CrtY is the lycopene beta-cyclase of the carotenoid-producing enterobacterium Pantoea ananatis (historically Erwinia uredovora), a 382-residue FAD-dependent enzyme of the lycopene cyclase family. It catalyses the double cyclization that converts the acyclic C40 hydrocarbon all-trans-lycopene into bicyclic all-trans-beta-carotene, forming one beta-ionone ring at each acyclic psi end by way of the monocyclic intermediate gamma-carotene. The reaction is a cycloisomerization (EC 5.5.1.19): despite the flavoprotein-like FAD/NAD(P)-binding fold, no net oxidation or reduction of the carotenoid takes place and no molecular oxygen is used or incorporated. Reduced FAD is the essential cofactor; it is proposed to stabilize a positively charged transition state or carbocation intermediate, while proton attack from water at C-2 of the folded psi end group initiates ring closure. NAD(P)H stimulates activity but is not incorporated and acts indirectly, most likely by maintaining FAD in its reduced state or as an allosteric activator. The enzyme is promiscuous at the ring-precursor level, also cyclizing neurosporene to 7,8-dihydro-beta-carotene via beta-zeacarotene and zeta-carotene to a tetrahydro-beta-carotene. CrtY is a membrane-associated peripheral enzyme of the inner (cytoplasmic) membrane, acting on its extremely hydrophobic membrane-embedded substrate. In the Pantoea crt gene cluster it acts on the lycopene supplied by the CrtE/CrtB/CrtI segment and hands beta-carotene to the hydroxylase CrtZ and glucosyltransferase CrtX, and it is a standard tool for reconstructing beta-carotene biosynthesis in heterologous hosts.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005886 plasma membrane | EXP PMID:20178989 The lycopene cyclase CrtY from Pantoea ananatis (formerly Er... | ACCEPT | Summary: Experimental localization of CrtY to the bacterial inner (cytoplasmic) membrane. In a Gram-negative bacterium the inner membrane is the plasma membrane, so GO:0005886 is the correct cellular-component term. Reason: UniProt records the subcellular location as "Cell inner membrane" with experimental evidence (ECO:0000269) attributed to PubMed:20178989, together with the note that the enzyme is membrane-bound. The cached copy of this paper is abstract-only and the abstract is mechanism-focused, so the membrane measurement lives in the full text the curator read; per the do-not-overrule-from-an-abstract-only-cache rule the experimental call is retained. The assignment is biologically coherent: lycopene and beta-carotene are extremely hydrophobic and partition into the membrane, so the cyclase acts at the membrane on a membrane-embedded substrate. The protein carries no signal peptide and no predicted transmembrane span, so it is best understood as a peripheral, membrane-associated enzyme; the plasma-membrane component term correctly captures where it functions without asserting an integral topology. Supporting Evidence: file:PANAN/crtY/crtY-uniprot.txt SUBCELLULAR LOCATION: Cell inner membrane file:PANAN/crtY/crtY-uniprot.txt Note=Membrane-bound. file:PANAN/crtY/crtY-deep-research-falcon.md membrane-associated/peripheral enzyme acting on a membrane-embedded substrate |
| GO:0005886 plasma membrane | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic assignment of the plasma-membrane term from the UniProt Swiss-Prot subcellular-location vocabulary mapping, producing the same cellular component that the experimental row asserts. Reason: This IEA is derived by mapping the curated subcellular-location controlled vocabulary term (UniProtKB-SubCell:SL-0037) to GO:0005886, and it is fully concordant with the experimental EXP row for the identical term. The underlying curated location (cell inner membrane) is itself experimentally grounded in PubMed:20178989, so the electronic route reaches the correct answer. Duplicate identifiers under different evidence codes carry independent provenance and are retained together rather than collapsed. Supporting Evidence: file:PANAN/crtY/crtY-uniprot.txt SUBCELLULAR LOCATION: Cell inner membrane file:PANAN/crtY/crtY-uniprot.txt KW Carotenoid biosynthesis; Cell inner membrane; Cell membrane; FAD; |
| GO:0016117 carotenoid biosynthetic process | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO assignment of the module-level carotenoid biosynthetic process from the lycopene cyclase signatures. CrtY catalyses the cyclization step of this pathway, so it participates in the process rather than merely being required for it. Reason: GO:0016117 is verified against QuickGO as the formation of carotenoids, and CrtY performs the lycopene-to-beta-carotene cyclization that is a genuine step of that pathway, so the participation test is satisfied by direct catalysis. The two supporting InterPro entries are lycopene-cyclase family signatures (IPR008461 CrtY_pseudo and IPR010108 Lycopene_cyclase_b/e), so the mapping is at the right level of specificity for the evidence. UniProt independently records the pathway assignment for this entry. This is a core biological process of the gene. Supporting Evidence: file:PANAN/crtY/crtY-uniprot.txt PATHWAY: Carotenoid biosynthesis; beta-carotene biosynthesis. file:PANAN/crtY/crtY-uniprot.txt Catalyzes the double cyclization reaction which converts file:PANAN/crtY/crtY-uniprot.txt InterPro; IPR010108; Lycopene_cyclase_b/e. |
| GO:0016705 oxidoreductase activity, acting on paired donors, with incorporation or reduction of molecular oxygen | IEA GO_REF:0000002 | REMOVE | Summary: InterPro2GO assigns an oxygen-dependent oxidoreductase activity from the lycopene-cyclase signature IPR010108, misled by the FAD/NAD(P)-binding fold. CrtY is a non-redox cycloisomerase that neither uses nor incorporates molecular oxygen, so this molecular-function term is biologically wrong. Reason: GO:0016705 is defined (verified against QuickGO) as catalysis of an oxidation-reduction reaction in which hydrogen or electrons are transferred from two donors and molecular oxygen is reduced or incorporated into a donor. Every element of that definition is contradicted by the established chemistry of CrtY. The enzyme is an isomerase (EC 5.5.1.19): its cyclization produces no net oxidation or reduction of the carotenoid, and no molecular oxygen is used. PubMed:20178989 showed that a mechanism relying on cryptic net electron transfer can be refuted and placed lycopene cyclase among the novel class of non-redox flavoproteins; PubMed:11943208 showed the hydrogen introduced at C-2 comes from water and not from NADPH, confirming proton-initiated acid-base catalysis rather than redox chemistry. The signature that produced this row, IPR010108, is the lycopene-cyclase family entry, and the same UniProt record matches an FAD/NAD(P)-binding superfamily fold (Gene3D 3.50.50.60, SUPFAM SSF51905); that fold is what misled the automatic mapping, because a dinucleotide-binding fold does not imply an oxygen-dependent redox reaction. Reduced FAD is required only to stabilize the carbocation-like transition state, not as a terminal oxidant. This is a fold-driven over-annotation that is contradicted by direct experiment. Propagation Review Root cause: PROPAGATION BAD Failure modes: FUNCTIONAL DIVERGENCE ROLE CONFLATION Sources checked: InterPro:IPR010108 · Lycopene_cyclase_b/e SOURCE BAD The lycopene-cyclase family entry carries an FAD/NAD(P)-binding-like fold; InterPro2GO read the flavoprotein architecture as an oxygen-dependent oxidoreductase, but the family reaction is a non-redox cycloisomerization that uses no molecular oxygen. Supporting Evidence: PMID:20178989 Lycopene cyclase, thus, ranks among the novel class of non-redox flavoproteins PMID:20178989 evidence was obtained based on which a catalytic mechanism relying on cryptic (net) electron transfer can be refuted PMID:11943208 the hydrogen atom introduced at C(2) in the cyclization comes from water and not from NADPH file:PANAN/crtY/crtY-uniprot.txt Gene3D; 3.50.50.60; FAD/NAD(P)-binding domain; 1. file:PANAN/crtY/crtY-deep-research-falcon.md no net oxidation or reduction of the carotenoid occurs |
| GO:0045436 lycopene beta cyclase activity | IEA GO_REF:0000120 | ACCEPT | Summary: The defining molecular function of the gene, assigned electronically from the CrtY family signature and the Rhea reaction, and independently established for this protein by direct biochemistry. Reason: GO:0045436 is verified against QuickGO as catalysis of the cyclization of beta rings at one or both ends of lycopene to form gamma-carotene or bicyclic beta-carotene, which is exactly the reaction CrtY performs. The row is grounded in the CrtY_pseudo signature IPR008461 and in RHEA:32219 (all-trans-lycopene = gamma-carotene), the first of the two cyclization half-reactions UniProt records for this entry (RHEA:32239 completes the second ring to all-trans-beta-carotene), under EC 5.5.1.19. The activity is not merely inferred: the original functional analysis of the crt cluster established that CrtY cyclizes lycopene to beta-carotene, and the reaction and cofactor have since been dissected biochemically. This is the gene's core catalytic function and matches the required function recorded for the Pantoea CrtY exemplar in the carotene backbone biosynthesis module. Supporting Evidence: file:PANAN/crtY/crtY-uniprot.txt Catalyzes the double cyclization reaction which converts file:PANAN/crtY/crtY-uniprot.txt Reaction=all-trans-lycopene = gamma-carotene file:PANAN/crtY/crtY-uniprot.txt Reaction=gamma-carotene = all-trans-beta-carotene; file:PANAN/crtY/crtY-uniprot.txt InterPro; IPR008461; CrtY_pseudo. |
| GO:1901812 beta-carotene biosynthetic process | IDA PMID:20178989 The lycopene cyclase CrtY from Pantoea ananatis (formerly Er... | NEW | Summary: Proposed specific product-process term. CrtY directly catalyses both beta-ring cyclizations that build beta-carotene from lycopene, so it does the chemical work of beta-carotene formation and participates in this process by catalysis. Reason: GO:1901812 beta-carotene biosynthetic process is verified against QuickGO (formation of beta-carotene) and, per the review brief, is a descendant of GO:0016117 in the current ontology. The participation test is met in its strongest form: CrtY itself performs the reactions that create beta-carotene, forming one beta-ionone ring at each psi end of lycopene via gamma-carotene, so this is catalysis of the named process and not a substrate/necessity artefact. The second cyclization (gamma-carotene to all-trans-beta-carotene, RHEA:32239) is recorded by UniProt with experimental evidence from PubMed:20178989. The term names the enzyme's actual physiological product and adds real specificity over the generic carotenoid term the gene already holds; although it descends from GO:0016117, the more specific term is the more informative statement for a directly catalysing enzyme. The companion carotene-level term GO:0016120 is deliberately not proposed: it would be an ancestor of GO:1901812 and, in the current ontology, is not a descendant of GO:0016117, so adding it would be redundant rather than added coverage. Supporting Evidence: file:PANAN/crtY/crtY-uniprot.txt Catalyzes the double cyclization reaction which converts file:PANAN/crtY/crtY-uniprot.txt lycopene to beta-carotene file:PANAN/crtY/crtY-uniprot.txt Reaction=gamma-carotene = all-trans-beta-carotene; file:PANAN/crtY/crtY-uniprot.txt PATHWAY: Carotenoid biosynthesis; beta-carotene biosynthesis. |
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Download this section (compressed HTML)Q: How does a reduced FAD cofactor promote a reaction with no net redox change in the carotenoid; does it stabilize a carbocation intermediate as proposed, and which active-site residues complete the acid-base machinery alongside Glu-199?
Q: What is the structural basis for the two-end double cyclization; does CrtY process both psi ends in a single active site sequentially via gamma-carotene, and what determines the strict beta-ring (versus epsilon-ring) outcome?
Q: How is CrtY positioned at the inner membrane and how does it acquire its membrane-embedded lycopene substrate and release beta-carotene to CrtZ; is there a physical or kinetic coupling among the crt-cluster enzymes?
Experiment: Determine the structure of CrtY with bound FAD and a non-reactive lycopene analogue by crystallography or cryo-EM, identify the FAD and substrate pockets, and test candidate catalytic residues (including the region around Glu-199) by alanine substitution, measuring the effect on activity, FAD binding and product ring stereochemistry.
Hypothesis: CrtY uses a flavoprotein fold to house an acid-base cycloisomerase active site in which reduced FAD stabilizes a positively charged intermediate while a network including Glu-199 provides the proton chemistry, and no residue performs oxygen-dependent redox catalysis.
Type: structure determination and site-directed mutagenesis
Experiment: Reconstitute purified CrtY with defined FAD redox states and with flavin analogues, quantify turnover and any change in flavin oxidation state during catalysis by spectroscopy and mass spectrometry, and test isotope incorporation from labelled NAD(P)H and from solvent water into the product to confirm the proton source and the absence of net redox chemistry.
Hypothesis: Reduced FAD is required for catalysis but is not consumed and effects no net oxidation or reduction of the carotenoid, and NAD(P)H acts only to regenerate or maintain reduced FAD rather than participating in the cyclization.
Type: cofactor and mechanism assay
Experiment: Fractionate Pantoea ananatis into soluble and membrane fractions with validated marker controls, locate CrtY by immunoblot, probe the strength and orientation of the membrane association by salt and carbonate extraction and protease-accessibility assays on sealed vesicles, and image a functional tagged allele in the native organism.
Hypothesis: CrtY is a peripheral enzyme of the cytoplasmic face of the inner membrane rather than a soluble cytoplasmic protein or an integral membrane protein.
Type: subcellular fractionation and imaging
Experiment: Assay purified CrtY against a graded panel of acyclic carotenes including lycopene, neurosporene and zeta-carotene, determine steady-state kinetics for each, and identify the mono- and bicyclic products by HPLC and mass spectrometry to map the tolerance for reduced terminal structures already reported qualitatively.
Hypothesis: CrtY cyclizes a range of psi and 7,8-dihydro-psi end groups but favours all-trans-lycopene as its physiological substrate.
Type: substrate specificity panel
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: No experimental three-dimensional structure of CrtY has been determined, so the residues that bind FAD and lycopene and the geometry that positions the psi end group for proton-initiated ring closure are not directly known.
OPEN BIOLOGY
What is known: The reaction, substrate, product, EC classification, FAD requirement and the non-redox mechanism are established by direct biochemistry, and one catalytic residue is identified (the E199A mutant loses activity while still binding FAD). What is missing is an atomic-resolution model of the active site.
Significance: A structure would explain how a flavoprotein fold supports a non-redox cycloisomerization, locate the acid-base machinery that Glu-199 belongs to, and guide engineering of ring specificity in heterologous carotenoid production.
What would resolve it: Crystallography or cryo-EM of CrtY with bound FAD and a lycopene analogue, combined with site-directed mutagenesis around the proposed catalytic residues.
Provenance (the field's own admissions):
Gap: The membrane association of CrtY has not been characterized at the level of topology or orientation: it is treated as a peripheral, membrane-associated enzyme, but whether and how it docks onto the cytoplasmic face of the inner membrane is not resolved.
OPEN BIOLOGY
What is known: The inner-membrane location is experimentally supported and the sequence carries no signal peptide or transmembrane span, so an integral multipass topology is excluded; the unresolved point is the nature of the peripheral interaction and how the hydrophobic substrate is accessed.
Significance: How CrtY engages the membrane bears on how the insoluble lycopene substrate is delivered and how beta-carotene is handed to the downstream membrane enzymes CrtZ and CrtX.
What would resolve it: Cell fractionation with membrane and soluble marker controls, protease-accessibility assays on inverted and right-side-out vesicles, and imaging of a functional tagged allele in the native organism.
Provenance (the field's own admissions):
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