, respectively) and α-linolenic (9-HPOT and 13-HPOT, respectively) acids. CYP74B34 specifically converted 9-HPOT and 13-HPOT into aldo acids (HPL products). The transformation of 13-HPOD led to the formation of aldo acids and epoxyalcohols [products of epoxyalcohol synthase (EAS) activity] as major and minor products, respectively. At the same time, conversion of 9-HPOD resulted in the formation of epoxyalcohols as the main products and aldo acids as the minor ones. Therefore, CYP74B34 is the first enzyme with a double HPL/EAS activity described in carrot. The presence of these catalytic activities was confirmed by analysis of the oxylipin profiles for the roots from young seedlings and mature plants. In addition, we substituted amino acid residues in one of the catalytically essential sites of the CYP74B34 and CYP74B33 proteins and investigated the properties of the obtained mutant enzymes.

From linoleic acid to hexanal and hexanol by whole cell catalysis with a lipoxygenase, hydroperoxide lyase and reductase cascade in Komagataella phaffii

Green leaf volatiles (GLVs) cover a group of mainly C6-and C9-aldehydes, -alcohols and -esters. Their name refers to their characteristic herbal and fruity scent, which is similar to that of freshly cut grass or vegetables. Lipoxygenases (LOXs) catalyze the peroxidation of unsaturated fatty acids. The resulting hydroperoxy fatty acids are then cleaved into aldehydes and oxo acids by fatty acid hydroperoxide lyases (HPLs). Herein, we equipped the yeast Komagataella phaffii with recombinant genes coding for LOX and HPL, to serve as a biocatalyst for GLV production. We expressed the well-known 13S-specific LOX gene from Pleurotus sapidus and a compatible HPL gene from Medicago truncatula. In bioconversions, glycerol induced strains formed 12.9 mM hexanal using whole cells, and 8 mM hexanol was produced with whole cells induced by methanol. We applied various inducible and constitutive promoters in bidirectional systems to influence the final ratio of LOX and HPL proteins. By implementing these recombinant enzymes in Komagataella phaffii, challenges such as biocatalyst supply and lack of product specificity can finally be overcome.

Synthesis of Polymer Precursor 12-Oxododecenoic Acid Utilizing Recombinant Papaya Hydroperoxide Lyase in an Enzyme Cascade

Hydroperoxide lyases (HPLs) catalyze the splitting of 13S-hydroperoxyoctadecadienoic acid (13S-HPODE) into the green note flavor hexanal and 12-oxo-9(Z)-dodecenoic acid, which is not yet used industrially. Here, HPL from Carica papaya (HPL_CP) was cloned and functionally expressed in Escherichia coli to investigate synthesis of 12-oxo-9(Z)-dodecenoic acid in detail. To improve the low catalytic activity of full-length HPL_CP, the hydrophobic, non-conserved N-terminal sequence was deleted. This enhanced enzyme activity from initial 10 to 40 U/l. With optimization of solubilization buffer, expression media enzyme activity was increased to 2700 U/l. The tetrameric enzyme was produced in a 1.5 l fermenter and enriched by affinity chromatography. The enzyme preparation possesses a slightly acidic pH optimum and a catalytic efficiency (k_cat/K_M) of 2.73 × 10⁶ s^-1·M^-1 towards 13S-HPODE. Interestingly, HPL_CP-N could be applied for the synthesis of 12-oxo-9(Z)-dodecenoic acid, and 1 mM of 13S-HPODE was transformed in just 10 s with a yield of 90%. At protein concentrations of 10 mg/ml, the slow formation of the 10(E)-isomer traumatin was observed, pointing to a non-enzymatic isomerization process. Bearing this in mind, a one-pot enzyme cascade starting from safflower oil was developed with consecutive addition of Pseudomonas fluorescens lipase, Glycine max lipoxygenase (LOX-1), and HPL_CP-N. A yield of 43% was obtained upon fast extraction of the reaction mixtures after 1 min of HPL_CP-N reaction. This work provides first insights into an enzyme cascade synthesis of 12-oxo-9(Z)-dodecenoic acid, which may serve as a bifunctional precursor for bio-based polymer synthesis.

Identification of the key genes contributing to the LOX-HPL volatile aldehyde biosynthesis pathway in jujube fruit

Jujube (Ziziphus jujuba Mill.) is a traditional popular fruit widely grown in China. The volatiles in jujube determine its unique flavor and the high fruit quality required by consumers. However, the biosynthesis of volatiles in jujube were remain unknown. By using gas chromatography-mass spectrometry, there were 46 volatile compounds were identified and determined from three representative jujube fruit types at six developmental stages, including the dry-used (Z. jujuba cv. 'Junzao'), the fresh-used (Z. jujuba cv. 'Dongzao'), and wild jujube (Z. jujuba var. spinosa Hu. cv. 'Qingjiansuanzao'). The aldehydes were identified as major volatile contributors to flavor, of which (E)-2-hexenal was the primary volatile in jujube fruit. Then LOX and HPL gene family were identified in jujube, which were involved in aldehyde biosynthesis through the lipoxygenase-hydroperoxide lyase (LOX-HPL) pathway. Gene expression analysis suggested that ZjLOX3, ZjLOX4, and ZjHPL1 were highly correlated with the accumulation of (E)-2-hexenal, and their proteins were localized to the nucleus and cytoplasm. Transient over-expression of ZjLOX3, ZjLOX4, and ZjHPL1 in jujube fruit significantly enhanced the accumulation of (E)-2-hexenal. Our study provides valuable information on the major volatiles and their biosynthesis in different types of jujube fruit. These results will help determine flavor improvements for future breeding.

Synthesis of Polymer Precursor 12-Oxododecenoic Acid Utilizing Recombinant Papaya Hydroperoxide Lyase in an Enzyme Cascade

Hydroperoxide lyases (HPLs) catalyze the splitting of 13S-hydroperoxyoctadecadienoic acid (13S-HPODE) into the green note flavor hexanal and 12-oxo-9(Z)-dodecenoic acid, which is not yet used industrially. Here, HPL from Carica papaya (HPL_CP) was cloned and functionally expressed in Escherichia coli to investigate synthesis of 12-oxo-9(Z)-dodecenoic acid in detail. To improve the low catalytic activity of full-length HPL_CP, the hydrophobic, non-conserved N-terminal sequence was deleted. This enhanced enzyme activity from initial 10 to 40 U/l. With optimization of solubilization buffer, expression media enzyme activity was increased to 2700 U/l. The tetrameric enzyme was produced in a 1.5 l fermenter and enriched by affinity chromatography. The enzyme preparation possesses a slightly acidic pH optimum and a catalytic efficiency (k_cat/K_M) of 2.73 × 10⁶ s^-1·M^-1 towards 13S-HPODE. Interestingly, HPL_CP-N could be applied for the synthesis of 12-oxo-9(Z)-dodecenoic acid, and 1 mM of 13S-HPODE was transformed in just 10 s with a yield of 90%. At protein concentrations of 10 mg/ml, the slow formation of the 10(E)-isomer traumatin was observed, pointing to a non-enzymatic isomerization process. Bearing this in mind, a one-pot enzyme cascade starting from safflower oil was developed with consecutive addition of Pseudomonas fluorescens lipase, Glycine max lipoxygenase (LOX-1), and HPL_CP-N. A yield of 43% was obtained upon fast extraction of the reaction mixtures after 1 min of HPL_CP-N reaction. This work provides first insights into an enzyme cascade synthesis of 12-oxo-9(Z)-dodecenoic acid, which may serve as a bifunctional precursor for bio-based polymer synthesis.

Processing of Airborne Green Leaf Volatiles for Their Glycosylation in the Exposed Plants

Green leaf volatiles (GLVs), the common constituents of herbivore-infested plant volatiles (HIPVs), play an important role in plant defense and function as chemical cues to communicate with other individuals in nature. Reportedly, in addition to endogenous GLVs, the absorbance of airborne GLVs emitted by infested neighboring plants also play a major role in plant defense. For example, the exclusive accumulation of (Z)-3-hexenyl vicianoside in the HIPV-exposed tomato plants occurs by the glycosylation of airborne (Z)-3-hexenol (Z3HOL); however, it is unclear how plants process the other absorbed GLVs. This study demonstrates that tomato plants dominantly accumulated GLV-glycosides after exposure to green leaf alcohols [Z3HOL, (E)-2-hexenol, and n-hexanol] using non-targeted LC-MS analysis. Three types of green leaf alcohols were independently glycosylated without isomerization or saturation/desaturation. Airborne green leaf aldehydes and esters were also glycosylated, probably through converting aldehydes and esters into alcohols. Further, we validated these findings in Arabidopsis mutants- (Z)-3-hexenal (Z3HAL) reductase (chr) mutant that inhibits the conversion of Z3HAL to Z3HOL and the acetyl-CoA:(Z)-3-hexen-1-ol acetyltransferase (chat) mutant that impairs the conversion of Z3HOL to (Z)-3-hexenyl acetate. Exposure of the chr and chat mutants to Z3HAL accumulated lower and higher amounts of glycosides than their corresponding wild types (Col-0 and Ler), respectively. These findings suggest that plants process the exogenous GLVs by the reductase(s) and the esterase(s), and a part of the processed GLVs contribute to glycoside accumulation. Overall, the study provides insights into the understanding of the communication of the plants within their ecosystem, which could help develop strategies to protect the crops and maintain a balanced ecosystem.

Green Leaf Volatile-Burst in Selaginella moellendorffii

Green leaf volatiles (GLVs) consist of six-carbon volatile aldehydes, alcohols, and their esters. They are formed from polyunsaturated fatty acids and are involved in the defense of plants against herbivores and pathogens. GLVs generally have low concentrations in intact healthy plant tissues, but the biosynthetic pathway to form GLVs is quickly activated by mechanical damage to tissues, an event called the GLV-burst. Most seed plants have the ability to implement GLV-burst; however, this potential in non-seed plants has not been extensively researched. In this study, we examined the GLV-burst capacity of monilophytes, lycophytes, and bryophytes, and confirmed that monilophytes and lycophytes showed substantial GLV-burst ability, while bryophytes did not, with a few exceptions. When the genome sequence of a model lycophyte, Selaginella moellendorffii was reviewed, 10 genes were found that showed high similarity with the non-canonical cytochrome P450 enzymes, CYP74s, specialized in oxylipin formation. Recombinant proteins expressed with Escherichia coli showed that one of them had the ability to encode allene oxide synthase, and another encoded hydroperoxide lyase (HPL), preferring linolenic acid 13-hydroperoxide, and it was inferred that this gene was responsible for GLV-burst in S. moellendorffii. Based on the phylogenetic tree constructed with CYP74s of non-seed and seed plants, we hypothesized that HPL was acquired independently in the lycophyte and seed plants through diversification of CYP74 genes.

Epoxyalcohol synthase activity of the CYP74B enzymes of higher plants

The CYP74B subfamily of fatty acid hydroperoxide transforming cytochromes P450 includes the most common plant enzymes. All CYP74Bs studied yet except the CYP74B16 (flax divinyl ether synthase, LuDES) and the CYP74B33 (carrot allene oxide synthase, DcAOS) are 13-hydroperoxide lyases (HPLs, synonym: hemiacetal synthases). The results of present work demonstrate that additional products (except the HPL products) of fatty acid hydroperoxides conversion by the recombinant StHPL (CYP74B3, Solanum tuberosum), MsHPL (CYP74B4v1, Medicago sativa), and CsHPL (CYP74B6, Cucumis sativus) are epoxyalcohols. MsHPL, StHPL, and CsHPL converted the 13-hydroperoxides of linoleic (13-HPOD) and α-linolenic acids (13-HPOT) primarily to the chain cleavage products. The minor by-products of 13-HPOD and 13-HPOT conversions by these enzymes were the oxiranyl carbinols, 11-hydroxy-12,13-epoxy-9-octadecenoic and 11-hydroxy-12,13-epoxy-9,15-octadecadienoic acid. At the same time, all enzymes studied converted 9-hydroperoxides into corresponding oxiranyl carbinols with HPL by-products. Thus, the results showed the additional epoxyalcohol synthase activity of studied CYP74B enzymes. The 13-HPOD conversion reliably resulted in smaller yields of the HPL products and bigger yields of the epoxyalcohols compared to the 13-HPOT transformation. Overall, the results show the dualistic HPL/EAS behaviour of studied CYP74B enzymes, depending on hydroperoxide isomerism and unsaturation.

Recombinant Lipoxygenases and Hydroperoxide Lyases for the Synthesis of Green Leaf Volatiles

Green leaf volatiles (GLVs) are mainly C₆- and in rare cases also C₉-aldehydes, -alcohols, and -esters, which are released by plants in response to biotic or abiotic stresses. These compounds are named for their characteristic smell reminiscent of freshly mowed grass. This review focuses on GLVs and the two major pathway enzymes responsible for their formation: lipoxygenases (LOXs) and fatty acid hydroperoxide lyases (HPLs). LOXs catalyze the peroxidation of unsaturated fatty acids, such as linoleic and α-linolenic acids. Hydroperoxy fatty acids are further converted by HPLs into aldehydes and oxo-acids. In many industrial applications, plant extracts have been used as LOX and HPL sources. However, these processes are limited by low enzyme concentration, stability, and specificity. Alternatively, recombinant enzymes can be used as biocatalysts for GLV synthesis. The increasing number of well-characterized enzymes efficiently expressed by microbial hosts will foster the development of innovative biocatalytic processes for GLV production.

Biocatalytic Synthesis of Natural Green Leaf Volatiles Using the Lipoxygenase Metabolic Pathway

In higher plants, the lipoxygenase enzymatic pathway combined actions of several enzymes to convert lipid substrates into signaling and defense molecules called phytooxylipins including short chain volatile aldehydes, alcohols, and esters, known as green leaf volatiles (GLVs). GLVs are synthesized from C18:2 and C18:3 fatty acids that are oxygenated by lipoxygenase (LOX) to form corresponding hydroperoxides, then the action of hydroperoxide lyase (HPL) produces C6 or C9 aldehydes that can undergo isomerization, dehydrogenation, and esterification. GLVs are commonly used as flavors to confer a fresh green odor of vegetable to perfumes, cosmetics, and food products. Given the increasing demand in these natural flavors, biocatalytic processes using the LOX pathway reactions constitute an interesting application. Vegetable oils, chosen for their lipid profile are converted in natural GLVs with high added value. This review describes the enzymatic reactions of GLVs biosynthesis in the plant, as well as the structural and functional properties of the enzymes involved. The various stages of the biocatalytic production processes are approached from the lipid substrate to the corresponding aldehyde or alcoholic aromas, as well as the biotechnological improvements to enhance the production potential of the enzymatic catalysts.

Detection of unprecedented allene oxide synthase member of CYP74B subfamily: CYP74B33 of carrot (Daucus carota)

Enzymes of CYP74 family widespread in higher plants control the metabolism of fatty acid hydroperoxides to numerous bioactive oxylipins. Hydroperoxide lyases (HPLs, synonym: hemiacetal synthases) of CYP74B subfamily belong to the most common CYP74 enzymes. HPLs isomerize the hydroperoxides to the short-lived hemiacetals, which are spontaneously decomposed to aldehydes and aldoacids. All CYP74Bs studied yet except the CYP74B16 (flax divinyl ether synthase, LuDES) possessed the 13-HPL activity. Present work reports the cloning of the expressed CYP74B33 gene of carrot (Daucus carota L.) and studies of catalytic properties of the recombinant CYP74B33 protein. In contrast to all CYP74B proteins studied yet, CYP74B33 behaved differently in few respects. Firstly, the preferred substrates of CYP74B33 are 9-hydroperoxides. Secondly and most importantly, CYP74B33 exhibits the 9-allene oxide synthase (AOS) activity. For example, the 9(S)-hydroperoxide of linoleic acid (9-HPOD) underwent the conversion to α-ketol via the short-lived allene oxide. Uncommonly, the 9-HPOD conversion affords a minority of cis-10-oxo-11-phytoenoic acid, which is also produced by CYP74C but not the CYP74A AOSs. The similar product patterns were observed upon the incubations of CYP74B33 with 9(S)-hydroperoxide of α-linolenic acid. The enzyme possessed a mixed HPL, AOS, and the epoxyalcohol synthase activity toward the 13-hydroperoxides, but the total activity was much lower than toward 9-hydroperoxides. Thus, the obtained results show that CYP74B33 is an unprecedented 9-AOS within the CYP74B subfamily.

Transcriptome analysis identifies strong candidate genes for ginsenoside biosynthesis and reveals its underlying molecular mechanism in Panax ginseng C.A. Meyer

Ginseng, Panax ginseng C.A. Meyer, is one of the most important medicinal herbs for human health and medicine in which ginsenosides are known to play critical roles. The genes from the cytochrome P450 (CYP) gene superfamily have been shown to play important roles in ginsenoside biosynthesis. Here we report genome-wide identification of the candidate PgCYP genes for ginsenoside biosynthesis, development of functional SNP markers for its manipulation and systems analysis of its underlying molecular mechanism. Correlation analysis identified 100 PgCYP genes, including all three published ginsenoside biosynthesis PgCYP genes, whose expressions were significantly correlated with the ginsenoside contents. Mutation association analysis identified that six of these 100 PgCYP genes contained SNPs/InDels that were significantly associated with ginsenosides biosynthesis (P ≤ 1.0e-04). These six PgCYP genes, along with all ten published ginsenoside biosynthesis genes from the PgCYP and other gene families, formed a strong co-expression network, even though they varied greatly in spatio-temporal expressions. Therefore, this study has identified six new ginsenoside biosynthesis candidate genes, provided a genome-wide insight into how they are involved in ginsenoside biosynthesis and developed a set of functional SNP markers useful for enhanced ginsenoside biosynthesis research and breeding in ginseng and related species.

Structural Variation, Functional Differentiation, and Activity Correlation of the Cytochrome P450 Gene Superfamily Revealed in Ginseng

Ginseng ( C.A. Mey.) is one of the most important medicinal herbs for human health and medicine, for which ginsenosides are the major bioactive components. The cytochrome P450 genes, , form a large gene superfamily; however, only three genes have been identified from ginseng and shown to be involved in ginsenoside biosynthesis, indicating the importance of the gene superfamily in the process. Here we report genome-wide identification and systems analysis of the genes in ginseng, defined as genes. We identified 414 genes, including the three published genes. These genes formed a superfamily consisting of 41 gene families, with a substantial diversity in phylogeny and dramatic variation in spatiotemporal expression. Gene ontology (GO) analysis categorized the gene superfamily into 12 functional subcategories distributing among all three primary functional categories, suggesting its functional differentiation. Nevertheless, the majority of its gene members expressed correlatively and tended to form a coexpression network and some of them were commonly regulated in expression across tissues and developmental stages. These results have led to genome-wide identification of genes useful for genome-wide identification of the genes involved in ginsenoside biosynthesis in ginseng and provided the first insight into how a gene superfamily functionally differentiates and acts correlatively in plants.

Functional evolution of biosynthetic enzymes that produce plant volatiles

Plants synthesize volatile compounds to attract pollinators. The volatiles emitted by flowers are often complex mixtures of organic compounds; pollinators are capable of distinctly recognizing different volatile compounds. Plants also produce volatile compounds to protect themselves against herbivores and pathogens. Some of the volatile compounds produced in floral and vegetative tissues are toxic to insects and microbes. To adapt changes in the environment, plants have evolved the ability to synthesize a unique set of volatiles. Intensive studies have identified and characterized the enzymes responsible for the formation of plant volatiles. In particular, many biosynthetic genes have been isolated and their enzymatic functions have been proposed. This review describes how plants have evolved the biosynthetic pathways leading to the formation of green leaf volatiles and phenylpropene volatiles.

Olive Recombinant Hydroperoxide Lyase, an Efficient Biocatalyst for Synthesis of Green Leaf Volatiles

Volatile C6-aldehydes are the main contributors to the characteristic odor of plants known as "green note" and are widely used by the flavor industry. Biotechnological processes were developed to fulfill the high demand in C6-aldehydes in natural flavorants and odorants. Recombinant hydroperoxide lyases (HPLs) constitute an interesting alternative to overcome drawbacks arising from the use of HPL from plant extracts. Thus, olive recombinant 13-HPL was assayed as biocatalysts to produce C6-aldehydes. Firstly, a cDNA encoding for olive HPL of Leccino variety was isolated and cloned in pQE-30 expression vector. In order to improve the enzyme solubility, its chloroplast transit peptide was deleted. Both enzymes (HPL wild type and HPL deleted) were expressed into Escherichia coli strain M15, purified, characterized, and then used for bioconversion of 13-hydroperoxides of linoleic and linolenic acids. Aldehydes produced were extracted, then identified and quantified using gas chromatography and mass spectrometry. Recombinant HPL wild type (HPLwt) allowed producing 5.61 mM of hexanal and 4.39 mM of 3Z-hexenal, corresponding to high conversion yields of 93.5 and 73 %, respectively. Using HPL deleted (HPLdel) instead of HPLwt failed to obtain greater quantities of hexanal or 3Z-hexenal. No undesirable products were formed, and no isomerization of 3Z-hexenal in 2E-hexenal occurred. The olive recombinant HPLwt appears to be a promising efficient biocatalyst for the production of C6-aldehydes.

Molecular Cloning and Characterization of Hydroperoxide Lyase Gene in the Leaves of Tea Plant (Camellia sinensis)

Hydroperoxide lyase (HPL, E.C. 4.1.2.) is the major enzyme in the biosynthesis of natural volatile aldehydes and alcohols in plants, however, little was known about HPL in tea plants (Camellia sinensis). A unique cDNA fragment was isolated by suppressive subtractive hybridization (SSH) from a tea plant subjected to herbivory by tea geometrid Ectropis obliqua. This full length cDNA acquired by RACE was 1476 bp and encoded 491 amino acids. DNA and protein BLAST searches showed high homology to HPL sequences from other plants. The His-tag expression vector pET-32a(+)/CsHPL was constructed and transferred into Escherichia coli Rosetta (DE3). The expression product of recombinant CsHPL in E. coli was about 60 kDa. The enzyme activity of CsHPL was 0.20 μmol·min(-1)·mg(-1). Quantitative RT-PCR analysis indicated CsHPL was strongly up-regulated in tea plants after Ectropis obliqua attack, suggesting that it may be an important candidate for defense against insects in tea plants.

Green Leaf Volatiles in Plant Signaling and Response

Most 'green' plants form green leaf volatiles (GLVs). GLVs are a familiar plant secondary metabolite, but knowledge of their physiological and ecological functions is limited. GLV formation is tightly suppressed when plant tissues are intact, but upon mechanical wounding, herbivore attack, or abiotic stresses, GLVs are formed rapidly, within seconds or minutes. Thus, this may be an important system for defense responses, allowing plants to protect themselves from damage as soon as possible. Because GLV formation in the natural environment is roughly related to the degree of stress in the plant life, sensing the amount of GLVs in the atmosphere might allow plants to recognize their surroundings. Because some plants respond to GLVs, they may communicate with GLVs. GLVs that contain α,β-unsaturated carbonyl groups might activate signaling systems regulated under the redox state of plant cells. Plasma membranes would also be targets of interactions with GLVs. Additionally, the metabolism of GLVs in plant cells after absorption from the atmosphere could also be classified as a plant-plant interaction.

Mechanistic insights into the catalytic reaction of plant allene oxide synthase (pAOS) via QM and QM/MM calculations

QM cluster and QM/MM protein models have been employed to understand aspects of the reaction mechanism of plant allene oxide synthase (pAOS). In this study we have investigated two reaction mechanisms for pAOS. The standard pAOS mechanism was contrasted with an alternative involving an additional active site molecule which has been shown to facilitate proton coupled electron transfer (PCET) in related systems. Firstly, we found that the results from QM/MM protein model are comparable with those from the QM cluster model, presumably due to the large active site used. Furthermore, the results from the QM cluster model show that the Fe(III) and Fe(IV) pathways for the standard mechanism have similar energetic and structural properties, indicating that the reaction mechanism may well proceed via both pathways. However, while the PCET process is facilitated by an additional active site bound water in other related families, in pAOS it is not, suggesting this type of process is not general to all closely related family members.

Isolation, expression, and characterization of a hydroperoxide lyase gene from cucumber

A full-length cDNA coding for hydroperoxide lyase (CsHPL) was isolated from cucumber fruits of No. 26 (Southern China type) and No.14-1 (Northern China type), which differed significantly in fruit flavor. The deduced amino acid sequences of CsHPL from both lines show the same and significant similarity to known plant HPLs and contain typical conserved domains of HPLs. The recombinant CsHPL was confirmed to have 9/13-HPL enzymatic activity. Gene expression levels of CsHPL were measured in different organs, especially in fruits of different development stages of both lines. The HPL activities of fruit were identified basing on the catalytic action of crude enzyme extracts incubating with 13-HPOD (13-hydroperoxy-(9Z,12E)-octadecadienoic acid) and 13-HPOD + 9-HPOD (9-hydroperoxy-(10E,12Z)-octadecadienoic acid), and volatile reaction products were analyzed by GC-MS (gas chromatography-mass spectrometry). CsHPL gene expression in No. 26 fruit occurred earlier than that of total HPL enzyme activity and 13-HPL enzyme activity, and that in No. 14-1 fruit was consistent with total HPL enzyme activity and 9-HPL enzyme activity. 13-HPL enzyme activities decreased significantly and the 9-HPL enzyme activities increased significantly with fruit ripening in both lines, which accounted for the higher content of C6 aldehydes at 0–6 day post-anthesis (dpa) and higher content of C9 aldehydes at 9–12 dpa.

Traumatin- and dinortraumatin-containing galactolipids in Arabidopsis: their formation in tissue-disrupted leaves as counterparts of green leaf volatiles

Green leaf volatiles (GLVs) consisting of six-carbon aldehydes, alcohols, and their esters, are biosynthesized through the action of fatty acid hydroperoxide lyase (HPL), which uses fatty acid hydroperoxides as substrates. GLVs form immediately after disruption of plant leaf tissues by herbivore attacks and mechanical wounding and play a role in defense against attackers that attempt to invade through the wounds. The fates and the physiological significance of the counterparts of the HPL reaction, the 12/10-carbon oxoacids that are formed from 18/16-carbon fatty acid 13-/11-hydroperoxides, respectively, are largely unknown. In this study, we detected monogalactosyl diacylglycerols (MGDGs) containing the 12/10-carbon HPL products in disrupted leaf tissues of Arabidopsis, cabbage, tobacco, tomato, and common bean. They were identified as an MGDG containing 12-oxo-9-hydroxy-(E)-10-dodecenoic acid and 10-oxo-7-hydroxy-(E)-8-decenoic acid and an MGDG containing two 12-oxo-9-hydroxy-(E)-10-dodecenoic acids as their acyl groups. Analyses of Arabidopsis mutants lacking HPL indicated that these MGDGs were formed enzymatically through an active HPL reaction. Thus, our results suggested that in disrupted leaf tissues, MGDG-hydroperoxides were cleaved by HPL to form volatile six-carbon aldehydes and non-volatile 12/10-carbon aldehyde-containing galactolipids. Based on these results, we propose a novel oxylipin pathway that does not require the lipase reaction to form GLVs.

Green leaf volatiles: a plant's multifunctional weapon against herbivores and pathogens

Plants cannot avoid being attacked by an almost infinite number of microorganisms and insects. Consequently, they arm themselves with molecular weapons against their attackers. Plant defense responses are the result of a complex signaling network, in which the hormones jasmonic acid (JA), salicylic acid (SA) and ethylene (ET) are the usual suspects under the magnifying glass when researchers investigate host-pest interactions. However, Green Leaf Volatiles (GLVs), C₆ molecules, which are very quickly produced and/or emitted upon herbivory or pathogen infection by almost every green plant, also play an important role in plant defenses. GLVs are semiochemicals used by insects to find their food or their conspecifics. They have also been reported to be fundamental in indirect defenses and to have a direct effect on pests, but these are not the only roles of GLVs. These volatiles, being probably one of the fastest weapons exploited, are also able to directly elicit or prime plant defense responses. Moreover, GLVs, via crosstalk with phytohormones, mostly JA, can influence the outcome of the plant’s defense response against pathogens. For all these reasons GLVs should be considered as co-protagonists in the play between plants and their attackers.

Heterologous production of pentane in the oleaginous yeast Yarrowia lipolytica

The complete biosynthetic replacement of petroleum transportation fuels requires a metabolic pathway capable of producing short chain n-alkanes. Here, we report and characterize a proof-of-concept pathway that enables microbial production of the C5 n-alkane, pentane. This pathway utilizes a soybean lipoxygenase enzyme to cleave linoleic acid to pentane and a tridecadienoic acid byproduct. Initial expression of the soybean lipoxygenase enzyme within a Yarrowia lipolytica host yielded 1.56 mg/L pentane. Efforts to improve pentane yield by increasing substrate availability and strongly overexpressing the lipoxygenase enzyme successfully increased pentane production three-fold to 4.98 mg/L. This work represents the first-ever microbial production of pentane and demonstrates that short chain n-alkane synthesis is conceivable in model cellular hosts. In this regard, we demonstrate the potential pliability of Y. lipolytica toward the biosynthetic production of value-added molecules from its generous fatty acid reserves.

Unusual cytochrome p450 enzymes and reactions

Cytochrome P450 enzymes primarily catalyze mixed-function oxidation reactions, plus some reductions and rearrangements of oxygenated species, e.g. prostaglandins. Most of these reactions can be rationalized in a paradigm involving Compound I, a high-valent iron-oxygen complex (FeO(3+)), to explain seemingly unusual reactions, including ring couplings, ring expansion and contraction, and fusion of substrates. Most P450s interact with flavoenzymes or iron-sulfur proteins to receive electrons from NAD(P)H. In some cases, P450s are fused to protein partners. Other P450s catalyze non-redox isomerization reactions. A number of permutations on the P450 theme reveal the diversity of cytochrome P450 form and function.

Impacts of diversification of cytochrome P450 on plant metabolism

Cytochrome P450 monooxygenases (P450s) catalyze a wide variety of monooxygenation reactions in primary and secondary metabolism in plants. The share of P450 genes in each plant genome is estimated to be up to 1%. This implies that the diversification of P450 has made a significant contribution to the ability to acquire the emergence of new metabolic pathways during land plant evolution. The P450 families conserved universally in land plants contribute to their chemical defense mechanisms. Several P450s are involved in the biosynthesis and catabolism of plant hormones. Species-specific P450 families are essential for the biosynthetic pathways of phytochemicals such as terpenoids and alkaloids. Genome wide analysis of the gene clusters including P450 genes will provide a clue to defining the metabolic roles of orphan P450s. Metabolic engineering with plant P450s is an important technology for large-scale production of valuable phytochemicals such as medicines.

Differential metabolisms of green leaf volatiles in injured and intact parts of a wounded leaf meet distinct ecophysiological requirements

Almost all terrestrial plants produce green leaf volatiles (GLVs), consisting of six-carbon (C6) aldehydes, alcohols and their esters, after mechanical wounding. C6 aldehydes deter enemies, but C6 alcohols and esters are rather inert. In this study, we address why the ability to produce various GLVs in wounded plant tissues has been conserved in the plant kingdom. The major product in completely disrupted Arabidopsis leaf tissues was (Z)-3-hexenal, while (Z)-3-hexenol and (Z)-3-hexenyl acetate were the main products formed in the intact parts of partially wounded leaves. (13)C-labeled C6 aldehydes placed on the disrupted part of a wounded leaf diffused into neighboring intact tissues and were reduced to C6 alcohols. The reduction of the aldehydes to alcohols was catalyzed by an NADPH-dependent reductase. When NADPH was supplemented to disrupted tissues, C6 aldehydes were reduced to C6 alcohols, indicating that C6 aldehydes accumulated because of insufficient NADPH. When the leaves were exposed to higher doses of C6 aldehydes, however, a substantial fraction of C6 aldehydes persisted in the leaves and damaged them, indicating potential toxicity of C6 aldehydes to the leaf cells. Thus, the production of C6 aldehydes and their differential metabolisms in wounded leaves has dual benefits. In disrupted tissues, C6 aldehydes and their α,β-unsaturated aldehyde derivatives accumulate to deter invaders. In intact cells, the aldehydes are reduced to minimize self-toxicity and allow healthy cells to survive. The metabolism of GLVs is thus efficiently designed to meet ecophysiological requirements of the microenvironments within a wounded leaf.

Diversification of P450 genes during land plant evolution

Plant cytochromes P450 (P450s) catalyze a wide variety of monooxygenation/hydroxylation reactions in primary and secondary metabolism. The number of P450 genes in plant genomes is estimated to be up to 1% of total gene annotations of each plant species. This implies that diversification within P450 gene superfamilies has led to the emergence of new metabolic pathways throughout land plant evolution. The conserved P450 families contribute to chemical defense mechanisms under terrestrial conditions and several are involved in hormone biosynthesis and catabolism. Species-specific P450 families are essential for the biosynthetic pathways of species-specialized metabolites. Future genome-wide analyses of P450 gene clusters and coexpression networks should help both in identifying the functions of many orphan P450s and in understanding the evolution of this versatile group of enzymes.

Plant cytochrome CYP74 family: biochemical features, endocellular localisation, activation mechanism in plant defence and improvements for industrial applications

Not just another P450: Shown here is a model of the overall structure of CYP74C3 with the putative membrane-binding region that is required for enzyme activation. Members of the CYP74 family of cytochrome P450 enzymes are specialised in the metabolism of hydroperoxides and play an important role in oxylipin metabolism, which is one of the main defence mechanisms employed by plants. In order to respond to their rapidly changing environments, plants have evolved complex signalling pathways, which enable tight control over stress responses. Recent work has shed new light on one of these pathways that involves the different classes of plant oxylipins that are produced through the CYP74 pathway. These phytochemicals play an important role in plant defence, and can act as direct antimicrobials or as signalling molecules that inducing the expression of defence genes. The fine-tuning regulation of defence responses, which depends on the precise cross-talk among different signalling pathways, has important consequences for plant fitness and is a new, challenging area of research. In this review we focus on new data relating to the physiological significance of different phyto-oxylipins and related enzymes. Moreover, recent advances in the biotechnological production of oxylipins are also discussed.

Oxylipins: structurally diverse metabolites from fatty acid oxidation

Oxylipins are lipophilic signaling molecules derived from the oxidation of polyunsaturated fatty acids. Initial fatty acid oxidation occurs mainly by the enzymatic or chemical formation of fatty acid hydroperoxides. An array of alternative reactions further converting fatty acid hydroperoxides gives rise to a multitude of oxylipin classes, many with reported signaling functions in plants. Oxylipins include the phytohormone, jasmonic acid, and a number of other molecules including hydroxy-, oxo- or keto-fatty acids or volatile aldehydes that may perform various biological roles as second messengers, messengers in inter-organismic signaling, or even as bactericidal agents. The structural diversity of oxylipins is further increased by esterification of the compounds in plastidial glycolipids, for instance the Arabidopsides, or by conjugation of oxylipins to amino acids or other metabolites. The enzymes involved in oxylipin metabolism are diverse and comprise a multitude of examples with interesting and unusual catalytic properties. In addition, the interplay of different subcellular compartments during oxylipin biosynthesis suggests complex mechanisms of regulation that are not well understood. This review aims at giving an overview of plant oxylipins and the multitude of enzymes responsible for their biosynthesis.

Role of the conserved distal heme asparagine of coral allene oxide synthase (Asn137) and human catalase (Asn148): mutations affect the rate but not the essential chemistry of the enzymatic transformations

A catalase-related allene oxide synthase (cAOS) and true catalases that metabolize hydrogen peroxide have similar structure around the heme. One of the distal heme residues considered to help control catalysis is a highly conserved asparagine. Here we addressed the role of this residue in metabolism of the natural substrate 8R-hydroperoxyeicosatetraenoic acid by cAOS and in H(2)O(2) breakdown by catalase. In cAOS, the mutations N137A, N137Q, N137S, N137D, and N137H drastically reduced the rate of reaction (to 0.8-4% of wild-type), yet the mutants all formed the allene oxide as product. This is remarkable because there are many potential heme-catalyzed transformations of fatty acid hydroperoxides and special enzymatic control must be required. In human catalase, the N148A, N148S, or N148D mutations only reduced rates to approximately 20% of wild-type. The distal heme Asn is not essential in either catalase or cAOS. Its conservation throughout evolution may relate to a role in optimizing catalysis.

The critical role of substrate-protein hydrogen bonding in the control of regioselective hydroxylation in p450cin

Cytochrome P450cin (CYP176A1) is a bacterial P450 isolated from Citrobacter braakii that catalyzes the hydroxylation of cineole to (S)-6beta-hydroxycineole. This initiates the biodegradation of cineole, enabling C. braakii to live on cineole as its sole source of carbon and energy. P450cin lacks the almost universally conserved threonine residue believed to be involved in dioxygen activation and instead contains an asparagine at this position (Asn-242). To investigate the role of Asn-242 in P450cin catalysis, it was converted to alanine, and the resultant mutant was characterized. The characteristic CO-bound spectrum and spectrally determined K(D) for substrate binding were unchanged in the mutant. The x-ray crystal structures of the substrate-free and -bound N242A mutant were determined and show that the only significant change is in a reorientation of the substrate such that (R)-6alpha-hydroxycineole should be a major product. Molecular dynamics simulations of both wild type and mutant are consistent with the change in regio- and stereoselectivity predicted from the crystal structure. The mutation has only a modest effect on enzyme activity and on the diversion of the NADPH-reducing equivalent toward unproductive peroxide formation. Product profile analysis shows that (R)-6alpha-hydroxycineole is the main product, which is consistent with the crystal structure. These results demonstrate that Asn-242 is not a functional replacement for the conserved threonine in other P450s but, rather, is critical in controlling regioselective substrate oxidation.

Mung bean lipoxygenase in the production of a C6-aldehyde. Natural green-note flavor generation via biotransformation

Mung bean was investigated as a novel source of lipoxygenase in the natural production of the green-note aroma compound hexanal. Lipoxygenase extracted from mung bean catalyzed the oxidative reaction of linoleic acid, after which the intermediate hydroperoxide compound was split via green bell pepper hydroperoxide lyase to produce hexanal. In comparison to soybean lipoxygenase, mung bean lipoxygenase was found to be a good substitute as it produced 15.4 mM (76% yield) hexanal while soybean gave 60% yield. The mung bean pH profile comprised a wide peak (optimum pH 6.5) representing lipoxygenase-2 and lipoxygenase-3 isozymes, whereas two narrower peaks representing lipoxygenase-1 and lipoxygenase-2/3 isozymes were observed for soybean (optimum pH 10). Extraction at pH 4.5 was preferred, at which specific lipoxygenase activity was also the highest.

α‐Tocopherol: A Multifaceted Molecule in Plants

alpha-Tocopherol, which belongs to the vitamin E group of compounds, is a lipophilic antioxidant that has a number of functions in plants. Synthesized from homogentisic acid and isopentenyl diphosphate in the chloroplast envelope, alpha-tocopherol is essential to maintain the integrity of photosynthetic membranes and plays a major role in photo- and antioxidant protection. alpha-Tocopherol scavenges lipid peroxy radicals, thereby preventing the propagation of lipid peroxidation, and protects lipids and other membrane components by physically quenching and reacting chemically with singlet oxygen. Moreover, given that alpha-tocopherol increases membrane rigidity, its concentration, together with that of the other membrane components, may be regulated to afford adequate fluidity for membrane function. Furthermore, recent studies on tocopherol-deficient plants indicate that alpha-tocopherol may affect cellular signaling in plants. Evidence thus far indicates that the effects of this compound in plant cellular signaling may be linked to the control of redox homeostasis. alpha-Tocopherol may influence cellular signaling by controlling the propagation of lipid peroxidation in chloroplasts, therefore modulating the formation of oxylipins such as the phytohormone jasmonic acid.

Characterization of a divinyl ether biosynthetic pathway specifically associated with pathogenesis in tobacco

In tobacco (Nicotiana tabacum), an elicitor- and pathogen-induced 9-lipoxygenase (LOX) gene, NtLOX1, is essential for full resistance to pathogens, notably to an incompatible race of Phytophthora parasitica var. nicotianae (Ppn race 0). In this work, we aimed to identify those oxylipins induced during attempted infection by Ppn race 0 and down-regulated in NtLOX1 antisense plants. Here we show that colneleic and colnelenic acids, which significantly inhibit germination of Ppn zoospores, are produced in roots of wild-type plants inoculated with Ppn, but are down-regulated in NtLOX1 antisense plants. A search for a tobacco gene encoding the enzyme involved in the formation of these divinyl ether (DVE) fatty acids resulted in the cloning and characterization of a DVE synthase (DES) clone (NtDES1). NtDES1 is a 9-DES, specifically converting fatty acid 9-hydroperoxides into DVE fatty acids. NtDES1 has the potential to act in combination with NtLOX1 because, in the presence of the two enzymes, linoleic and linolenic acids were converted in vitro into colneleic and colnelenic acids, respectively. In addition, the pattern of NtDES1 gene expression was quite similar to that of NtLOX1. Their transcripts were undetected in healthy tissues from different plant organs, and accumulated locally and transiently after elicitation and fungal infection, but not after wounding. Visualization of NtDES1-yellow fluorescent protein and NtLOX1-cyan fluorescent protein fusion proteins in tobacco leaves indicated that both localize in the cytosol and are excluded from plastids, consistent with the presumed location of the 9-LOX pathway in plants and the lack of transit peptides for NtLOX1 and NtDES1, respectively. Our data suggest that, in tobacco, NtDES1 and NtLOX1 act together and form DVEs in response to pathogen attack and that this class of oxylipins modulates in vivo the outcome of the tobacco-Ppn race 0 interaction.

Changing green leaf volatile biosynthesis in plants: an approach for improving plant resistance against both herbivores and pathogens

Green leaf volatiles (GLVs) are commonly emitted by green plants, and their production is drastically enhanced when they are under biotic stress. To clarify the ecological function of naturally emitted GLVs, we studied the response of Arabidopsis, whose GLV biosynthesis had been modified, when subjected to herbivory or a pathogenic infection. There was a significant increase in GLV production after herbivory by cabbage white butterfly larvae and pathogen (gray mold) infection in hydroperoxide lyase (HPL) sense Arabidopsis compared with WT controls. The HPL sense modification resulted in the plant being more attractive to the parasitic wasp Cotesia glomerata, leading to higher mortality of the herbivores. The HPL sense modification also resulted in greater inhibition of growth of the fungus. By contrast, HPL antisense Arabidopsis produced fewer GLVs, attracted fewer parasitoids, and was more susceptible to the pathogens than the WT control. These data show that (i) one of the ecological functions of GLV biosynthesis related to resistance against both herbivores and pathogens, and (ii) the genetic modification of GLV biosynthesis could be a unique approach for improving plant resistance against such biotic stresses.

Hydroperoxide lyases (CYP74C and CYP74B) catalyze the homolytic isomerization of fatty acid hydroperoxides into hemiacetals

The conversion of linoleic acid 9-hydroperoxide (9-HPOD) by recombinant melon (Cucumis melo L.) hydroperoxide lyase (HPL, CYP74C subfamily) was studied. Short (5 s-1 min) incubations at 0 degrees C followed by rapid extraction and trimethylsilylation made it possible to trap a new unstable (t(1/2) <30 s) product, i.e. the hemiacetal (1'E,3'Z)-9-hydroxy-9-(1',3'-nonadienyloxy)-nonanoic acid. Identification was performed by GC-MS analysis and substantiated by the formation of trimethylsilyl 9-trimethylsilyloxy-9-nonyloxy-nonanoate upon catalytic hydrogenation and by (2)H-labelling experiments. Both (18)O atoms of [(18)O(2)-hydroperoxy]9-HPOD were incorporated into the hemiacetal. Along with the hemiacetal, three chain-cleavage products, i.e. the enol (1E,3Z)-nonadienol and the hydrates of 3(Z)-nonenal and 9-oxononanoic acid, were trapped as their trimethylsilyl derivatives. The kinetics of (18)O incorporation from [(18)O(2)]9-HPOD provided strong evidence that the cleavage products originated in the hemiacetal. Linolenic and linoleic acid 13-hydroperoxides served as substrates for recombinant HPLs of melon, alfalfa (Medicago sativa) and guava (Psidium guajava), and in each case hemiacetals and enols were detectable by the trapping technique. The data obtained demonstrated that CYP74C and CYP74B HPLs act as isomerases performing a homolytic rearrangement of fatty acid hydroperoxides into short-lived hemiacetals which upon decomposition produce 3(Z)-nonenal, 3(Z)-hexenal and other short chain aldehydes.

Rice HYDROPEROXIDE LYASES with unique expression patterns generate distinct aldehyde signatures in Arabidopsis

HYDROPEROXIDE LYASE (HPL) genes encode enzymes that catalyze the cleavage of fatty acid hydroperoxides into aldehydes and oxoacids. There are three HPLs in rice (Oryza sativa), designated OsHPL1 through OsHPL3. To explore the possibility of differential functional activities among these genes, we have examined their expression patterns and biochemical properties of their encoded products. Transcript analysis indicates that these genes have distinct patterns and levels of expression. OsHPL1 is ubiquitously expressed, OsHPL2 is expressed in the leaves and leaf sheaths, whereas OsHPL3 is wound inducible and expressed exclusively in leaves. OsHPLs also differ in their substrate preference as determined by in vitro enzyme assays using 9-/13-hydroperoxy linolenic and 9-/13-hydroperoxy linoleic acids as substrates. OsHPL1 and OsHPL2 metabolize 9-/13-hydroperoxides, whereas OsHPL3 metabolizes 13-hydroperoxy linolenic acid exclusively. Sequence alignments of the HPL enzymes have identified signature residues potentially responsible for the substrate specificity/preference of these enzymes. All three OsHPLs are chloroplast localized as determined by chloroplast import assays and green fluorescent protein (GFP) fusion studies. Aldehyde measurements in transgenic Arabidopsis (Arabidopsis thaliana) plants overexpressing individual OsHPL-GFP fusions indicate that all rice HPLs are functional in a heterologous system, and each of them generates a distinct signature of the metabolites. Interestingly, these aldehydes were only detectable in leaves, but not in roots, despite similar levels of OsHPL-GFP proteins in both tissues. Similarly, there were undetectable levels of aldehydes in rice roots, in spite of the presence of OsHPL1 transcripts. Together, these data suggest that additional tissue-specific mechanism(s) beyond transcript and HPL enzyme abundance, regulate the levels of HPL-derived metabolites.

Watermelon (Citrullus lanatus) hydroperoxide lyase greatly increases C6 aldehyde formation in transgenic leaves

Fatty acid hydroperoxide lyase (HL) is the key enzyme for the production of the "green note"compounds, leaf aldehyde [(2E)-hexenal] and leaf alcohol [(3Z)-hexenol], in plant tissues. A cDNA encoding HL was cloned from leaves of watermelon (Citrullus lanatus) and expressed in Nicotiana tabacum. The enzyme is 3 times more active with 13-hydroperoxylinolenic acid than with 13-hydroperoxylinoleic acid. The activity against 9-hydroperoxides of polyunsaturated fatty acids is minimal. Enzyme activity of the watermelon HL in the transgenic leaves was approximately 50 times higher than endogenous HL activity in the wild-type N. tabacum plants. When compared with Arabidopsis HL also expressed in N. tabacum, the highest HL activity is 10 times higher in watermelon HL overexpressing leaves than in Arabidopsis HL overexpressers.