Pterocarpan synthase (PTS) structures suggest a common quinone methide-stabilizing function in dirigent proteins and proteins with dirigent-like domains.

The biochemical activities of dirigent proteins (DPs) give rise to distinct complex classes of plant phenolics. DPs apparently began to emerge during the aquatic-to-land transition, with phylogenetic analyses revealing the presence of numerous DP subfamilies in the plant kingdom. The vast majority (>95%) of DPs in these large multigene families still await discovery of their biochemical functions. Here, we elucidated the 3D structures of two pterocarpan-forming proteins with dirigent-like domains. Both proteins stereospecifically convert distinct diastereomeric chiral isoflavonoid precursors to the chiral pterocarpans, (-)- and (+)-medicarpin, respectively. Their 3D structures enabled comparisons with stereoselective lignan- and aromatic terpenoid-forming DP orthologs. Each protein provides entry into diverse plant natural products classes, and our experiments suggest a common biochemical mechanism in binding and stabilizing distinct plant phenol-derived mono- and bis-quinone methide intermediates during different C-C and C-O bond-forming processes. These observations provide key insights into both their appearance and functional diversification of DPs during land plant evolution/adaptation. The proposed biochemical mechanisms based on our findings provide important clues to how additional physiological roles for DPs and proteins harboring dirigent-like domains can now be rationally and systematically identified.

De novo sequencing and native mass spectrometry revealed hetero-association of dirigent protein homologs and potential interacting proteins in Forsythia × intermedia.

The discovery of dirigent proteins (DPs) and their functions in plant phenol biochemistry was made over two decades ago with Forsythia × intermedia. Stereo-selective, DP-guided, monolignol-derived radical coupling in vitro was then reported to afford the optically active lignan, (+)-pinoresinol from coniferyl alcohol, provided one-electron oxidase/oxidant capacity was present. It later became evident that DPs have several distinct sub-families, presumably with different functions. Some known DPs require other essential enzymes/proteins (e.g. oxidases) for their functions. However, the lack of a fully sequenced genome for Forsythia × intermedia made it difficult to profile other components co-purified with the (+)-pinoresinol forming DP. Herein, we used an integrated bottom-up, top-down, and native mass spectrometry (MS) approach to de novo sequence the extracted proteins via adaptation of our initial report of DP solubilization and purification. Using publicly available transcriptome and genomic data from closely related species, we identified 14 proteins that were putatively associated with either DP function or the cell wall. Although their co-occurrence after extraction and chromatographic separation is suggestive for potential protein-protein interactions, none were found to form stable protein complexes with DPs in native MS under the specific experimental conditions we have explored. Interestingly, two new DP homologs were found and they formed hetero-trimers. Molecular dynamics simulations suggested that similar hetero-trimers were possible between Arabidopsis DP homologs with comparable sequence similarities. Nevertheless, our integrated mass spectrometry method development helped prepare for future investigations directed to the discovery of novel proteins and protein-protein interactions. These advantages can be highly beneficial for plant and microbial research where fully sequenced genomes may not be readily available.

Allyl/propenyl phenol synthases from the creosote bush and engineering production of specialty/commodity chemicals, eugenol/isoeugenol, in Escherichia coli.

The creosote bush (Larrea tridentata) harbors members of the monolignol acyltransferase, allylphenol synthase, and propenylphenol synthase gene families, whose products together are able to catalyze distinct regiospecific conversions of various monolignols into their corresponding allyl- and propenyl-phenols, respectively. In this study, co-expression of a monolignol acyltransferase with either substrate versatile allylphenol or propenylphenol synthases in Escherichia coli established that various monolignol substrates were efficiently converted into their corresponding allyl/propenyl phenols, as well as providing proof of concept for efficacious conversion in a bacterial platform. This capability thus potentially provides an alternate source to these important plant phytochemicals, whether for flavor/fragrance and fine chemicals, or ultimately as commodities, e.g., for renewable energy or other intermediate chemical purposes. Previous reports had indicated that specific and highly conserved amino acid residues 84 (Phe or Val) and 87 (Ile or Tyr) of two highly homologous allyl/propenyl phenol synthases (circa 96% identity) from a Clarkia species mainly dictate their distinct regiospecific catalyzed conversions to afford either allyl- or propenyl-phenols, respectively. However, several other allyl/propenyl phenol synthase homologs isolated by us have established that the two corresponding amino acid 84 and 87 residues are not, in fact, conserved.

Assessment of a putative proton relay in Arabidopsis cinnamyl alcohol dehydrogenase catalysis.

Extended proton relay systems have been proposed for various alcohol dehydrogenases, including the Arabidopsis thaliana cinnamyl alcohol dehydrogenases (AtCADs). Following a previous structural biology investigation of AtCAD5, the potential roles of three amino acid residues in a putative proton relay system, namely Thr49, His52 and Asp57, in AtCAD5, were investigated herein. Using site-directed mutagenesis, kinetic and isothermal titration calorimetry (ITC) analyses, it was established that the Thr49 residue was essential for overall catalytic conversion, whereas His52 and Asp57 residues were not. Mutation of the Thr49 residue to Ala resulted in near abolition of catalysis, with thermodynamic data indicating a negative enthalpic change (ΔH), as well as a significant decrease in binding affinity with NADPH, in contrast to wild type AtCAD5. Mutation of His52 and Asp57 residues by Ala did not significantly change either catalytic efficiency or thermodynamic parameters. Therefore, only the Thr49 residue is demonstrably essential for catalytic function. ITC analyses also suggested that for AtCAD5 catalysis, NADPH was bound first followed by p-coumaryl aldehyde.

Pinus taeda phenylpropenal double-bond reductase: purification, cDNA cloning, heterologous expression in Escherichia coli, and subcellular localization in P. taeda.

A phenylpropenal double-bond reductase (PPDBR) was obtained from cell suspension cultures of loblolly pine (Pinus taeda L.). Following trypsin digestion and amino acid sequencing, the cDNA encoding this protein was subsequently cloned, with the functional recombinant protein expressed in Escherichia coli and characterized. PPDBR readily converted both dehydrodiconiferyl and coniferyl aldehydes into dihydrodehydrodiconiferyl and dihydroconiferyl aldehydes, when NADPH was added as cofactor. However, it was unable to reduce directly either the double bond of dehydrodiconiferyl or coniferyl alcohols in the presence of NADPH. During this reductive step, the corresponding 4-proR hydrogen was abstracted from [4R-3H]-NADPH during hydride transfer. This is thus the first report of a double-bond reductase involved in phenylpropanoid metabolism, and which is presumed to be involved in plant defense. In situ mRNA hybridization indicated that the PPDBR transcripts in P. taeda stem sections were localized to the vascular cambium, as well as to radial and axial parenchyma cell types. Additionally, using P. taeda cell suspension culture crude protein extracts, dehydrodiconiferyl and coniferyl alcohols could be dehydrogenated to afford dehydrodiconiferyl and coniferyl aldehydes. Furthermore, these same extracts were able to convert dihydrodehydrodiconiferyl and dihydroconiferyl aldehydes into the corresponding alcohols. Taken together, these results indicate that in the crude extracts dehydrodiconiferyl and coniferyl alcohols can be converted to dihydrodehydrodiconiferyl and dihydroconiferyl alcohols through a three-step process, i.e. by initial phenylpropenol oxidation, then sequential PPDBR and phenylpropanal reductions, respectively.

Reassessment of effects on lignification and vascular development in the irx4 Arabidopsis mutant.

The Arabidopsis thaliana irregular xylem4 (irx4) cinnamoyl-CoA reductase 1 (CCR1) mutant was reassessed for its purported exclusive rate-limiting or key effects on lignification. Analyses of gross growth characteristics and stem cross-section anatomy, from seedling emergence to senescence, revealed that stunted irx4 mutant lines were developmentally delayed, which in turn indirectly but predictably led to modest reductions (ca. 10-15%) in overall lignin amounts. Such developmental changes are not generally observed in suppression of other monolignol pathway forming enzymes (e.g., 4-coumarate CoA ligase) even when accompanied by significant reductions in lignin amounts. With the greatly arrested development of the irx4 mutant, formation of the lignin-derived syringyl moieties was also predictably delayed (by about 1-2 weeks), although at maturation the final guaiacyl:syringyl ratios were essentially identical to wild-type. No evidence was obtained for so-called abnormal lignin precursors being incorporated into the lignin, as shown by solid-state 13C NMR spectroscopic analysis in contrast to a claim to the contrary [Jones, L., Ennos, A.R., Turner, S.R., 2001. Cloning and characterization of irregular xylem4 (irx4): a severely lignin-deficient mutant of Arabidopsis. Plant J. 26, 205-216]. A previous claim of an "abnormal" lignin present in stunted CCR downregulated tobacco was also not substantiated, with only trace differences being noted in the presumed cell-wall constituent levels. More importantly, a linear correlation between total lignin amounts and lignin-derived fragmentation products was observed at all stages of Arabidopsis growth/development in both wild-type and irx4 mutant lines, regardless of lignin content, i.e., in harmony with an exquisitely controlled and predictable macromolecular assembly process. Recombinant CCR1 displayed fairly broad substrate versatility for all phenylpropanoid CoA substrates, with both feruloyl and 5-hydroxyferuloyl CoA being the best substrates. Taken together, these data indicate that other CCR isoforms are apparently capable of generating monolignol-derived lignified elements in irx4 when CCR1 is impaired, i.e., indicative of a functionally redundant CCR metabolic network operative in Arabidopsis. Other dwarfed phenotypes have also been observed following downregulation/disruption of unrelated metabolic processes but which also involve CoA ester metabolism, i.e., with hydroxymethylglutaryl CoA reductases in Arabidopsis and a bacterial enoyl CoA hydratase/lyase overexpressed in tobacco. Although the reasons for dwarfing in each case are unknown, a common mechanism for the various pleiotropic effects is proposed through perturbation of CoASH pool levels. Finally, this study demonstrates the need for progressive analyses over the lifespan of an organism, rather than at a single time point which cannot reveal the progressive developmental changes occurring.

Characterization in vitro and in vivo of the putative multigene 4-coumarate:CoA ligase network in Arabidopsis: syringyl lignin and sinapate/sinapyl alcohol derivative formation.

A recent in silico analysis revealed that the Arabidopsis genome has 14 genes annotated as putative 4-coumarate:CoA ligase isoforms or homologues. Of these, 11 were selected for detailed functional analysis in vitro, using all known possible phenylpropanoid pathway intermediates (p-coumaric, caffeic, ferulic, 5-hydroxyferulic and sinapic acids), as well as cinnamic acid. Of the 11 recombinant proteins so obtained, four were catalytically active in vitro, with fairly broad substrate specificities, confirming that the 4CL gene family in Arabidopsis has only four members. This finding is in agreement with our previous phylogenetic analyses, and again illustrates the need for comprehensive characterization of all putative 4CLs, rather than piecemeal analysis of selected gene members. All 11 proteins were expressed with a C-terminal His6-tag and functionally characterized, with one, At4CL1, expressed in native form for kinetic property comparisons. Of the 11 putative His6-tagged 4CLs, isoform At4CL1 best utilized p-coumaric, caffeic, ferulic and 5-hydroxyferulic acids as substrates, whereas At4CL2 readily transformed p-coumaric and caffeic acids into the corresponding CoA esters, while ferulic and 5-hydroxyferulic acids were converted quite poorly. At4CL3 also displayed broad substrate specificity efficiently converting p-coumaric, caffeic and ferulic acids into their CoA esters, whereas 5-hydroxyferulic acid was not as effectively utilized. By contrast, while At4CL5 is the only isoform capable of ligating sinapic acid, the two preferred substrates were 5-hydroxyferulic and caffeic acids. Indeed, both At4CL1 and At4CL5 most effectively utilized 5-hydroxyferulic acid with kenz approximately 10-fold higher than that for At4CL2 and At4CL3. The remaining seven 4CL-like homologues had no measurable catalytic activity (at approximately 100 microg protein concentrations), again bringing into sharp focus both the advantages to, and the limitations of, current database annotations, and the need to unambiguously demonstrate true enzyme function. Lastly, although At4CL5 is able to convert both 5-hydroxyferulic and sinapic acids into the corresponding CoA esters, the physiological significance of the latter observation in vitro was in question, i.e. particularly since other 4CL isoforms can effectively convert 5-hydroxyferulic acid into 5-hydroxyferuloyl CoA. Hence, homozygous lines containing T-DNA or enhancer trap inserts (knockouts) for 4cl5 were selected by screening, with Arabidopsis stem sections from each mutant line subjected to detailed analyses for both lignin monomeric compositions and contents, and sinapate/sinapyl alcohol derivative formation, at different stages of growth and development until maturation. The data so obtained revealed that this "knockout" had no significant effect on either lignin content or monomeric composition, or on the accumulation of sinapate/sinapyl alcohol derivatives. The results from the present study indicate that formation of syringyl lignins and sinapate/sinapyl alcohol derivatives result primarily from methylation of 5-hydroxyferuloyl CoA or derivatives thereof rather than sinapic acid ligation. That is, no specific physiological role for At4CL5 in direct sinapic acid CoA ligation could be identified. How the putative overlapping 4CL metabolic networks are in fact organized in planta at various stages of growth and development will be the subject of future inquiry.

Transgenic hybrid poplar for sustainable and scalable production of the commodity/specialty chemical, 2-phenylethanol.

Fast growing hybrid poplar offers the means for sustainable production of specialty and commodity chemicals, in addition to rapid biomass production for lignocellulosic deconstruction. Herein we describe transformation of fast-growing transgenic hybrid poplar lines to produce 2-phenylethanol, this being an important fragrance, flavor, aroma, and commodity chemical. It is also readily converted into styrene or ethyl benzene, the latter being an important commodity aviation fuel component. Introducing this biochemical pathway into hybrid poplars marks the beginnings of developing a platform for a sustainable chemical delivery system to afford this and other valuable specialty/commodity chemicals at the scale and cost needed. These modified plant lines mainly sequester 2-phenylethanol via carbohydrate and other covalently linked derivatives, thereby providing an additional advantage of effective storage until needed. The future potential of this technology is discussed. MALDI metabolite tissue imaging also established localization of these metabolites in the leaf vasculature.

The arogenate dehydratase gene family: towards understanding differential regulation of carbon flux through phenylalanine into primary versus secondary metabolic pathways.

Phe is formed from arogenate in planta through the action of arogenate dehydratase (ADT), and there are six ADT isoenzymes in the "model" vascular plant species Arabidopsis thaliana. This raised the possibility that specific ADTs may be differentially regulated so as to control Phe biosynthesis for protein synthesis vs its much more massive deployment for phenylpropanoid metabolism. In our previous reverse genetics study using 25 single/multiple ADT knockout (KO) lines, a subset of these knockouts was differentially reduced in their lignin contents. In the current investigation, it was hypothesized that Phe pool sizes might correlate well with reduction in lignin contents in the affected KO lines. The free amino acid contents of these KO lines were thus comprehensively analyzed in stem, leaf and root tissues, over a growth/developmental time course from 3 to 8 weeks until senescence. The data obtained were then compared to, and contrasted with, the differential extent of lignin deposition occurring in the various lines. Relative changes in pool sizes were also analyzed by performing a pairwise confirmatory factor analysis for Phe:Tyr, Phe:Trp and Tyr:Trp, following determination of the deviation from the mean for Phe, Tyr and Trp in each plant line. It was found that the Phe pool sizes measured were differentially reduced only in lignin-deficient lines, and in tissues and at time points where lignin biosynthesis was constitutively highly active (in wild type lines) under the growth conditions employed. In contrast, this trend was not evident across all ADT KO lines, possibly due to maintenance of Phe pools by non-targeted isoenzymes, or by feedback mechanisms known to be in place.

Phenylalanine biosynthesis in Arabidopsis thaliana. Identification and characterization of arogenate dehydratases.

There is much uncertainty as to whether plants use arogenate, phenylpyruvate, or both as obligatory intermediates in Phe biosynthesis, an essential dietary amino acid for humans. This is because both prephenate and arogenate have been reported to undergo decarboxylative dehydration in plants via the action of either arogenate (ADT) or prephenate (PDT) dehydratases; however, neither enzyme(s) nor encoding gene(s) have been isolated and/or functionally characterized. An in silico data mining approach was thus undertaken to attempt to identify the dehydratase(s) involved in Phe formation in Arabidopsis, based on sequence similarity of PDT-like and ACT-like domains in bacteria. This data mining approach suggested that there are six PDT-like homologues in Arabidopsis, whose phylogenetic analyses separated them into three distinct subgroups. All six genes were cloned and subsequently established to be expressed in all tissues examined. Each was then expressed as a Nus fusion recombinant protein in Escherichia coli, with their substrate specificities measured in vitro. Three of the resulting recombinant proteins, encoded by ADT1 (At1g11790), ADT2 (At3g07630), and ADT6 (At1g08250), more efficiently utilized arogenate than prephenate, whereas the remaining three, ADT3 (At2g27820), ADT4 (At3g44720), and ADT5 (At5g22630) essentially only employed arogenate. ADT1, ADT2, and ADT6 had k(cat)/Km values of 1050, 7650, and 1560 M(-1) S(-1) for arogenate versus 38, 240, and 16 M(-1) S(-1) for prephenate, respectively. By contrast, the remaining three, ADT3, ADT4, and ADT5, had k(cat)/Km values of 1140, 490, and 620 M(-1) S(-1), with prephenate not serving as a substrate unless excess recombinant protein (>150 microg/assay) was used. All six genes, and their corresponding proteins, are thus provisionally classified as arogenate dehydratases and designated ADT1-ADT6.

Secoisolariciresinol dehydrogenase: mode of catalysis and stereospecificity of hydride transfer in Podophyllum peltatum.

Secoisolariciresinol dehydrogenase (SDH) catalyzes the NAD+ dependent enantiospecific conversion of secoisolariciresinol into matairesinol. In Podophyllum species, (-)-matairesinol is metabolized into the antiviral compound, podophyllotoxin, which can be semi-synthetically converted into the anticancer agents, etoposide, teniposide and Etopophos. Matairesinol is also a precursor of the cancer-preventative "mammalian" lignan, enterolactone, formed in the gut following ingestion of, for example, various high fiber dietary foods, as well as being an intermediate to numerous defense compounds in vascular plants. This study investigated the mode of enantiospecific Podophyllum SDH catalysis, the order of binding, and the stereospecificity of hydride abstraction/transfer from secoisolariciresinol to NAD+. SDH contains a highly conserved catalytic triad (Ser153, Tyr167 and Lys171), whose activity was abolished with site-directed mutagenesis of Tyr167Ala and Lys171Ala, whereas mutagenesis of Ser153Ala only resulted in a much reduced catalytic activity. Isothermal titration calorimetry measurements indicated that NAD+ binds first followed by the substrate, (-)-secoisolariciresinol. Additionally, for hydride transfer, the incoming hydride abstracted from the substrate takes up the pro-S position in the NADH formed. Taken together, a catalytic mechanism for the overall enantiospecific conversion of (-)-secoisolariciresinol into (-)-matairesinol is proposed.

Mechanistic and structural studies of apoform, binary, and ternary complexes of the Arabidopsis alkenal double bond reductase At5g16970.

In this study, we determined the crystal structures of the apoform, binary, and ternary complexes of the Arabidopsis alkenal double bond reductase encoded by At5g16970. This protein, one of 11 homologues in Arabidopsis thaliana, is most closely related to the Pinus taeda phenylpropenal double bond reductase, involved in, for example, heartwood formation. Both enzymes also have essential roles in plant defense, and can function by catalyzing the reduction of the 7-8-double bond of phenylpropanal substrates, such as p-coumaryl and coniferyl aldehydes in vitro. At5g16970 is also capable of reducing toxic substrates with the same alkenal functionality, such as 4-hydroxy-(2E)-nonenal. The overall fold of At5g16970 is similar to that of the zinc-independent medium chain dehydrogenase/reductase superfamily, the members of which have two domains and are dimeric in nature, i.e. in contrast to their original classification as being zinc-containing oxidoreductases. As provisionally anticipated from the kinetic data, the shape of the binding pocket can readily accommodate p-coumaryl aldehyde, coniferyl aldehyde, 4-hydroxy-(2E)-nonenal, and 2-alkenals. However, the enzyme kinetic data among these potential substrates differ, favoring p-coumaryl aldehyde. Tyr-260 is provisionally proposed to function as a general acid/base for hydride transfer. A catalytic mechanism for this reduction, and its applicability to related important detoxification mammalian proteins, is also proposed.

Functional reclassification of the putative cinnamyl alcohol dehydrogenase multigene family in Arabidopsis.

Of 17 genes annotated in the Arabidopsis genome database as cinnamyl alcohol dehydrogenase (CAD) homologues, an in silico analysis revealed that 8 genes were misannotated. Of the remaining nine, six were catalytically competent for NADPH-dependent reduction of p-coumaryl, caffeyl, coniferyl, 5-hydroxyconiferyl, and sinapyl aldehydes, whereas three displayed very low activity and only at very high substrate concentrations. Of the nine putative CADs, two (AtCAD5 and AtCAD4) had the highest activity and homology (approximately 83% similarity) relative to bona fide CADs from other species. AtCAD5 used all five substrates effectively, whereas AtCAD4 (of lower overall catalytic capacity) poorly used sinapyl aldehyde; the corresponding 270-fold decrease in k(enz) resulted from higher K(m) and lower k(cat) values, respectively. No CAD homologue displayed a specific requirement for sinapyl aldehyde, which was in direct contrast with unfounded claims for a so-called sinapyl alcohol dehydrogenase in angiosperms. AtCAD2, 3, as well as AtCAD7 and 8 (highest homology to sinapyl alcohol dehydrogenase) were catalytically less active overall by at least an order of magnitude, due to increased K(m) and lower k(cat) values. Accordingly, alternative and/or bifunctional metabolic roles of these proteins in plant defense cannot be ruled out. Comprehensive analyses of lignified tissues of various Arabidopsis knockout mutants (for AtCAD5, 6, and 9) at different stages of growth/development indicated the presence of functionally redundant CAD metabolic networks. Moreover, disruption of AtCAD5 expression had only a small effect on either overall lignin amounts deposited, or on syringyl-guaiacyl compositions, despite being the most catalytically active form in vitro.

Crystal structures of pinoresinol-lariciresinol and phenylcoumaran benzylic ether reductases and their relationship to isoflavone reductases.

Despite the importance of plant lignans and isoflavonoids in human health protection (e.g. for both treatment and prevention of onset of various cancers) as well as in plant biology (e.g. in defense functions and in heartwood development), systematic studies on the enzymes involved in their biosynthesis have only recently begun. In this investigation, three NADPH-dependent aromatic alcohol reductases were comprehensively studied, namely pinoresinol-lariciresinol reductase (PLR), phenylcoumaran benzylic ether reductase (PCBER), and isoflavone reductase (IFR), which are involved in central steps to the various important bioactive lignans and isoflavonoids. Of particular interest was in determining how differing regio- and enantiospecificities are achieved with the different enzymes, despite each apparently going through similar enone intermediates. Initially, the three-dimensional x-ray crystal structures of both PLR_Tp1 and PCBER_Pt1 were solved and refined to 2.5 and 2.2 A resolutions, respectively. Not only do they share high gene sequence similarity, but their structures are similar, having a continuous alpha/beta NADPH-binding domain and a smaller substrate-binding domain. IFR (whose crystal structure is not yet obtained) was also compared (modeled) with PLR and PCBER and was deduced to have the same overall basic structure. The basis for the distinct enantio-specific and regio-specific reactions of PCBER, PLR, and IFR, as well as the reaction mechanism and participating residues involved (as identified by site-directed mutagenesis), are discussed.