Kinetic Characterization of Prenyl-Flavin Synthase from Saccharomyces cerevisiae

Characterization of the N5-dimethylallyl-FMN Intermediate in the Biosynthesis of Prenylated-FMN Catalyzed by UbiX

Prenylated-FMN (prFMN) is the cofactor used by the UbiD-like family of decarboxylases that catalyzes the decarboxylation of various aromatic and unsaturated carboxylic acids. prFMN is synthesized from reduced FMN and dimethylallyl phosphate (DMAP) by a specialized prenyl transferase, UbiX. UbiX catalyzes the sequential formation of two bonds, the first between N5 of the flavin and C1 of DMAP, and the second between C6 of the flavin and C3 of DMAP. We have examined the reaction of UbiX with both FMN and riboflavin. Although UbiX converts FMN to prFMN, we show that significant amounts of the N5-dimethylallyl-FMN intermediate are released from the enzyme during catalysis. With riboflavin as the substrate, UbiX catalyzes only a partial reaction, resulting in only N5-dimethylallyl-riboflavin being formed. Purification of the N5-dimethylallyl-FMN adduct allowed its structure to be verified by 1H NMR spectroscopy and its reactivity to be investigated. Surprisingly, whereas reduced prFMN oxidizes in seconds to form the stable prFMN semiquinone radical when exposed to air, N5-dimethylallyl-FMN oxidizes much more slowly over several hours; in this case, oxidation is accompanied by spontaneous hydrolysis to regenerate FMN. These studies highlight the important contribution that cyclization of the prenyl-derived ring of prFMN makes to the cofactor's biological activity.

Negative Cooperativity in the Mechanism of Prenylated-Flavin-Dependent Ferulic Acid Decarboxylase: A Proposal for a "Two-Stroke" Decarboxylation Cycle

Ferulic acid decarboxylase (FDC) catalyzes the reversible carboxylation of various substituted phenylacrylic acids to produce the correspondingly substituted styrenes and CO₂. FDC is a member of the UbiD family of enzymes that use prenylated-FMN (prFMN) to catalyze decarboxylation reactions on aromatic rings and C-C double bonds. Although a growing number of prFMN-dependent enzymes have been identified, the mechanism of the reaction remains poorly understood. Here, we present a detailed pre-steady-state kinetic analysis of the FDC-catalyzed reaction of prFMN with both styrene and phenylacrylic acid. Based on the pattern of reactivity observed, we propose a "two-stroke" kinetic model in which negative cooperativity between the two subunits of the FDC homodimer plays an important and previously unrecognized role in catalysis. In this model, catalysis is initiated at the high-affinity active site, which reacts with phenylacrylate to yield, after decarboxylation, the covalently bound styrene-prFMN cycloadduct. In the second stage of the catalytic cycle, binding of the second substrate molecule to the low-affinity active site drives a conformational switch that interconverts the high-affinity and low-affinity active sites. This switching of affinity couples the energetically unfavorable cycloelimination of styrene from the first site with the energetically favorable cycloaddition and decarboxylation of phenylacrylate at the second site. We note as a caveat that, at this point, the complexity of the FDC kinetics leaves open other mechanistic interpretations and that further experiments will be needed to more firmly establish or refute our proposal.

Unravelling interactions between active site residues and DMAP in the initial steps of prenylated flavin mononucleotide biosynthesis catalyzed by PaUbiX

BACKGROUND

Prenylated flavin mononucleotide (prFMN) is a recently discovered, heavily modified flavin compound. It is the only known cofactor that enables enzymatic 1,3-dipolar cycloaddition reactions. It is produced by enzymes from the UbiX family, from flavin mononucleotide and either dimethylallyl mono- or diphosphate. prFMN biosynthesis is currently reported to be initiated by protonation of the substrate by Glu140.

METHODS

Computational chemistry methods are applied herein - Constant pH MD, classical MD simulations, and QM cluster optimizations.

RESULTS

Glu140 competes for a single proton with Lys129 prior to prFMN biosynthesis, but it is the latter that adopted a protonated state. Once the prenyl-FMN adduct is formed, Glu140 occurs in a protonated state far more often, while the occupancy of protonated Lys129 does not change. Lys129, Glu140, and Arg122 seem to play a key role in either stabilizing or protonating DMAP phosphate group within the PaUbiX active site throughout initial steps of prFMN biosynthesis.

CONCLUSIONS

The role of Lys129 in the functioning of PaUbiX is reported for the first time. Glu140 is unlikely to act as a proton donor in prFMN biosynthesis. Instead, Lys129 and Arg122 fulfil this role. Glu140 still plays a role in contributing to hydrogen-bond network. This behavior is most likely conserved throughout the UbiX family due to the structural similarity of the active sites of those proteins.

SIGNIFICANCE

Mechanistic insights into a crucial biochemical process, the biosynthesis of prFMN, are provided. This study, although purely computational, extends and perfectly complements the knowledge obtained in classical laboratory experiments.

Structural insights into UbiD reversible decarboxylation

The ubiquitous UbiX-UbiD system is associated with a wide range of microbial (de)carboxylation reactions. Recent X-ray crystallographic studies have contributed to elucidating the enigmatic mechanism underpinning the conversion of α,β-unsaturated acids by this system. The UbiD component utilises a unique cofactor, prenylated flavin (prFMN), generated by the bespoke action of the associated UbiX flavin prenyltransferase. Structure determination of a range of UbiX/UbiD representatives has revealed a generic mode of action for both the flavin-to-prFMN metamorphosis and the (de)carboxylation. In contrast to the conserved UbiX, the UbiD superfamily is associated with a versatile substrate range. The latter is reflected in the considerable variety of UbiD quaternary structure, dynamic behaviour and active site architecture. Directed evolution of UbiD enzymes has taken advantage of this apparent malleability to generate new variants supporting in vivo hydrocarbon production. Other applications include coupling UbiD to carboxylic acid reductase to convert alkenes into α,β-unsaturated aldehydes via enzymatic CO₂ fixation.

Isopentenyl diphosphate/dimethylallyl diphosphate-specific Nudix hydrolase from the methanogenic archaeon Methanosarcina mazei

Nudix hydrolases typically catalyze the hydrolysis of nucleoside diphosphate linked to moiety X and yield nucleoside monophosphate and X-phosphate, while some of them hydrolyze a terminal diphosphate group of non-nucleosidic compounds and convert it into a phosphate group. Although the number of Nudix hydrolases is usually limited in archaea comparing with those in bacteria and eukaryotes, the physiological functions of most archaeal Nudix hydrolases remain unknown. In this study, a Nudix hydrolase family protein, MM_2582, from the methanogenic archaeon Methanosarcina mazei was recombinantly expressed in Escherichia coli, purified, and characterized. This recombinant protein shows higher hydrolase activity toward isopentenyl diphosphate and short-chain prenyl diphosphates than that toward nucleosidic compounds. Kinetic studies demonstrated that the archaeal enzyme prefers isopentenyl diphosphate and dimethylallyl diphosphate, which suggests its role in the biosynthesis of prenylated flavin mononucleotide, a recently discovered coenzyme that is required, for example, in the archaea-specific modified mevalonate pathway.

The In Vitro Production of prFMN for Reconstitution of UbiD Enzymes

Prenylated flavin (prFMN) is a modified FMN cofactor, the isoalloxazine is extended by an additional six membered nonaromatic ring. The modification confers azomethine ylide characteristics on the oxidised prFMN, allowing it to support the reversible nonoxidative decarboxylation of unsaturated acids by the UbiD family of decarboxylases. In absence of a chemical synthesis route for prFMN, enzymatic production by the flavin prenyltransferase, UbiX, is required for in vitro reconstitution of prFMN-dependent enzymes. Here we provide an overview of the methods for producing prFMN in vitro using the flavin prenyltransferase UbiX, and the subsequent reconstitution and activation of UbiD enzymes.

Kinetic Analysis of Transient Intermediates in the Mechanism of Prenyl-Flavin-Dependent Ferulic Acid Decarboxylase

Ferulic acid decarboxylase catalyzes the decarboxylation of various substituted phenylacrylic acids to their corresponding styrene derivatives and CO₂ using the recently discovered cofactor prenylated FMN (prFMN). The mechanism involves an unusual 1,3-dipolar cycloaddition reaction between prFMN and the substrate to generate a cycloadduct capable of undergoing decarboxylation. Using native mass spectrometry, we show the enzyme forms a stable prFMN-styrene cycloadduct that accumulates on the enzyme during turnover. Pre-steady state kinetic analysis of the reaction using ultraviolet-visible stopped-flow spectroscopy reveals a complex pattern of kinetic behavior, best described by a half-of-sites model involving negative cooperativity between the two subunits of the dimeric enzyme. For the reactive site, the cycloadduct of prFMN with phenylacylic acid is formed with a k_app of 131 s^-1. This intermediate converts to the prFMN-styrene cycloadduct with a k_app of 75 s^-1. Cycloelimination of the prFMN-styrene cycloadduct to generate styrene and free enzyme appears to determine k_cat for the overall reaction, which is 11.3 s^-1.

Biochemistry of prenylated-FMN enzymes

The reversible (de)carboxylation of unsaturated carboxylic acids is carried out by the UbiX-UbiD system, ubiquitously present in microbes. The biochemical basis of this challenging reaction has recently been uncovered by the discovery of the UbiD cofactor, prenylated FMN (prFMN). This heavily modified flavin is synthesized by the flavin prenyltransferase UbiX, which catalyzes the non-metal dependent prenyl transfer from dimethylallyl(pyro)phosphate (DMAP(P)) to the flavin N5 and C6 positions, creating a fourth non-aromatic ring. Following prenylation, prFMN undergoes oxidative maturation to form the iminium species required for UbiD activity. prFMN^iminium acts as a prostethic group and is bound via metal ion mediated interactions between UbiD and the prFMN^iminium phosphate moiety. The modified isoalloxazine ring is place adjacent to the E(D)-R-E UbiD signature sequent motif. The fungal ferulic acid decarboxylase Fdc from Aspergillus niger has emerged as a UbiD-model system, and has yielded atomic level insight into the prFMN^iminium mediated (de)carboxylation. A wealth of data now supports a mechanism reliant on reversible 1,3 dipolar cycloaddition between substrate and cofactor for this enzyme. This poses the intriguing question whether a similar mechanism is used by all UbiD enzymes, especially those that act as carboxylases on inherently more difficult substrates such as phenylphosphate or benzene/naphthalene. Indeed, considerable variability in terms of oligomerization, domain motion and active site structure is now reported for the UbiD family.

Reconstruction of the "Archaeal" Mevalonate Pathway from the Methanogenic Archaeon Methanosarcina mazei in Escherichia coli Cells

The mevalonate pathway is a well-known metabolic route that provides biosynthetic precursors for myriad isoprenoids. An unexpected variety of the pathway has been discovered from recent studies on microorganisms, mainly on archaea. The most recently discovered example, called the "archaeal" mevalonate pathway, is a modified version of the canonical eukaryotic mevalonate pathway and was elucidated in our previous study using the hyperthermophilic archaeon Aeropyrum pernix This pathway comprises four known enzymes that can produce mevalonate 5-phosphate from acetyl coenzyme A, two recently discovered enzymes designated phosphomevalonate dehydratase and anhydromevalonate phosphate decarboxylase, and two more known enzymes, i.e., isopentenyl phosphate kinase and isopentenyl pyrophosphate:dimethylallyl pyrophosphate isomerase. To show its wide distribution in archaea and to confirm if its enzyme configuration is identical among species, the putative genes of a lower portion of the pathway-from mevalonate to isopentenyl pyrophosphate-were isolated from the methanogenic archaeon Methanosarcina mazei, which is taxonomically distant from A. pernix, and were introduced into an engineered Escherichia coli strain that produces lycopene, a red carotenoid pigment. Lycopene production, as a measure of isoprenoid productivity, was enhanced when the cells were grown semianaerobically with the supplementation of mevalonolactone, which demonstrates that the archaeal pathway can function in bacterial cells to convert mevalonate into isopentenyl pyrophosphate. Gene deletion and complementation analysis using the carotenogenic E. coli strain suggests that both phosphomevalonate dehydratase and anhydromevalonate phosphate decarboxylase from M. mazei are required for the enhancement of lycopene production.IMPORTANCE Two enzymes that have recently been identified from the hyperthermophilic archaeon A. pernix as components of the archaeal mevalonate pathway do not require ATP for their reactions. This pathway, therefore, might consume less energy than other mevalonate pathways to produce precursors for isoprenoids. Thus, the pathway might be applicable to metabolic engineering and production of valuable isoprenoids that have application as pharmaceuticals. The archaeal mevalonate pathway was successfully reconstructed in E. coli cells by introducing several genes from the methanogenic or hyperthermophilic archaeon, which demonstrated that the pathway requires the same components even in distantly related archaeal species and can function in bacterial cells.

The UbiX flavin prenyltransferase reaction mechanism resembles class I terpene cyclase chemistry

The UbiX-UbiD enzymes are widespread in microbes, acting in concert to decarboxylate alpha-beta unsaturated carboxylic acids using a highly modified flavin cofactor, prenylated FMN (prFMN). UbiX serves as the flavin prenyltransferase, extending the isoalloxazine ring system with a fourth non-aromatic ring, derived from sequential linkage between a dimethylallyl moiety and the FMN N5 and C6. Using structure determination and solution studies of both dimethylallyl monophosphate (DMAP) and dimethyallyl pyrophosphate (DMAPP) dependent UbiX enzymes, we reveal the first step, N5-C1' bond formation, is contingent on the presence of a dimethylallyl substrate moiety. Hence, an SN1 mechanism similar to other prenyltransferases is proposed. Selected variants of the (pyro)phosphate binding site are unable to catalyse subsequent Friedel-Crafts alkylation of the flavin C6, but can be rescued by addition of (pyro)phosphate. Thus, retention of the (pyro)phosphate leaving group is required for C6-C3' bond formation, resembling pyrophosphate initiated class I terpene cyclase reaction chemistry.

Heterologous production, reconstitution and EPR spectroscopic analysis of prFMN dependent enzymes

The recent discovery of the prenylated FMN (prFMN) cofactor has led to a renewed interest in the prFMN-dependent UbiD family of enzymes. The latter catalyses the reversible decarboxylation of alpha-beta unsaturated carboxylic acids and features widely in microbial metabolism. The flavin prenyltransferase UbiX synthesizes prFMN from reduced FMN and phosphorylated dimethylallyl precursors. Oxidative maturation of the resulting prFMN^reduced species to the active prFMN^iminium form is required for UbiD activity. Heterologous production of active holo-UbiD requires co-expression of UbiX, but the levels of prFMN incorporation and oxidative maturation appear variable. Detailed protocols and strategies for in vitro reconstitution and oxidative maturation of UbiD are presented that can yield an alternative source of active holo-UbiD for biochemical studies.

Flavin metamorphosis: cofactor transformation through prenylation

Prenylated flavin (prFMN) is a recently discovered cofactor that underpins catalysis in the ubiquitous microbial UbiDX system. UbiX acts as a flavin prenyltransferase while UbiD is a prFMN-dependent reversible (de)carboxylase. The extensive modification of flavin by prenylation, and the consecutive oxidation to the prFMNiminium azomethine ylide, leads to cofactor metamorphosis. While prFMN is no longer able to perform N5-based classical flavin chemistry, it is capable of forming cycloadducts with dipolarophiles, long-lived C4a-based radical species as well as undergoing extensive light driven isomerization. An ever-expanding range of distinct prFMN forms hints at the possibility of novel prFMN driven biochemistry yet to be discovered.

Ligand-Driven Conformational Dynamics Influences Selectivity of UbiX

Up until now, it has remained elusive as to why the flavin prenyltransferase UbiX requires dimethylallyl monophosphate (DMAP) as one of its cosubstrates instead of dimethylallyl pyrophosphate (DMAPP), even though the former is not used in metabolic pathways, while the latter is a common isoprenoid precursor. Herein, mainly on the basis of molecular dynamics (MD) simulations, we demonstrate that the selectivity of UbiX may be governed by its conformational dynamics. The hydrogen-bonding network of UbiX does not facilitate a proper encompassing of DMAPP. This induces significant conformational changes of the enzyme that result mostly in unreactive trajectories, whereas DMAP remains at a catalytically competent position throughout the performed simulations. Within the presented study, we provide a justification for the atypical selectivity of UbiX.

Biochemical and Structural Characterization of TtnD, a Prenylated FMN-Dependent Decarboxylase from the Tautomycetin Biosynthetic Pathway

Tautomycetin (TTN) is a polyketide natural product featuring a terminal alkene. Functional characterization of the genes within the ttn gene cluster from Streptomyces griseochromogenes established the biosynthesis of the TTN polyketide backbone, its dialkylmaleic anhydride moiety, the coupling of the two moieties to form the nascent intermediate TTN F-1, and the tailoring steps converting TTN F-1 to TTN. Here, we report biochemical and structural characterization of TtnD, a prenylated FMN (prFMN)-dependent decarboxylase belonging to the UbiD family that catalyzes the penultimate step of TTN biosynthesis. TtnD catalyzes decarboxylation of TTN D-1 to TTN I-1, utilizing prFMN as a cofactor generated by the TtnC flavin prenyltransferase; both TtnD and TtnC are encoded within the ttn biosynthetic gene cluster. TtnD exhibits substrate promiscuity but accepts only TTN D-1 congeners that feature an α,β-unsaturated acid, supporting the [3+2] cycloaddition mechanism during catalysis that requires the double bond of an α,β-unsaturated acid substrate. TtnD shares a similar overall structure with other members of the UbiD family but forms a homotetramer in solution. Each protomer is composed of three domains with the active site located between the middle and C-terminal domains; R169-E272-E277, constituting the catalytic triad, and E228, involved in Mn(II)-mediated binding of prFMN, were confirmed by site-directed mutagenesis. TtnD represents the first example of a prFMN-dependent decarboxylase involved in polyketide biosynthesis, expanding the substrate scope of the UbiD family of decarboxylases beyond simple aromatic and cinnamic acids. TtnD and its homologues are widespread in nature and could be exploited as biocatalysts for organic synthesis.

An Engineered Aro1 Protein Degradation Approach for Increased cis,cis-Muconic Acid Biosynthesis in Saccharomyces cerevisiae

Muconic acid (MA) is a chemical building block and precursor to adipic and terephthalic acids used in the production of nylon and polyethylene terephthalate polymer families. Global demand for these important materials, coupled to their dependence on petrochemical resources, provides substantial motivation for the microbial synthesis of MA and its derivatives. In this context, the Saccharomyces cerevisiae yeast shikimate pathway can be sourced as a precursor for the formation of MA. Here we report a novel strategy to balance MA pathway performance with aromatic amino acid prototrophy by destabilizing Aro1 through C-terminal degron tagging. Coupling of a composite MA production pathway to degron-tagged Aro1 in an aro3Δ aro4Δ mutant background led to the accumulation of 5.6 g/liter protocatechuic acid (PCA). However, metabolites downstream of PCA were not detected, despite the inclusion of genes mediating their biosynthesis. Because CEN.PK family strains of S. cerevisiae lack the activity of Pad1, a key enzyme supporting PCA decarboxylase activity, chromosomal expression of intact PAD1 alleviated this bottleneck, resulting in nearly stoichiometric conversion (95%) of PCA to downstream products. In a fed-batch bioreactor, the resulting strain produced 1.2 g/liter MA under prototrophic conditions and 5.1 g/liter MA when supplemented with amino acids, corresponding to a yield of 58 mg/g sugar.IMPORTANCE Previous efforts to engineer a heterologous MA pathway in Saccharomyces cerevisiae have been hindered by a bottleneck at the PCA decarboxylation step and the creation of aromatic amino acid auxotrophy through deleterious manipulation of the pentafunctional Aro1 protein. In light of these studies, this work was undertaken with the central objective of preserving amino acid prototrophy, which we achieved by employing an Aro1 degradation strategy. Moreover, resolution of the key PCA decarboxylase bottleneck, as detailed herein, advances our understanding of yeast MA biosynthesis and will guide future strain engineering efforts. These strategies resulted in the highest titer reported to date for muconic acid produced in yeast. Overall, our study showcases the effectiveness of careful tuning of yeast Aro1 activity and the importance of host-pathway dynamics.

Formation and Cleavage of C-C Bonds by Enzymatic Oxidation-Reduction Reactions

Many oxidation-reduction (redox) enzymes, particularly oxygenases, have roles in reactions that make and break C-C bonds. The list includes cytochrome P450 and other heme-based monooxygenases, heme-based dioxygenases, nonheme iron mono- and dioxygenases, flavoproteins, radical S-adenosylmethionine enzymes, copper enzymes, and peroxidases. Reactions involve steroids, intermediary metabolism, secondary natural products, drugs, and industrial and agricultural chemicals. Many C-C bonds are formed via either (i) coupling of diradicals or (ii) generation of unstable products that rearrange. C-C cleavage reactions involve several themes: (i) rearrangement of unstable oxidized products produced by the enzymes, (ii) oxidation and collapse of radicals or cations via rearrangement, (iii) oxygenation to yield products that are readily hydrolyzed by other enzymes, and (iv) activation of O₂ in systems in which the binding of a substrate facilitates O₂ activation. Many of the enzymes involve metals, but of these, iron is clearly predominant.