Directed evolution to generate cycloartenol synthase mutants that produce lanosterol

Structure Elucidation and Biosynthesis of Nannosterols A and B, Myxobacterial Sterols from Nannocystis sp. MNa10993

Myxobacteria represent an underinvestigated source of chemically diverse and biologically active secondary metabolites. Here, we report the discovery, isolation, structure elucidation, and biological evaluation of two new bacterial sterols, termed nannosterols A and B (1, 2), from the terrestrial myxobacterium Nannocystis sp. (MNa10993). Nannosterols feature a cholestanol core with numerous modifications including a secondary alcohol at position C-15, a terminal vicinal diol side chain at C-24-C-25 (1, 2), and a hydroxy group at the angular methyl group at C-18 (2), which is unprecedented for bacterial sterols. Another rare chemical feature of bacterial triterpenoids is a ketone group at position C-7, which is also displayed by 1 and 2. The combined exploration based on myxobacterial high-resolution secondary metabolome data and genomic in silico investigations exposed the nannosterols as frequently produced sterols within the myxobacterial suborder of Nannocystineae. The discovery of the nannosterols provides insights into the biosynthesis of these new myxobacterial sterols, with implications in understanding the evolution of sterol production by prokaryotes.

Immunomodulatory Molecular Mechanisms of Luffa cylindrica for Downy Mildews Resistance Induced by Growth-Promoting Endophytic Fungi

Downy mildew (DM), caused by P. cubensis, is harmful to cucurbits including luffa, with increased shortcomings associated with its control through cultural practices, chemical fungicides, and resistant cultivars; there is a prompt need for an effective, eco-friendly, economical, and safe biocontrol approach. Current research is therefore dealt with the biocontrol of luffa DM1 through the endophytic fungi (EF) consortium. Results revealed that T. harzianum (ThM9) and T. virens (TvA1) showed pathogen-dependent inducible metabolic production of squalene and gliotoxins by higher gene expression induction of SQS1/ERG9 (squalene synthase) and GliP (non-ribosomal peptide synthetase). Gene expression of lytic enzymes of EF was also induced with subsequently higher enzyme activities upon confrontation with P. cubensis. EF-inoculated luffa seeds showed efficient germination with enhanced growth potential and vigor of seedlings. EF-inoculated plants showed an increased level of growth-promoting hormone GA with higher gene expression of GA2OX8. EF-pre-inoculated seedlings were resistant to DM and showed an increased GSH content and antioxidant enzyme activities (SOD, CAT, POD). The level of MDA, H₂O_2, REL, and disease severity was reduced by EF. ACC, JA, ABA, and SA were overproduced along with higher gene expression of LOX, ERF, NCED2, and PAL. Expression of defense-marker genes (PPO, CAT2, SOD, APX, PER5, LOX, NBS-LRR, PSY, CAS, Ubi, MLP43) was also modulated in EF-inoculated infected plants. Current research supported the use of EF inoculation to effectively escalate the systemic immunity against DM corresponding to the significant promotion of induced systemic resistance (ISR) and systemic acquired resistance (SAR) responses through initiating the defense mechanism by SA, ABA, ET, and JA biosynthesis and signaling pathways in luffa.

Molecular cloning and functional characterization of multiple ApOSCs from Andrographis paniculata

Triterpenoids have been described in Andrographis paniculata. Oleanolic acid exhibits high biological activity and is widely used in the clinic, and β-sitosterol not only has good biological activity but also plays an important physiological role in plants. However, analysis of the biosynthetic pathway of triterpenoids in Andrographis paniculata has not been reported. Here, we provide the first report of the isolation and identification of nine 2, 3-oxidosqualene cyclases (ApOSC3 to ApOSC11) from A. paniculata. The results showed that ApOSC4 represented a monofunctional synthase that could convert 2, 3-oxidosqualene to β-amyrin. ApOSC5 as a bifunctional 2, 3-oxidosqualene cyclases, could transfer 2, 3-oxidosqualene to β-amyrin and α-amyrin. ApOSC6 to ApOSC8 composed the multifunctional 2, 3-oxidosqualene cyclases that could convert 2, 3-oxidosqualene to β-amyrin, α-amyrin and one or two undetermined triterpenoids. This study provides a better understanding of the biosynthetic pathway of triterpenoids in A. paniculata, and the discovery of multifunctional 2, 3-oxidosqualene cyclases ApOSC5 to ApOSC8 of the facilitates knowledge of the compounds diversity in A. paniculata.

Phytosterol Profiles, Genomes and Enzymes - An Overview

The remarkable diversity of sterol biosynthetic capacities described in living organisms is enriched at a fast pace by a growing number of sequenced genomes. Whereas analytical chemistry has produced a wealth of sterol profiles of species in diverse taxonomic groups including seed and non-seed plants, algae, phytoplanktonic species and other unicellular eukaryotes, functional assays and validation of candidate genes unveils new enzymes and new pathways besides canonical biosynthetic schemes. An overview of the current landscape of sterol pathways in the tree of life is tentatively assembled in a series of sterolotypes that encompass major groups and provides also peculiar features of sterol profiles in bacteria, fungi, plants, and algae.

Site-directed mutagenesis and substrate compatibility to reveal the structure-function relationships of plant oxidosqualene cyclases

Covering: up to May 2020Oxidosqualene cyclases (OSCs) catalyze one of the most complex polycyclization reactions in nature, using the linear 2,3-oxidosqualene to generate an array of triterpene skeletons in plants. Despite the structural diversity of the products, the protein sequences of plant OSCs are highly conserved, where a few key amino acids could govern the product selectivity. Due to the absence of crystal structures, site-directed mutagenesis and substrate structural modification become key approaches to understand the cyclization mechanism. In this review, 98 mutation sites in 25 plant OSCs have been summarized, and the conserved key residues have been identified by sequence alignment. Structure-function relationships are further discussed. Meanwhile, the substrate selectivity has been summarized to probe the active site cavity of plant OSCs. A total of 77 references are included.

A cycloartenol synthase from the steroidal saponin biosynthesis pathway of Paris polyphylla

Steroidal saponins named polyphyllin are the major active components of Paris polyphylla. Cycloartenol synthase (CAS) is a key enzyme that catalyzes the formation of the sterol scaffold. In this study, we cloned a putative CAS gene from Paris polyphylla. Heterologous expression in yeast indicated that PpCAS can convert 2,3-oxidosqualene into cycloartenol. qRT-PCR analysis showed that the expression of PpCAS was highest in leaves and lowest in roots. To our best knowledge, this is the first report of the functional characterization of cycloartenol synthase from Paris polyphylla, which lays the foundation for further analysis of the biosynthesis pathway of polyphyllins.[Formula: see text].

New insights into diosgenin biosynthesis pathway and its regulation in Trigonella foenum-graecum L

INTRODUCTION

Throughout history, thousands of medicinal and aromatic plants have been widely utilised by people worldwide. Owing to them possessing of valuable compounds with little side effects in comparison with chemical drugs, herbs have been of interest to humans for a number of purposes. Diosgenin, driven from fenugreek, Trigonella foenum-graecum L., has extensively drawn scientist's attention owing to having curable properties and being a precursor of steroid hormones synthesis. Nonetheless, complete knowledge about the biosynthesis pathway of this metabolite is still elusive.

OBJECTIVE

In the present research, we isolated the full-length CDS of 14 genes involving in diosgenin formation and measured their expression rate in various genotypes, which had illustrated different amount of diosgenin.

METHODOLOGY

The genes were successfully isolated, and functional motifs were also assessed using in silico approaches.

RESULTS

Moreover, combining transcript and metabolite analysis revealed that there are many genes playing the role in diosgenin formation, some of which are highly influential. Among them, ∆²⁴ -reductase, which converts cycloartenol to cycloartanol, is the first-committed and rate-limiting enzyme in this pathway. Additionally, no transcripts indicating to the presence or expression of lanosterol synthase were detected, contradicting the previous hypothesis about the biosynthetic pathway of diosgenin in fenugreek.

CONCLUSION

Considering all these, therefore, we propose the most possible pathway of diosgenin. This knowledge will then pave the way toward cloning the genes as well as engineering the diosgenin biosynthesis pathway.

Distinct triterpene synthases in the laticifers of Euphorbia lathyris

Euphorbia lathyris was proposed about fifty years ago as a potential agroenergetic crop. The tremendous amounts of triterpenes present in its latex has driven investigations for transforming this particular biological fluid into an industrial hydrocarbon source. The huge accumulation of terpenes in the latex of many plant species represent a challenging question regarding cellular homeostasis. In fact, the enzymes, the mechanisms and the controllers that tune the amount of products accumulated in specialized compartments (to fulfill ecological roles) or deposited at important sites (as essential factors) are not known. Here, we have isolated oxidosqualene cyclases highly expressed in the latex of Euphorbia lathyris. This triterpene biosynthetic machinery is made of distinct paralogous enzymes responsible for the massive accumulation of steroidal and non-steroidal tetracyclic triterpenes. More than eighty years after the isolation of butyrospermol from shea butter (Heilbronn IM, Moffet GL, and Spring FS J. Chem. Soc. 1934, 1583), a butyrospermol synthase is characterized in this work using yeast and in folia heterologous expression assays.

Identification of Key Amino Acid Residues Determining Product Specificity of 2,3-Oxidosqualene Cyclase in Siraitia grosvenorii

Sterols and triterpenes are structurally diverse bioactive molecules generated through cyclization of linear 2,3-oxidosqualene. Based on carbocationic intermediates generated during the initial substrate preorganization step, oxidosqualene cyclases (OSCs) are roughly segregated into a dammarenyl cation group that predominantly catalyzes triterpenoid precursor products and a protosteryl cation group which mostly generates sterol precursor products. The mechanism of conversion between two scaffolds is not well understood. Previously, we have characterized a promiscuous OSC from Siraitia grosvenorii (SgCS) that synthesizes a novel cucurbitane-type triterpene cucurbitadienol as its main product. By integration of homology modeling, molecular docking and site-directed mutagenesis, we discover that five key amino acid residues (Asp486, Cys487, Cys565, Tyr535, and His260) may be responsible for interconversions between chair–boat–chair and chair–chair–chair conformations. The discovery of euphol, dihydrolanosterol, dihydroxyeuphol and tirucallenol unlocks a new path to triterpene diversity in nature. Our findings also reveal mechanistic insights into the cyclization of oxidosqualene into cucurbitane-type and lanostane-type skeletons, and provide a new strategy to identify key residues determining OSC specificity.

Lanosterol synthase-like is involved with differential accumulation of steroidal glycoalkaloids in potato

Phytosterol homeostasis may be maintained in leaves through diversion of intermediates into glycoalkaloid biosynthesis, whereas in tuber flesh, excess intermediates are catalyzed by tuber-specific StLAS - like , resulting in low tuber glycoalkaloids. Lanosterol synthase (LAS) and cycloartenol synthase (CAS) are phylogenetically related enzymes. Cycloartenol is the accepted precursor leading to cholesterol and phytosterols, and in potato, to steroidal glycoalkaloid (SGA) biosynthesis. LAS was also shown to synthesize some plant sterols, albeit at trace amounts, questioning its role in sterol homeostasis. Presently, a potato LAS-related gene (StLAS-like) was identified and its activity verified in a yeast complementation assay. A transgenic approach with targeted gene expression and metabolic profiling of sterols and SGAs was used. Analyses of StLAS-like transcript levels and StLAS-like-promoter::GUS reporter assays indicated specific expression in tuber flesh tissue. Overexpression of Arabidopsis AtLAS in leaves where the endogenic StLAS-like is not expressed, resulted with increased SGA level and reduced phytosterol level, while in the tuber flesh SGA level was reduced. StLAS-like expression only in tuber flesh may explain the differential accumulation of SGAs in commercial cultivars-low in tubers, high in leaves. In leaves, to maintain phytosterol homeostasis, an excess of intermediates may be diverted into SGA biosynthesis, whereas in tuber flesh these intermediates are catalyzed by tuber-specific StLAS-like instead, resulting in low levels of SGA.

Synthesis of arborane triterpenols by a bacterial oxidosqualene cyclase

Cyclic triterpenoids are a broad class of polycyclic lipids produced by bacteria and eukaryotes. They are biologically relevant for their roles in cellular physiology, including membrane structure and function, and biochemically relevant for their exquisite enzymatic cyclization mechanism. Cyclic triterpenoids are also geobiologically significant as they are readily preserved in sediments and are used as biomarkers for ancient life throughout Earth's history. Isoarborinol is one such triterpenoid whose only known biological sources are certain angiosperms and whose diagenetic derivatives (arboranes) are often used as indicators of terrestrial input into aquatic environments. However, the occurrence of arborane biomarkers in Permian and Triassic sediments, which predates the accepted origin of angiosperms, suggests that microbial sources of these lipids may also exist. In this study, we identify two isoarborinol-like lipids, eudoraenol and adriaticol, produced by the aerobic marine heterotrophic bacterium Eudoraea adriatica Phylogenetic analysis demonstrates that the E. adriatica eudoraenol synthase is an oxidosqualene cyclase homologous to bacterial lanosterol synthases and distinct from plant triterpenoid synthases. Using an Escherichia coli heterologous sterol expression system, we demonstrate that substitution of four amino acid residues in a bacterial lanosterol synthase enabled synthesis of pentacyclic arborinols in addition to tetracyclic sterols. This variant provides valuable mechanistic insight into triterpenoid synthesis and reveals diagnostic amino acid residues to differentiate between sterol and arborinol synthases in genomic and metagenomic datasets. Our data suggest that there may be additional bacterial arborinol producers in marine and freshwater environments that could expand our understanding of these geologically informative lipids.

The Oxidosqualene Cyclase from the Oomycete Saprolegnia parasitica Synthesizes Lanosterol as a Single Product

The first committed step of sterol biosynthesis is the cyclisation of 2,3-oxidosqualene to form either lanosterol (LA) or cycloartenol (CA). This is catalyzed by an oxidosqualene cyclase (OSC). LA and CA are subsequently converted into various sterols by a series of enzyme reactions. The specificity of the OSC therefore determines the final composition of the end sterols of an organism. Despite the functional importance of OSCs, the determinants of their specificity are not well understood. In sterol-synthesizing oomycetes, recent bioinformatics, and metabolite analysis suggest that LA is produced. However, this catalytic activity has never been experimentally demonstrated. Here, we show that the OSC of the oomycete Saprolegnia parasitica, a severe pathogen of salmonid fish, has an uncommon sequence in a conserved motif important for specificity. We present phylogenetic analysis revealing that this sequence is common to sterol-synthesizing oomycetes, as well as some plants, and hypothesize as to the evolutionary origin of some microbial sequences. We also demonstrate for the first time that a recombinant form of the OSC from S. parasitica produces LA exclusively. Our data pave the way for a detailed structural characterization of the protein and the possible development of specific inhibitors of oomycete OSCs for disease control in aquaculture.

Sterol Synthesis in Diverse Bacteria

Sterols are essential components of eukaryotic cells whose biosynthesis and function has been studied extensively. Sterols are also recognized as the diagenetic precursors of steranes preserved in sedimentary rocks where they can function as geological proxies for eukaryotic organisms and/or aerobic metabolisms and environments. However, production of these lipids is not restricted to the eukaryotic domain as a few bacterial species also synthesize sterols. Phylogenomic studies have identified genes encoding homologs of sterol biosynthesis proteins in the genomes of several additional species, indicating that sterol production may be more widespread in the bacterial domain than previously thought. Although the occurrence of sterol synthesis genes in a genome indicates the potential for sterol production, it provides neither conclusive evidence of sterol synthesis nor information about the composition and abundance of basic and modified sterols that are actually being produced. Here, we coupled bioinformatics with lipid analyses to investigate the scope of bacterial sterol production. We identified oxidosqualene cyclase (Osc), which catalyzes the initial cyclization of oxidosqualene to the basic sterol structure, in 34 bacterial genomes from five phyla (Bacteroidetes, Cyanobacteria, Planctomycetes, Proteobacteria, and Verrucomicrobia) and in 176 metagenomes. Our data indicate that bacterial sterol synthesis likely occurs in diverse organisms and environments and also provides evidence that there are as yet uncultured groups of bacterial sterol producers. Phylogenetic analysis of bacterial and eukaryotic Osc sequences confirmed a complex evolutionary history of sterol synthesis in this domain. Finally, we characterized the lipids produced by Osc-containing bacteria and found that we could generally predict the ability to synthesize sterols. However, predicting the final modified sterol based on our current knowledge of sterol synthesis was difficult. Some bacteria produced demethylated and saturated sterol products even though they lacked homologs of the eukaryotic proteins required for these modifications emphasizing that several aspects of bacterial sterol synthesis are still completely unknown.

Plant oxidosqualene metabolism: cycloartenol synthase-dependent sterol biosynthesis in Nicotiana benthamiana

The plant sterol pathway exhibits a major biosynthetic difference as compared with that of metazoans. The committed sterol precursor is the pentacyclic cycloartenol (9β,19-cyclolanost-24-en-3β-ol) and not lanosterol (lanosta-8,24-dien-3β-ol), as it was shown in the late sixties. However, plant genome mining over the last years revealed the general presence of lanosterol synthases encoding sequences (LAS1) in the oxidosqualene cyclase repertoire, in addition to cycloartenol synthases (CAS1) and to non-steroidal triterpene synthases that contribute to the metabolic diversity of C30H50O compounds on earth. Furthermore, plant LAS1 proteins have been unambiguously identified by peptidic signatures and by their capacity to complement the yeast lanosterol synthase deficiency. A dual pathway for the synthesis of sterols through lanosterol and cycloartenol was reported in the model Arabidopsis thaliana, though the contribution of a lanosterol pathway to the production of 24-alkyl-Δ(5)-sterols was quite marginal (Ohyama et al. (2009) PNAS 106, 725). To investigate further the physiological relevance of CAS1 and LAS1 genes in plants, we have silenced their expression in Nicotiana benthamiana. We used virus induced gene silencing (VIGS) based on gene specific sequences from a Nicotiana tabacum CAS1 or derived from the solgenomics initiative (http://solgenomics.net/) to challenge the respective roles of CAS1 and LAS1. In this report, we show a CAS1-specific functional sterol pathway in engineered yeast, and a strict dependence on CAS1 of tobacco sterol biosynthesis.

Cloning and Functional Expression of Cycloartenol Synthases from Mangrove SpeciesRhizophora stylosaGriff. andKandelia candel(L.) Druce

To obtain cDNAs encoding oxidosqualene cyclase (OSC), we cloned two cDNAs, KcCAS and RsCAS, from roots of Kandelia candel (L.) Druce and leaves of Rhizophora stylosa Griff. by homology based PCR method respectively. The deduced amino acid sequences of both OSCs showed 82% homology to cycloartenol synthases from Lotus japonicus (OSC5) and Ricinus cummunis (RcCAS), suggesting that these are cycloartenol synthases of K. candel and R. stylosa. The genes obtained were expressed in a lanosterol synthase deficient Saccharomyces cerevisiae (ERG7) strain, GIL77. GC-MS analysis identified the accumulated reaction product in the yeast transformant to be cycloartenol, indicating that both KcCAS and RsCAS encode cycloartenol synthase.

Saponin Synthesis and Function

Saponins are one of the most numerous and diverse groups of plant natural products. They serve a range of ecological roles including plant defence against disease and herbivores and possibly as allelopathic agents in competitive interactions between plants. Some saponins are also important pharmaceuticals, and the underexplored biodiversity of plant saponins is likely to prove to be a vital resource for future drug discovery. The biological activity of saponins is normally attributed to the amphipathic properties of these molecules, which consist of a hydrophobic triterpene or sterol backbone and a hydrophilic carbohydrate chain, although some saponins are known to have potent biological activities that are dependent on other aspects of their structure. This chapter will focus on the biological activity and the synthesis of some of the best-studied examples of plant saponins and on recent developments in the identification of the genes and enzymes responsible for saponin synthesis.

The triterpene cyclase protein family: a systematic analysis

Triterpene cyclases catalyze a broad range of cyclization reactions to form polycyclic triterpenes. Triterpene cyclases that convert squalene to hopene are named squalene-hopene cyclases (SHC) and triterpene cyclases that convert oxidosqualene are named oxidosqualene cyclases (OSC). Many sequences have been published, but there is only one structure available for each of SHCs and OSCs. Although they catalyze a similar reaction, the sequence similarity between SHCs and OSCs is low. A family classification based on phylogenetic analysis revealed 20 homologous families which are grouped into two superfamilies, SHCs and OSCs. Based on this family assignment, the Triterpene Cyclase Engineering Database (TTCED) was established. It integrates available information on sequence and structure of 639 triterpene cyclases as well as on structurally and functionally relevant amino acids. Family specific multiple sequence alignments were generated to identify the functionally relevant residues. Based on sequence alignments, conserved residues in SHCs and OSCs were analyzed and compared to experimentally confirmed mutational data. Functional schematic models of the central cavities of OSCs and SHCs were derived from structure comparison and sequence conservation analysis. These models demonstrate the high similarity of the substrate binding cavity of SHCs and OSCs and the equivalences of the respective residues. The TTCED is a novel source for comprehensive information on the triterpene cyclase family, including a compilation of previously described mutational data. The schematic models present the conservation analysis in a readily available fashion and facilitate the correlation of residues to a specific function or substrate interaction.

Protein engineering towards natural product synthesis and diversification

A dazzling array of enzymes is used by nature in making structurally complex natural products. These enzymes constitute a molecular toolbox that may be used in the construction and fine-tuning of pharmaceutically active molecules. Aided by technological advancements in protein engineering, it is now possible to tailor the activities and specificities of these enzymes as biocatalysts in the production of both natural products and their unnatural derivatives. These efforts are crucial in drug discovery and development, where there is a continuous quest for more potent agents. Both rational and random evolution techniques have been utilized in engineering these enzymes. This review will highlight some examples from several large families of natural products.

Unusually cyclized triterpenes: occurrence, biosynthesis and chemical synthesis

The biosynthetic origin of most of triterpenes lies in cascade cyclizations and rearrangements of the acyclic precursors squalene (S) and 2,3-oxidosqualene (OS), processes leading to tetra- and pentacyclic triterpene skeleta. Apart from these, a number of triterpenoid structures derived from cyclization processes, that are different from those leading to tetra- and pentacyclic triterpenes, are also found in Nature. We have defined these processes as unusual cyclizations, and grouped them in three blocks, namely, incomplete cyclizations of the corresponding S-derived precursors, cyclizations of S or OS towards polycyclic triterpenes and subsequent cleavage of the preformed ring systems, and two independent cyclizations of the S- or OS-derived precursor. Apart from the molecules obtained from intact organisms, we will also consider the compounds obtained from in vitro cyclizations promoted by enzyme systems. After establishing which compounds could unambiguously be grouped under the term 'unusually cyclized triterpenes', this review moves on to the advances achieved in this kind of structure during the last ten years. These advances are presented in three parts. The first one presents the structure and biological properties of the unusual triterpenes reported in the last decade. The second part considers the main biosynthetic pathways which justify the formation of these triterpenes from their corresponding acyclic precursors. Finally, we look at the achievements made in different synthetic strategies directed at some of these molecules. One hundred and twenty-three references are cited.

Triterpene synthases from the Okinawan mangrove tribe, Rhizophoraceae

Oleanane-type triterpene is one of the most widespread triterpenes found in plants, together with the lupane type, and these two types often occur together in the same plant. Bruguiera gymnorrhiza (L.) Lamk. and Rhizophora stylosa Griff. (Rhizophoraceae) are known to produce both types of triterpenes. Four oxidosqualene cyclase cDNAs were cloned from the leaves of B. gymnorrhiza and R. stylosa by a homology-based PCR method. The ORFs of full-length clones termed BgbAS (2280 bp, coding for 759 amino acids), BgLUS (2286 bp, coding for 761 amino acids), RsM1 (2280 bp, coding for 759 amino acids) and RsM2 (2316 bp coding for 771 amino acids) were ligated into yeast expression plasmid pYES2 under the control of the GAL1 promoter. Expression of BgbAS and BgLUS in GIL77 resulted in the production of beta-amyrin and lupeol, suggesting that these genes encode beta-amyrin and lupeol synthase (LUS), respectively. Furthermore, RsM1 produced germanicol, beta-amyrin, and lupeol in the ratio of 63 : 33 : 4, whereas RsM2 produced taraxerol, beta-amyrin, and lupeol in the proportions 70 : 17 : 13. This result indicates that these are multifunctional triterpene synthases. Phylogenetic analysis and sequence comparisons revealed that BgbAS and RsM1 demonstrated high similarities (78-93%) to beta-amyrin synthases, and were located in the same branch as beta-amyrin synthase. BgLUS formed a new branch for lupeol synthase that was closely related to the beta-amyrin synthase cluster, whereas RsM2 was found in the first branch of the multifunctional triterpene synthase evolved from lupeol to beta-amyrin synthase. Based on these sequence comparisons and product profiles, we discuss the molecular evolution of triterpene synthases and the involvement of these genes in the formation of terpenoids in mangrove leaves.

Cation-pi interaction in the polyolefin cyclization cascade uncovered by incorporating unnatural amino acids into the catalytic sites of squalene cyclase

It has been assumed that the pi-electrons of aromatic residues in the catalytic sites of triterpene cyclases stabilize the cationic intermediates formed during the polycyclization cascade of squalene or oxidosqualene, but no definitive experimental evidence has been given. To validate this cation-pi interaction, natural and unnatural aromatic amino acids were site-specifically incorporated into squalene-hopene cyclase (SHC) from Alicyclobacillus acidocaldarius and the kinetic data of the mutants were compared with that of the wild-type SHC. The catalytic sites of Phe365 and Phe605 were substituted with O-methyltyrosine, tyrosine, and tryptophan, which have higher cation-pi binding energies than phenylalanine. These replacements actually increased the SHC activity at low temperature, but decreased the activity at high temperature, as compared with the wild-type SHC. This decreased activity is due to the disorganization of the protein architecture caused by the introduction of the amino acids more bulky than phenylalanine. Then, mono-, di-, and trifluorophenylalanines were incorporated at positions 365 and 605; these amino acids reduce cation-pi binding energies but have van der Waals radii similar to that of phenylalanine. The activities of the SHC variants with fluorophenylalanines were found to be inversely proportional to the number of the fluorine atoms on the aromatic ring and clearly correlated with the cation-pi binding energies of the ring moiety. No serious structural alteration was observed for these variants even at high temperature. These results unambiguously show that the pi-electron density of residues 365 and 605 has a crucial role for the efficient polycyclization reaction by SHC. This is the first report to demonstrate experimentally the involvement of cation-pi interaction in triterpene biosynthesis.

Molecular Genetics of Plant Sterol Backbone Synthesis

Sterols, which are biosynthesized via the cytoplasmic mevalonate (MVA) pathway, are important structural components of the plasma membrane and precursors of steroid hormones in both vertebrates and plants. Ergosterol and cholesterol are the major sterols in yeast and vertebrates, respectively. In contrast, plants produce a wide variety of phytosterols, which have various functions in plant development. Although the general biosynthetic pathway to plant sterols has been defined, the details of the biochemical, physiological, and developmental functions of genes involved in the biosynthetic network and their regulation are not well understood. Molecular genetic analyses are an effective approach to use when studying these fascinating problems. Since three enzymes, 3-hydroxy-3-methylglutaryl CoA reductase, farnesyl diphosphate synthase, and lanosterol synthase, have been functionally characterized in planta, we reviewed recent progress on these enzymes. Arabidopsis T-DNA and transposon insertion mutants are now widely available. The use of molecular genetics, molecular biology, and bioorganic chemical approaches on these mutants, as well as inhibitors of the MVA pathway, should help us to understand plant sterol biosynthesis comprehensively.

Phenylalanine 445 within Oxidosqualene−Lanosterol Cyclase from Saccharomyces cerevisiae Influences C-Ring Cyclization and Deprotonation Reactions

[reaction: see text] We describe the Saccharomyces cerevisiae oxidosqualene-lanosterol cyclase Phe445 site-saturated mutants that generate truncated tricyclic and altered deprotonation product profiles. Among these mutants, only polar side-chain group substitutions genetically complemented yeast viability and produced spatially related product diversity, supporting the Johnson model that cation-pi interactions between a carbocationic intermediate and an enzyme can be replaced by an electrostatic or polar side chain to stabilize the cationic intermediate, but with product differentiation.

Steroids, triterpenoids and molecular oxygen

There is a close connection between modern-day biosynthesis of particular triterpenoid biomarkers and presence of molecular oxygen in the environment. Thus, the detection of steroid and triterpenoid hydrocarbons far back in Earth history has been used to infer the antiquity of oxygenic photosynthesis. This prompts the question: were these compounds produced similarly in the past? In this paper, we address this question with a review of the current state of knowledge surrounding the oxygen requirement for steroid biosynthesis and phylogenetic patterns in the distribution of steroid and triterpenoid biosynthetic pathways. The hopanoid and steroid biosynthetic pathways are very highly conserved within the bacterial and eukaryotic domains, respectively. Bacteriohopanepolyols are produced by a wide range of bacteria, and are methylated in significant abundance at the C2 position by oxygen-producing cyanobacteria. On the other hand, sterol biosynthesis is sparsely distributed in distantly related bacterial taxa and the pathways do not produce the wide range of products that characterize eukaryotes. In particular, evidence for sterol biosynthesis by cyanobacteria appears flawed. Our experiments show that cyanobacterial cultures are easily contaminated by sterol-producing rust fungi, which can be eliminated by treatment with cycloheximide affording sterol-free samples. Sterols are ubiquitous features of eukaryotic membranes, and it appears likely that the initial steps in sterol biosynthesis were present in their modern form in the last common ancestor of eukaryotes. Eleven molecules of O2 are required by four enzymes to produce one molecule of cholesterol. Thermodynamic arguments, optimization of function and parsimony all indicate that an ancestral anaerobic pathway is highly unlikely. The known geological record of molecular fossils, especially steranes and triterpanes, is notable for the limited number of structural motifs that have been observed. With a few exceptions, the carbon skeletons are the same as those found in the lipids of extant organisms and no demonstrably extinct structures have been reported. Furthermore, their patterns of occurrence over billion year time-scales correlate strongly with environments of deposition. Accordingly, biomarkers are excellent indicators of environmental conditions even though the taxonomic affinities of all biomarkers cannot be precisely specified. Biomarkers are ultimately tied to biochemicals with very specific functional properties, and interpretations of the biomarker record will benefit from increased understanding of the biological roles of geologically durable molecules.

Lanosterol Synthase in Dicotyledonous Plants

Sterols are important as structural components of plasma membranes and precursors of steroidal hormones in both animals and plants. Plant sterols show a wide structural variety and significant structural differences from those of animals. To elucidate the origin of structural diversity in plant sterols, their biosynthesis has been extensively studied [Benveniste (2004) Annu. Rev. Plant. Biol. 55: 429, Schaller (2004) Plant Physiol. Biochem. 42: 465]. The differences in the biosynthesis of sterols between plants and animals begin at the step of cyclization of 2,3-oxidosqualene, which is cyclized to lanosterol in animals and to cycloartenol in plants. However, here we show that plants also have the ability to synthesize lanosterol directly from 2,3-oxidosqualene, which may lead to a new pathway to plant sterols. The Arabidopsis gene At3g45130, designated LAS1, encodes a functional lanosterol synthase in plants. A phylogenetic tree showed that LAS1 belongs to the previously uncharacterized branch of oxidosqualene cyclases, which differs from the cycloartenol synthase branch. Panax PNZ on the same branch was also shown to be a lanosterol synthase in a yeast heterologous expression system. The higher diversity of plant sterols may require two biosynthetic routes in steroidal backbone formation.

Tryptophan 232 within Oxidosqualene−Lanosterol Cyclase from Saccharomyces cerevisiae Influences Rearrangement and Deprotonation but Not Cyclization Reactions

[reaction: see text] Oxidosqualene-lanosterol cyclases convert oxidosqualene to lanosterol in yeast and mammals. Site-saturated mutants' construction of Saccharomyces cerevisiae oxidosqualene-lanosterol cyclase, at Trp232 exchanges against proteinogenic amino acids, and product profiles are shown. All mutants, except Lys and Arg, produced protosta-12,24-dien-3beta-ol, lanosterol, and parkeol. Overall, Trp232 plays a catalytic role in the influence of rearrangement process and determination of deprotonation position but does not involve intervention in the cyclization steps.

A Putative Precursor of Isomalabaricane Triterpenoids from Lanosterol Synthase Mutants

[reaction: see text]. Known lanosterol synthase mutants produce monocyclic or tetracyclic byproducts from oxidosqualene. We describe Erg7 Tyr510 mutants that cause partial substrate misfolding and generate a tricyclic byproduct. This novel triterpene, (13alphaH)-isomalabarica-14(27),17E,21-trien-3beta-ol, is the likely biosynthetic precursor of isomalabaricane triterpenoids in sponges. The results suggest the facile evolution of protective triterpenoids in sessile animals.

Biogenesis, molecular regulation and function of plant isoprenoids

Isoprenoids represent the oldest class of known low molecular-mass natural products synthesized by plants. Their biogenesis in plastids, mitochondria and the endoplasmic reticulum-cytosol proceed invariably from the C5 building blocks, isopentenyl diphosphate and/or dimethylallyl diphosphate according to complex and reiterated mechanisms. Compounds derived from the pathway exhibit a diverse spectrum of biological functions. This review centers on advances obtained in the field based on combined use of biochemical, molecular biology and genetic approaches. The function and evolutionary implications of this metabolism are discussed in relation with seminal informations gathered from distantly but related organisms.

Triterpenoids

This review covers the isolation and structure determination of triterpenoids including squalene derivatives, lanostanes, cycloartanes, dammaranes, euphanes, tirucallanes, tetranortriterpenoids, quassinoids, lupanes, oleannes, friedelanes, ursanes, hopanes, isomalabaricanes and saponins. The literature from January to December 2002 is reviewed and 221 references are used.

Catalytic Promiscuity in Biocatalysis: Using Old Enzymes to Form New Bonds and Follow New Pathways

Biocatalysis has expanded rapidly in the last decades with the discoveries of highly stereoselective enzymes with broad substrate specificity. A new frontier for biocatalysis is broad reaction specificity, where enzymes catalyze alternate reactions. Although often under-appreciated, catalytic promiscuity has a natural role in evolution and occasionally in the biosynthesis of secondary metabolites. Examples of catalytic promiscuity with current or potential applications in synthesis are reviewed here. Combined with protein engineering, the catalytic promiscuity of enzymes may broadly extend their usefulness in organic synthesis.

On the origins of triterpenoid skeletal diversity

The triterpenoids are a large group of natural products derived from C(30) precursors. Nearly 200 different triterpene skeletons are known from natural sources or enzymatic reactions that are structurally consistent with being cyclization products of squalene, oxidosqualene, or bis-oxidosqualene. This review categorizes each of these structures and provides mechanisms for their formation.

Squalene–hopene cyclase: final deprotonation reaction, conformational analysis for the cyclization of (3R,S)-2,3-oxidosqualene and further evidence for the requirement of an isopropylidene moiety both for initiation of the polycyclization cascade and for the formation of the 5-membered E-ring

To provide insight into the polycyclization mechanism of squalene by squalene-hopene cyclase (SHC) from Alicyclobacilus acidocaldarius, some analogs of nor- and bisnorsqualenes were synthesized including the deuterium-labeled squalenes and incubated with the wild-type SHC, leading to the following inferences. (1) The deprotonation reaction for the introduction of the double bond of the hopene skeleton occurs exclusively from the Z-methyl group on the terminal double bond of squalene. (2) 3R-Oxidosqualene was folded in a boat conformation for the A-ring construction, while the 3S-form was in a chair structure. (3) The terminal two methyl groups are indispensable both for the formation of the 5-membered E-ring of the hopene skeleton and for the initiation of the polycyclization cascade, but the terminal Z-methyl group has a more crucial role for the construction of the 5-membered E-ring than the E-methyl group. (4) Some of the novel terpene skeletons, 36, 37, 39 and 40, were created from the analogs employed in this investigation.

Optimising enzyme function by directed evolution

The past decade has seen a revolution in our ability to engineer designer enzymes using genetic tools that mimic evolution on a laboratory timescale. Many excellent examples of directed evolution applied to a wide range of enzymes have clearly demonstrated its future role in adapting enzymes for use in the chemical industry. Recent advances in 'smart' library design and computational screening are now permitting much deeper searches of sequence space, which potentially increases the extent to which enzyme function can be modified.

Mutagenesis approaches to deduce structure-function relationships in terpene synthases

This review highlights mutagenesis studies of terpene synthases, specifically sesquiterpene synthases and oxidosqualene cyclases. Mutagenesis studies of these enzymes have provided mechanistic insights, structure-function relationships for specific enzymatic residues, novel terpene structures and enzymes with novel activities. The literature through 2002 is reviewed and 113 references cited.

The role of sterols in plant growth and development

Sterols found in all eukaryotic organisms are membrane components which regulate the fluidity and the permeability of phospholipid bilayers. Certain sterols in minute amounts, such as campesterol in Arabidopsis thaliana, are precursors of oxidized steroids acting as growth hormones collectively named brassinosteroids. The crucial importance of brassinosteroids upon growth and development has been established through the study of a set of dwarf mutants affected in brassinosteroid synthesis or perception. Some of these dwarfs are, in fact, deficient in the final steps of sterol biosynthesis and their developmental phenotypes are primarily caused by a depletion in the sterol precursor for brassinosteroids. Recently, the characterization of genes encoding sterol biosynthetic enzymes and the isolation of novel plant lines affected in the expression of those genes, either by insertional or classical mutagenesis, overexpression or cosuppression, have shed new light on the involvement of sterols in biological processes such as embryonic development, cell and plant growth, and fertility, which will be presented and discussed in this review article.

In VitroEvolution of Proteins for Drug Development

Thanks to biotechnology, proteins are becoming increasingly important tools to fight disease, both as therapeutics in their own right and as catalysts for the synthesis of small molecule drugs. However, the properties of these proteins are not necessarily optimal for their intended tasks. In vitro evolution is a set of technologies useful to address their shortcomings. Moreover, in vitro evolution can help illuminate natural evolutionary pathways, thus potentially enabling prediction of drug resistance evolution. We consider here recent developments in the area of in vitro evolution, as well as its application to proteins of interest to medical science.