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CRT is the gene cluster responsible for the biosynthesis of carotenoids. Those genes are found in eubacteria, [1] in algae [2] and are cryptic in Streptomyces griseus . [3]
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CRT is the gene cluster responsible for the biosynthesis of carotenoids. Those genes are found in eubacteria, [1] in algae [2] and are cryptic in Streptomyces griseus . [3]
The CRT gene cluster consists of twenty-five genes such as crtA, crtB, crtC, crtD, crtE, crtF, crtG, crtH, crtI, crtO, crtP, crtR, crtT, crtU, crtV, and crtY, crtZ. These genes play a role in varying stages of the Astaxanthin biosynthesis and Carotenoid biosynthesis (Table 1). [4]
crtE encodes for an enzyme known as geranylgeranyl diphosphate synthase known to catalyze the condensation reaction of isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) into geranylgeranyl diphosphate (GGDP). [5] [6] Two GGDP molecules are subsequently converted into a single phytoene molecule by phytoene synthase, an enzyme encoded by crtB, known as PSY in Chlorophyta. [2] [5] [6] The following desaturation of phytoene into ζ-carotene is catalyzed by the phytoene desaturase encoded by crtI, crtP, and/or PDS. [2] [5] [6] ζ -carotene can also be obtained through phytoene using the carotene 2,4-desaturase enzyme (crtD). [2] [7] Depending on the species, varying carotenoids are accumulated following these steps. [1] [8]
Spirilloxanthin is obtained from lycopene following a hydration, desaturation, and methylation reaction. These reactions are catalyzed by carotene hydratase (crtC), carotene 3,4- desaturase (crtD), and carotene methyltransferase (crtF), respectively. [6] [1]
Lycopene is cyclized through two enzymes lycopene cyclase and β-C-4-oxygenase/β-carotene ketolase encoded on the crtY (in Chlorophyta) /crtL (in cyanobacteria), and crtW, respectively. crtY cyclizes lycopene into β-carotene, which is subsequently oxygenated by crtW to form canthaxanthin. [6]
Zeaxanthin and lutein are obtained through hydroxylation of α- and β-carotene. [1] Hydroxylation of Zeaxanthin occurs by β-carotene hydroxylase an enzyme encoded on the crtR (in cyanobacteria) and crtZ gene (in Chlorophyta). [6]
Zeaxanthin can be further processed to obtain zeaxanthin-diglucoside by Zeaxanthin glucosyl transferase (crtX).
Echinenone is obtained from β -carotene through the catalyzing enzyme β-C-4-oxygenase/β-carotene ketolase (crtO). [9] CrtO, also known as bkt2 in Chlorophyta, is also involved in the conversion of other carotenoids into Canthaxanthin, 3-Hydroxyechinenone, 3'-Hydroxyechinenone, Adonixanthin, and Astaxanthin. [9] [10] CrtZ, similarly to crtO, is also capable of converting carotenoids into β-cryptoxanthin, Zeaxanthin, 3-Hydroxyechinenone, 3'-Hydroxyechinenone, Astaxanthin, Adonixanthin, and Adonirubin. [9]
crtH catalyzes the isomerization of cis-carotenes into trans-carotenes through carotenoid isomerase. [2]
crtG encodes for carotenoid 2,2'- β-hydroxylase, this enzyme leads to the formation of 2-hydroxylated and 2,2′-dihydroxylated products in E coli . [11]
| Gene | Enzyme | Catalyzed reaction |
|---|---|---|
| crtE | GGDP synthase | IPP and DMAPP conversion to GGDP |
| crtB (PSY*) | Phytoene Synthase | GGDP conversion to phytoene |
| crtP (PDS*) | Phytoene desaturase | Conversion of phytoene into ζ- carotene |
| crtI | Phytoeine desaturase | Conversion of phytoene into ζ- carotene |
| crtQ | ζ- carotene desaturase | Desaturation of ζ- carotene to lycopene |
| crtH | Carotenoid isomerase | Isomeration of cis to trans carotones |
| crtY (lcy-E) | Lycopene cyclase | Cyclization of lycopene |
| crtL (lcy-B) | Lycopene cyclase | Cyclization of lycopene |
| crtD | Carotene 3,4-desaturase | Conversion of phytoene to ζ-carotene |
| crtA | Spheroidene monooxygenase | Conversion of spheroidene to spheroidenone |
| crtR+ | β-carotene hydroxylase (various Cyanobacteria) | Hydroxylation of β-carotene to zeaxanthin |
| crtZ* | β-carotene hydroxylase (various Chlorophyta) | Hydroxylation of β-carotene to zeaxanthin |
| crtX | Zeaxanthin glucosyl transferase | Conversion of zeaxanthin to zeaxanthin-diglucoside |
| crtW (bkt2*) | β-C-4-oxygenase/β-carotene ketolase | Conversion of β-carotene to canthaxanthin |
| crtO | β-C-4-oxygenase/β-carotene ketolase | Conversion of β-carotene to echinenone |
| crtC | Carotene hydratase | Conversion of neurosporene to demethylspheroidene and lycopene to hydroxy derivatives |
| crtG | Carotenoid 2,2′-β-hydroxylase | Conversion of myxol to 2-hydroxymyxol and zeaxanthin to nostoxanthin |
| crtK | Carotenoid regulation | - |
| * In Chlorophyta, + In cyanobacteria | ||
Previous studies have indicated through phylogenetic analysis that evolutionary patterns of crt genes are characterized by horizontal gene transfer and gene duplication events. [12]
Horizontal gene transfer has been hypothesized to have occurred between cyanobacteria and Chlorophyta, as similarities in these genes have been found across taxa. [12] Note, however, that some cyanobacteria retained their nature. Horizontal gene transfer among species occurred with a high probability in genes involved in the initial steps of the carotenoid biosynthesis pathway such as crtE, crtB, crtY, crtL, PSY, and crtQ. These genes are often well conserved while others involved in the later stages of Carotenoid biosynthesis such as crtW and crtO are less conserved. [1] The less conserved nature of these genes allowed for the expansion of the carotenoid biosynthesis pathway and its end products. Amino acid variations within crt genes have evolved due to purifying and adaptive selection. [2]
Gene duplications are suspected to have occurred due to the presence of multiple copies of ctr clusters or genes within a single species. [2] An example of this can be seen in the Bradyrhizobium ORS278 strain, where initial crt genes can be found (excluding crtC, crtD, and crtF genes) as well as a second crt gene cluster. This second gene cluster has been shown to also be involved in carotenoid biosynthesis using its crt paralogs. [6] [13]
Carotenoids are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, corn, tomatoes, canaries, flamingos, salmon, lobster, shrimp, and daffodils. Over 1,100 identified carotenoids can be further categorized into two classes – xanthophylls and carotenes.
Astaxanthin is a keto-carotenoid within a group of chemical compounds known as terpenes. Astaxanthin is a metabolite of zeaxanthin and canthaxanthin, containing both hydroxyl and ketone functional groups. It is a lipid-soluble pigment with red coloring properties, which result from the extended chain of conjugated double bonds at the center of the compound.
Canthaxanthin is a keto-carotenoid pigment widely distributed in nature. Carotenoids belong to a larger class of phytochemicals known as terpenoids. The chemical formula of canthaxanthin is C40H52O2. It was first isolated in edible mushrooms. It has also been found in green algae, bacteria, crustaceans, and bioaccumulates in fish such as carp, golden grey mullet, seabream and trush wrasse.
Zeaxanthin is one of the most common carotenoids in nature, and is used in the xanthophyll cycle. Synthesized in plants and some micro-organisms, it is the pigment that gives paprika, corn, saffron, goji (wolfberries), and many other plants and microbes their characteristic color.
In enzymology, a carotene 7,8-desaturase (EC 1.14.99.30) is an enzyme that catalyzes the chemical reaction
Damascenones are a series of closely related chemical compounds that are components of a variety of essential oils. The damascenones belong to a family of chemicals known as rose ketones, which also includes damascones and ionones. beta-Damascenone is a major contributor to the aroma of roses, despite its very low concentration, and is an important fragrance chemical used in perfumery.
Phytoene synthase is a transferase enzyme involved in the biosynthesis of carotenoids. It catalyzes the conversion of geranylgeranyl pyrophosphate to phytoene. This enzyme catalyses the following chemical reaction
15-cis-phytoene desaturases, are enzymes involved in the carotenoid biosynthesis in plants and cyanobacteria. Phytoene desaturases are membrane-bound enzymes localized in plastids and introduce two double bonds into their colorless substrate phytoene by dehydrogenation and isomerize two additional double bonds. This reaction starts a biochemical pathway involving three further enzymes called the poly-cis pathway and leads to the red colored lycopene. The homologous phytoene desaturase found in bacteria and fungi (CrtI) converts phytoene directly to lycopene by an all-trans pathway.
9,9'-dicis-zeta-carotene desaturase is an enzyme with systematic name 9,9'-dicis-zeta-corotene:quinone oxidoreductase. This enzyme catalyses the following chemical reaction
4,4'-Diapophytoene desaturase is an enzyme with systematic name 15-cis-4,4'-diapophytoene:FAD oxidoreductase. This enzyme catalyses the following chemical reaction
All-trans-zeta-carotene desaturase is an enzyme with systematic name all-trans-zeta-carotene:acceptor oxidoreductase. This enzyme catalyses the following chemical reaction
Phytoene desaturase (neurosporene-forming) is an enzyme with systematic name 15-cis-phytoene:acceptor oxidoreductase (neurosporene-forming). This enzyme catalyses the following chemical reaction
Phytoene desaturase (zeta-carotene-forming) is an enzyme with systematic name 15-cis-phytoene:acceptor oxidoreductase (zeta-carotene-forming). This enzyme catalyses the following chemical reaction
Phytoene desaturase (3,4-didehydrolycopene-forming) is an enzyme with systematic name 15-cis-phytoene:acceptor oxidoreductase (3,4-didehydrolycopene-forming). This enzyme catalyses the following chemical reaction
Phytoene desaturase (lycopene-forming) are enzymes found in archaea, bacteria and fungi that are involved in carotenoid biosynthesis. They catalyze the conversion of colorless 15-cis-phytoene into a bright red lycopene in a biochemical pathway called the poly-trans pathway. The same process in plants and cyanobacteria utilizes four separate enzymes in a poly-cis pathway.
Beta-carotene 3-hydroxylase (EC 1.14.13.129, beta-carotene 3,3'-monooxygenase, CrtZ) is an enzyme with systematic name beta-carotene,NADH:oxygen 3-oxidoreductase . This enzyme catalyses the following chemical reaction
The squalene/phytoene synthase family represents proteins that catalyze the head-to-head condensation of C15 and C20 prenyl units (i.e. farnesyl diphosphate and genranylgeranyl diphosphate). This enzymatic step constitutes part of steroid and carotenoid biosynthesis pathway. Squalene synthase EC (SQS) and Phytoene synthase EC (PSY) are two well-known examples of this protein family and share a number of functional similarities. These similarities are also reflected in their primary structure. In particular three well conserved regions are shared by SQS and PSY; they could be involved in substrate binding and/or the catalytic mechanism. SQS catalyzes the conversion of two molecules of farnesyl diphosphate (FPP) into squalene. It is the first committed step in the cholesterol biosynthetic pathway. The reaction carried out by SQS is catalyzed in two separate steps: the first is a head-to-head condensation of the two molecules of FPP to form presqualene diphosphate; this intermediate is then rearranged in a NADP-dependent reduction, to form squalene:
Prolycopene isomerase is an enzyme with systematic name 7,9,7',9'-tetracis-lycopene cis-trans-isomerase. This enzyme catalyses the following chemical reaction
Lycopene β-cyclase is an enzyme with systematic name carotenoid beta-end group lyase (decyclizing). This enzyme catalyses the following chemical reaction
Phytoene desaturase may refer to: