Biochemical characterization of purified zeta-carotene desaturase from Anabaena PCC 7120 after expression in Escherichia coli

Diversity and origin of carotenoid biosynthesis: its history of coevolution towards plant photosynthesis

The development of photosynthesis was a highlight in the progression of bacteria. In addition to the photosystems with their structural proteins, the photosynthesis apparatus consists of different cofactors including essential carotenoids. Thus, the evolution of the carotenoid pathways in relation to the functionality of the resulting structures in photosynthesis is the focus of this review. Analysis of carotenoid pathway genes indicates early evolutionary roots in prokaryotes. The pathway complexity leading to a multitude of structures is a result of gene acquisition, including their functional modifications, emergence of novel genes and gene exchange between species. Along with the progression of photosynthesis, carotenoid pathways coevolved with photosynthesis according to their advancing functionality. Cyanobacteria, with their oxygenic photosynthesis, became a landmark for evolutionary events including carotenogenesis. Concurrent with endosymbiosis, the cyanobacterial carotenoid pathways were inherited into algal plastids. In the lineage leading to Chlorophyta and plants, carotenoids evolved to their prominent role in protection and regulation of light energy input as constituents of a highly efficient light-harvesting complex.

Multiplicity of carotene patterns derives from competition between phytoene desaturase diversification and biological environments

Phytoene desaturases catalyse from two to six desaturation reactions on phytoene, generating a large diversity of molecules that can then be cyclised and produce, depending on the organism, many different carotenoids. We constructed a phylogenetic tree of a subset of phytoene desaturases from the CrtI family for which functional data was available. We expressed in a bacterial system eight codon optimized CrtI enzymes from different clades. Analysis of the phytoene desaturation reactions on crude extracts showed that three CrtI enzymes can catalyse up to six desaturations, forming tetradehydrolycopene. Kinetic data generated using a subset of five purified enzymes demonstrate the existence of characteristic patterns of desaturated molecules associated with various CrtI clades. The kinetic data was also analysed using a classical Michaelis–Menten kinetic model, showing that variations in the reaction rates and binding constants could explain the various carotene patterns observed. Competition between lycopene cyclase and the phytoene desaturases modified the distribution between carotene intermediates when expressed in yeast in the context of the full β-carotene production pathway. Our results demonstrate that the desaturation patterns of carotene molecules in various biological environments cannot be fully inferred from phytoene desaturases classification but is governed both by evolutionary-linked variations in the desaturation rates and competition between desaturation and cyclisation steps.

Biosynthesis of Carotenoids in Plants: Enzymes and Color

Carotenoids are the most important biocolor isoprenoids responsible for yellow, orange and red colors found in nature. In plants, they are synthesized in plastids of photosynthetic and sink organs and are essential molecules for photosynthesis, photo-oxidative damage protection and phytohormone synthesis. Carotenoids also play important roles in human health and nutrition acting as vitamin A precursors and antioxidants. Biochemical and biophysical approaches in different plants models have provided significant advances in understanding the structural and functional roles of carotenoids in plants as well as the key points of regulation in their biosynthesis. To date, different plant models have been used to characterize the key genes and their regulation, which has increased the knowledge of the carotenoid metabolic pathway in plants. In this chapter a description of each step in the carotenoid synthesis pathway is presented and discussed.

The biosynthetic pathway for myxol-2' fucoside (myxoxanthophyll) in the cyanobacterium Synechococcus sp. strain PCC 7002

Synechococcus sp. strain PCC 7002 produces a variety of carotenoids, which comprise predominantly dicylic beta-carotene and two dicyclic xanthophylls, zeaxanthin and synechoxanthin. However, this cyanobacterium also produces a monocyclic myxoxanthophyll, which was identified as myxol-2' fucoside. Compared to the carotenoid glycosides produced by diverse microorganisms, cyanobacterial myxoxanthophyll and closely related compounds are unusual because they are glycosylated on the 2'-OH rather than on the 1'-OH position of the psi end of the molecule. In this study, the genes encoding two enzymes that modify the psi end of myxoxanthophyll in Synechococcus sp. strain PCC 7002 were identified. Mutational and biochemical studies showed that open reading frame SynPCC7002_A2032, renamed cruF, encodes a 1',2'-hydroxylase [corrected] and that open reading frame SynPCC7002_A2031, renamed cruG, encodes a 2'-O-glycosyltransferase. The enzymatic activity of CruF was verified by chemical characterization of the carotenoid products synthesized when cruF was expressed in a lycopene-producing strain of Escherichia coli. Database searches showed that homologs of cruF and cruG occur in the genomes of all sequenced cyanobacterial strains that are known to produce myxol or the acylic xanthophyll oscillaxanthin. The genomes of many other bacteria that produce hydroxylated carotenoids but do not contain crtC homologs also contain cruF orthologs. Based upon observable intermediates, a complete biosynthetic pathway for myxoxanthophyll is proposed. This study expands the suite of enzymes available for metabolic engineering of carotenoid biosynthetic pathways for biotechnological applications.

Evolution of carotene desaturation: the complication of a simple pathway

In a series of desaturation reactions, the trienoic structures of phytoene and diapophytoene are extended to a maximum of 15 or 11 conjugated double bonds, respectively. After the cloning of several genes from bacteria and eukaryotes, the desaturation reactions were first analyzed in a heterologous host by functional genetic complementation. In addition, different desaturases were heterologously expressed and the reactions studied in vitro. This revealed that in archaea, non-photosynthetic prokaryotes and fungi the desaturases differ significantly from convergently evolved desaturases in cyanobacteria, Chlorobaculum (old name Chlorobium) species and eukaryotic photosynthetic organisms including plants. Detailed analysis of the desaturation reactions including the determination of the substrates converted by the enzymes, the intermediates and the products formed in the reactions revealed the bacterial all-trans desaturation pathway catalyzed by a single enzyme and the cyanobacterial/plant type poly-cis desaturation pathway which involves two closely related desaturases. This indicates that in the course of evolution of carotenogenesis from bacteria via cyanobacteria to plants, the simple situation of one enzyme for the entire reaction sequence from phytoene to all-trans lycopene changed to a more complex process. Three individual enzymes, newly acquired phytoene and zeta-carotene desaturases, as well as a carotene isomerase which is phylogenetically related to CrtI are involved. Only the CrtI-type enzymes seem to have the property to catalyze cis to trans conversion of carotenes.

Isorenieratene biosynthesis in green sulfur bacteria requires the cooperative actions of two carotenoid cyclases

The cyclization of lycopene to gamma- or beta-carotene is a major branch point in the biosynthesis of carotenoids in photosynthetic bacteria. Four families of carotenoid cyclases are known, and each family includes both mono- and dicyclases, which catalyze the formation of gamma- and beta-carotene, respectively. Green sulfur bacteria (GSB) synthesize aromatic carotenoids, of which the most commonly occurring types are the monocyclic chlorobactene and the dicyclic isorenieratene. Recently, the cruA gene, encoding a conserved hypothetical protein found in the genomes of all GSB and some cyanobacteria, was identified as a lycopene cyclase. Further genomic analyses have found that all available fully sequenced genomes of GSB encode an ortholog of cruA. Additionally, the genomes of all isorenieratene-producing species of GSB encode a cruA paralog, now named cruB. The cruA gene from the chlorobactene-producing GSB species Chlorobaculum tepidum and both cruA and cruB from the brown-colored, isorenieratene-producing GSB species Chlorobium phaeobacteroides strain DSM 266(T) were heterologously expressed in lycopene- and neurosporene-producing strains of Escherichia coli, and the cruB gene of Chlorobium clathratiforme strain DSM 5477(T) was also heterologously expressed in C. tepidum by inserting the gene at the bchU locus. The results show that CruA is probably a lycopene monocyclase in all GSB and that CruB is a gamma-carotene cyclase in isorenieratene-producing species. Consequently, the branch point for the synthesis of mono- and dicyclic carotenoids in GSB seems to be the modification of gamma-carotene, rather than the cyclization of lycopene as occurs in cyanobacteria.

The biochemical basis for structural diversity in the carotenoids of chlorophototrophic bacteria

Ongoing work has led to the identification of most of the biochemical steps in carotenoid biosynthesis in chlorophototrophic bacteria. In carotenogenesis, a relatively small number of modifications leads to a great diversity of carotenoid structures. This review examines the individual steps in the pathway, discusses how each contributes to structural diversity among carotenoids, and summarizes recent progress in elucidating the biosynthetic pathways for carotenoids in chlorophototrophs.

Identification of a fourth family of lycopene cyclases in photosynthetic bacteria

A fourth and large family of lycopene cyclases was identified in photosynthetic prokaryotes. The first member of this family, encoded by the cruA gene of the green sulfur bacterium Chlorobium tepidum, was identified in a complementation assay with a lycopene-producing strain of Escherichia coli. Orthologs of cruA are found in all available green sulfur bacterial genomes and in all cyanobacterial genomes that lack genes encoding CrtL- or CrtY-type lycopene cyclases. The cyanobacterium Synechococcus sp. PCC 7002 has two homologs of CruA, denoted CruA and CruP, and both were shown to have lycopene cyclase activity. Although all characterized lycopene cyclases in plants are CrtL-type proteins, genes orthologous to cruP also occur in plant genomes. The CruA- and CruP-type carotenoid cyclases are members of the FixC dehydrogenase superfamily and are distantly related to CrtL- and CrtY-type lycopene cyclases. Identification of these cyclases fills a major gap in the carotenoid biosynthetic pathways of green sulfur bacteria and cyanobacteria.

Genetic manipulation of carotenoid biosynthesis in the green sulfur bacterium Chlorobium tepidum

The green sulfur bacterium Chlorobium tepidum is a strict anaerobe and an obligate photoautotroph. On the basis of sequence similarity with known enzymes or sequence motifs, nine open reading frames encoding putative enzymes of carotenoid biosynthesis were identified in the genome sequence of C. tepidum, and all nine genes were inactivated. Analysis of the carotenoid composition in the resulting mutants allowed the genes encoding the following six enzymes to be identified: phytoene synthase (crtB/CT1386), phytoene desaturase (crtP/CT0807), zeta-carotene desaturase (crtQ/CT1414), gamma-carotene desaturase (crtU/CT0323), carotenoid 1',2'-hydratase (crtC/CT0301), and carotenoid cis-trans isomerase (crtH/CT0649). Three mutants (CT0180, CT1357, and CT1416 mutants) did not exhibit a discernible phenotype. The carotenoid biosynthetic pathway in C. tepidum is similar to that in cyanobacteria and plants by converting phytoene into lycopene using two plant-like desaturases (CrtP and CrtQ) and a plant-like cis-trans isomerase (CrtH) and thus differs from the pathway known in all other bacteria. In contrast to the situation in cyanobacteria and plants, the construction of a crtB mutant completely lacking carotenoids demonstrates that carotenoids are not essential for photosynthetic growth of green sulfur bacteria. However, the bacteriochlorophyll a contents of mutants lacking colored carotenoids (crtB, crtP, and crtQ mutants) were decreased from that of the wild type, and these mutants exhibited a significant growth rate defect under all light intensities tested. Therefore, colored carotenoids may have both structural and photoprotection roles in green sulfur bacteria. The ability to manipulate the carotenoid composition so dramatically in C. tepidum offers excellent possibilities for studying the roles of carotenoids in the light-harvesting chlorosome antenna and iron-sulfur-type (photosystem I-like) reaction center. The phylogeny of carotenogenic enzymes in green sulfur bacteria and green filamentous bacteria is also discussed.

Carotenoid Biosynthesis and Biotechnological Application

A survey is given on the carotenoid biosynthetic pathway leading to beta-carotene and its oxidation products in bacteria and plants. This includes the synthesis of prenyl pyrophosphates via the mevalonate or the 1-deoxyxylulose-5-phosphate pathways as well as the reaction sequences of carotenoid formation and interconversion together with the properties of the enzymes involved. Biotechnological application of this knowledge resulted in the development of heterologous carotenoid production systems using bacteria and fungi with metabolic engineered precursor supply and crop plants with manipulated carotenoid biosynthesis. The recent developments in engineering crops with increased carotenoid contents are covered.

New sequence motifs in flavoproteins: evidence for common ancestry and tools to predict structure

We describe two new sequence motifs, present in several families of flavoproteins. The "GG motif" (RxGGRxxS/T) is found shortly after the betaalphabetadinucleotide-binding motif (DBM) in L-amino acid oxidases, achacin and aplysianin-A, monoamine oxidases, corticosteroid-binding proteins, and tryptophan 2-monooxygenases. Other disperse sequence similarities between these families suggest a common origin. A GG motif is also found in protoporphyrinogen oxidase and carotenoid desaturases and, reduced to the central GG doublet, in the THI4 protein, dTDP-4-dehydrorhamnose reductase, soluble fumarate reductase, steroid dehydrogenases, Rab GDP-dissociation inhibitor, and in most flavoproteins with two dinucleotide-binding domains (glutathione reductase, glutamate synthase, flavin-containing monooxygenase, trimethylamine dehydrogenase...). In the latter families, an "ATG motif" (oxhhhATG) is found in both the FAD- and NAD(P)H-binding domains, forming the fourth beta-strand of the Rossman fold and the connecting loop. On the basis of these and previously described motifs, we present a classification of dinucleotide-binding proteins that could also serve as an evolutionary scheme. Like the DBM, the ATG motif appears to predate the divergence of NAD(P)H- and FAD-binding proteins. We propose that flavoproteins have evolved from a well-differentiated NAD(P)H-binding protein. The bulk of the substrate-binding domain was formed by an insertion after the fourth beta-strand, either of a closely related NAD(P)H-binding domain or of a domain of completely different origin.

Catalytic properties of an expressed and purified higher plant type zeta-carotene desaturase from Capsicum annuum

The zeta-carotene desaturase from Capsicum annuum (EC 1.14.99.-) was expressed in Escherichia coli, purified and characterized biochemically. The enzyme acts as a monomer with lipophilic quinones as cofactors. Km values for the substrate zeta-carotene or the intermediate neurosporene in the two-step desaturation reaction are almost identical. Product analysis showed that different lycopene isomers are formed, including substantial amounts of the all-trans form, together with 7,7',9,9'-tetracis prolycopene via the corresponding neurosporene isomers. The application of different geometric isomers as substrates revealed that the zeta-carotene desaturase has no preference for certain isomers and that the nature of the isomers formed during catalysis depends strictly on the isomeric composition of the substrate.

Two Arabidopsis thaliana carotene desaturases, phytoene desaturase and zeta-carotene desaturase, expressed in Escherichia coli, catalyze a poly-cis pathway to yield pro-lycopene

We have expressed in Escerichia coli the enzymes geranylgeranyl diphosphate synthase and phytoene synthase, from the soil bacterium Erwinia stewartii, and the two carotene desaturases phytoene desaturase and carotene zeta-carotene desaturase from Arabidopsis thaliana. We show that pro-lycopene (7,9,7',9'-tetra-cis)-lycopene is the main end product of the plant desaturation pathway in these cells. In addition, light is required in this system. Whereas in the dark mainly zeta-carotene, the phytoene desaturase product, accumulates, illumination leads to activation of this intermediate caused by its photoisomerization. zeta-Carotene then meets the stereospecific requirements of zeta-carotene desaturase and pro-lycopene is formed. In contrast, a strain of E. coli carrying geranylgeranyl diphosphate synthase, phytoene desaturase and the bacterial carotene desaturase CrtI, which mediates lycopene formation from phytoene, does not require light, nor is a poly-cis-lycopene species formed. The stereoselectivity of the plant-type desaturation pathway expressed in E. coli is the same as previously shown with chromoplast membranes. As the phytoene desaturase and zeta-carotene desaturase used originate from a system not capable of developing chromoplasts, this indicates that the poly-cis pathway of carotene desaturation may have a wider occurrence than initially believed.

Purification and biochemical characterization of a hydroxyneurosporene desaturase involved in the biosynthetic pathway of the carotenoid spheroidene in Rhodobacter sphaeroides

Hydroxyneurosporene desaturase is involved in the carotenoid biosynthetic pathway of Rhodobacter species. The gene encoding this enzyme was expressed in Escherichia coli, purified, and biochemically characterized. The resulting protein contained an N-terminal six-histidine extension which derived from the cloning vector; this allowed for a one-step purification of the enzyme to homogeneity after solubilization with Nonidet P-40. The hydrogen acceptor in the C-3,4 desaturation reaction was molecular oxygen. NAD+, NADP+, and flavin adenine dinucleotide had no influence on enzymatic activity. Different acyclic 1-hydroxycarotenoids were tested as substrates. Very good conversion was achieved with 1-hydroxyneurosporene and 1-hydroxylycopene, whereas 1-hydroxy-gamma-carotene and 1,1'-dihydroxylycopene were much less effective. From 1'-hydroxy-3,4-didehydrolycopene only trace amounts of product were obtained, and 1-methoxyneurosporene was not converted by purified hydroxyneurosporene desaturase. A Km of 13.4 microM was determined for 1-hydroxyneurosporene.

Genetics of eubacterial carotenoid biosynthesis: a colorful tale

Carotenoids represent one of the most widely distributed and structurally diverse classes of natural pigments, with important functions in photosynthesis, nutrition, and protection against photooxidative damage. In the eubacterial community, yellow, orange, and red carotenoids are produced by anoxygenic photosynthetic bacteria, cyanobacteria, and certain species of nonphotosynthetic bacteria. Many eukaryotes, including all algae and plants, as well as some fungi, also synthesize these pigments. In noncarotenogenic organisms, such as mammals, birds, amphibians, fish, crustaceans, and insects, dietary carotenoids and their metabolites also serve important biological roles. Within the last decade, major advances have been made in the elucidation of the molecular genetics, the biochemistry, and the regulation of eubacterial carotenoid biosynthesis. These developments have important implications for eukaryotes, and they make increasingly attractive the genetic manipulation of carotenoid content for biotechnological purposes.

Phytoene desaturase: heterologous expression in an active state, purification, and biochemical properties

Conditions were developed for heterologous expression in Escherichia coli of the membrane-bound cyanobacterial/plant-type phytoene desaturase (PDS) from Synechococcus in an active form. Decrease of growth temperature for the transformant to 28 degrees C resulted in an increase of proteins in a supernatant fraction obtained after pressure disruption (20 MPa) of cells and centrifugation. This supernatant in which the highest PDS activity was found was used for purification to a homogeneous protein by ammonium sulfate precipitation and DEAE chromatography. The purified PDS was employed to determine substrate specificity and cofactor requirement. Substrates in addition to phytoene were phytofluene and 1,2-epoxy phytoene which were converted to zeta-carotene and the corresponding 1,2-epoxide. The reaction was stimulated by NAD, NADP, and oxygen. The K(m) values determined for phytoene and NADP were 3.5 microM and 14.3 mM, respectively.