Two Distinct Mechanisms for C-C Desaturation by Iron(II)- and 2-(Oxo)glutarate-Dependent Oxygenases: Importance of α-Heteroatom Assistance

An Unusual Ferryl Intermediate and Its Implications for the Mechanism of Oxacyclization by the Loline-Producing Iron(II)- and 2-Oxoglutarate-Dependent Oxygenase, LolO

N-Acetylnorloline synthase (LolO) is one of several iron(II)- and 2-oxoglutarate-dependent (Fe/2OG) oxygenases that catalyze sequential reactions of different types in the biosynthesis of valuable natural products. LolO hydroxylates C2 of 1-exo-acetamidopyrrolizidine before coupling the C2-bonded oxygen to C7 to form the tricyclic loline core. Each reaction requires cleavage of a C-H bond by an oxoiron(IV) (ferryl) intermediate; however, different carbons are targeted, and the carbon radicals have different fates. Prior studies indicated that the substrate-cofactor disposition (SCD) controls the site of H· abstraction and can affect the reaction outcome. These indications led us to determine whether a change in SCD from the first to the second LolO reaction might contribute to the observed reactivity switch. Whereas the single ferryl complex in the C2 hydroxylation reaction was previously shown to have typical Mössbauer parameters, one of two ferryl complexes to accumulate during the oxacyclization reaction has the highest isomer shift seen to date for such a complex and abstracts H· from C7 ∼ 20 times faster than does the first ferryl complex in its previously reported off-pathway hydroxylation of C7. The detectable hydroxylation of C7 in competition with cyclization by the second ferryl complex is not enhanced in 2H2O solvent, suggesting that the C2 hydroxyl is deprotonated prior to C7-H cleavage. These observations are consistent with the coordination of the C2 oxygen to the ferryl complex, which may reorient its oxo ligand, the substrate, or both to positions more favorable for C7-H cleavage and oxacyclization.

Controllable multi-halogenation of a non-native substrate by SyrB2 iron halogenase

Geminal, multi-halogenated functional groups are widespread in natural products and pharmaceuticals, yet no synthetic methodologies exist that enable selective multi-halogenation of unactivated C-H bonds. Biocatalysts are powerful tools for late-stage C-H functionalization, as they operate with high degrees of regio-, chemo-, and stereoselectivity. 2-oxoglutarate (2OG)-dependent non-heme iron halogenases chlorinate and brominate aliphatic C-H bonds offering a solution for achieving these challenging transformations. Here, we describe the ability of a non-heme iron halogenase, SyrB2, to controllably halogenate non-native substrate alpha-aminobutyric acid (Aba) to yield mono-chlorinated, di-chlorinated, and tri-chlorinated products. These chemoselective outcomes are achieved by controlling the loading of 2OG cofactor and SyrB2 biocatalyst. By using a ferredoxin-based biological reductant for electron transfer to the catalytic center of SyrB2, we demonstrate order-of-magnitude enhancement in the yield of tri-chlorinated product that were previously inaccessible using any single halogenase enzyme. We also apply these strategies to broaden SyrB2's reactivity scope to include multi-bromination and demonstrate chemoenzymatic conversion of the ethyl side chain in Aba to an ethylyne functional group. We show how steric hindrance induced by the successive addition of halogen atoms on Aba's C4 carbon dictates the degree of multi-halogenation by hampering C3-C4 bond rotation within SyrB2's catalytic pocket. Overall, our work showcases the synthetic potential of iron halogenases to facilitate multi-C-H functionalization chemistry.

Computational Study Into the Oxidative Ring-Closure Mechanism During the Biosynthesis of Deoxypodophyllotoxin

The nonheme iron dioxygenase deoxypodophyllotoxin synthase performs an oxidative ring-closure reaction as part of natural product synthesis in plants. How the enzyme enables the oxidative ring-closure reaction of (-)-yatein and avoids substrate hydroxylation remains unknown. To gain insight into the reaction mechanism and understand the details of the pathways leading to products and by-products we performed a comprehensive computational study. The work shows that substrate is bound tightly into the substrate binding pocket with the C7'-H bond closest to the iron(IV)-oxo species. The reaction proceeds through a radical mechanism starting with hydrogen atom abstraction from the C7'-H position followed by ring-closure and a final hydrogen transfer to form iron(II)-water and deoxypodophyllotoxin. Alternative mechanisms including substrate hydroxylation and an electron transfer pathway were explored but found to be higher in energy. The mechanism is guided by electrostatic perturbations of charged residues in the second-coordination sphere that prevent alternative pathways.

Guanidine production by plant homoarginine-6-hydroxylases

Metabolism and biological functions of the nitrogen-rich compound guanidine have long been neglected. The discovery of four classes of guanidine-sensing riboswitches and two pathways for guanidine degradation in bacteria hint at widespread sources of unconjugated guanidine in nature. So far, only three enzymes from a narrow range of bacteria and fungi have been shown to produce guanidine, with the ethylene-forming enzyme (EFE) as the most prominent example. Here, we show that a related class of Fe2+- and 2-oxoglutarate-dependent dioxygenases (2-ODD-C23) highly conserved among plants and algae catalyze the hydroxylation of homoarginine at the C6-position. Spontaneous decay of 6-hydroxyhomoarginine yields guanidine and 2-aminoadipate-6-semialdehyde. The latter can be reduced to pipecolate by pyrroline-5-carboxylate reductase but more likely is oxidized to aminoadipate by aldehyde dehydrogenase ALDH7B in vivo. Arabidopsis has three 2-ODD-C23 isoforms, among which Din11 is unusual because it also accepted arginine as substrate, which was not the case for the other 2-ODD-C23 isoforms from Arabidopsis or other plants. In contrast to EFE, none of the three Arabidopsis enzymes produced ethylene. Guanidine contents were typically between 10 and 20 nmol*(g fresh weight)-1 in Arabidopsis but increased to 100 or 300 nmol*(g fresh weight)-1 after homoarginine feeding or treatment with Din11-inducing methyljasmonate, respectively. In 2-ODD-C23 triple mutants, the guanidine content was strongly reduced, whereas it increased in overexpression plants. We discuss the implications of the finding of widespread guanidine-producing enzymes in photosynthetic eukaryotes as a so far underestimated branch of the bio-geochemical nitrogen cycle and propose possible functions of natural guanidine production.

How Do Differences in Electronic Structure Affect the Use of Vanadium Intermediates as Mimics in Nonheme Iron Hydroxylases?

We study active-site models of nonheme iron hydroxylases and their vanadium-based mimics using density functional theory to determine if vanadyl is a faithful structural mimic. We identify crucial structural and energetic differences between ferryl and vanadyl isomers owing to the differences in their ground electronic states, i.e., high spin (HS) for Fe and low spin (LS) for V. For the succinate cofactor bound to the ferryl intermediate, we predict facile interconversion between monodentate and bidentate coordination isomers for ferryl species but difficult rearrangement for vanadyl mimics. We study isomerization of the oxo intermediate between axial and equatorial positions and find the ferryl potential energy surface to be characterized by a large barrier of ca. 10 kcal/mol that is completely absent for the vanadyl mimic. This analysis reveals even starker contrasts between Fe and V in hydroxylases than those observed for this metal substitution in nonheme halogenases. Analysis of the relative bond strengths of coordinating carboxylate ligands for Fe and V reveals that all of the ligands show stronger binding to V than Fe owing to the LS ground state of V in contrast to the HS ground state of Fe, highlighting the limitations of vanadyl mimics of native nonheme iron hydroxylases.

Heterologous Biosynthesis of the Sterol O-Acyltransferase Inhibitor Helvamide Unveils an α-Ketoglutarate-Dependent Cross-Linking Oxygenase

We have identified the biosynthetic gene cluster (hvm) for the sterol O-acyltransferase inhibitor helvamide (1) from the genome of Aspergillus rugulosus MST-FP2007. Heterologous expression of hvm in A. nidulans produced a previously unreported analog helvamide B (5). An α-ketoglutarate-dependent oxygenase Hvm1 was shown to catalyze intramolecular cyclization of 1 to yield 5. The biosynthetic branch to the related hancockiamides and helvamides was found to be controlled by the substrate selectivity of monomodular nonribosomal peptide synthetases.

Poly-γ-glutamylation of biomolecules

Poly-γ-glutamate tails are a distinctive feature of archaeal, bacterial, and eukaryotic cofactors, including the folates and F420. Despite decades of research, key mechanistic questions remain as to how enzymes successively add glutamates to poly-γ-glutamate chains while maintaining cofactor specificity. Here, we show how poly-γ-glutamylation of folate and F420 by folylpolyglutamate synthases and γ-glutamyl ligases, non-homologous enzymes, occurs via processive addition of L-glutamate onto growing γ-glutamyl chain termini. We further reveal structural snapshots of the archaeal γ-glutamyl ligase (CofE) in action, crucially including a bulged-chain product that shows how the cofactor is retained while successive glutamates are added to the chain terminus. This bulging substrate model of processive poly-γ-glutamylation by terminal extension is arguably ubiquitous in such biopolymerisation reactions, including addition to folates, and demonstrates convergent evolution in diverse species from archaea to humans.

Catalytic divergencies in the mechanism of L-arginine hydroxylating nonheme iron enzymes

Many enzymes in nature utilize a free arginine (L-Arg) amino acid to initiate the biosynthesis of natural products. Examples include nitric oxide synthases, which generate NO from L-Arg for blood pressure control, and various arginine hydroxylases involved in antibiotic biosynthesis. Among the groups of arginine hydroxylases, several enzymes utilize a nonheme iron(II) active site and let L-Arg react with dioxygen and α-ketoglutarate to perform either C3-hydroxylation, C4-hydroxylation, C5-hydroxylation, or C4-C5-desaturation. How these seemingly similar enzymes can react with high specificity and selectivity to form different products remains unknown. Over the past few years, our groups have investigated the mechanisms of L-Arg-activating nonheme iron dioxygenases, including the viomycin biosynthesis enzyme VioC, the naphthyridinomycin biosynthesis enzyme NapI, and the streptothricin biosynthesis enzyme OrfP, using computational approaches and applied molecular dynamics, quantum mechanics on cluster models, and quantum mechanics/molecular mechanics (QM/MM) approaches. These studies not only highlight the differences in substrate and oxidant binding and positioning but also emphasize on electronic and electrostatic differences in the substrate-binding pockets of the enzymes. In particular, due to charge differences in the active site structures, there are changes in the local electric field and electric dipole moment orientations that either strengthen or weaken specific substrate C-H bonds. The local field effects, therefore, influence and guide reaction selectivity and specificity and give the enzymes their unique reactivity patterns. Computational work using either QM/MM or density functional theory (DFT) on cluster models can provide valuable insights into catalytic reaction mechanisms and produce accurate and reliable data that can be used to engineer proteins and synthetic catalysts to perform novel reaction pathways.

Electronic Structure and Reactivity of Mononuclear Nonheme Iron-Peroxo Complexes as a Biomimetic Model of Rieske Oxygenases: Ring Size Effects of Macrocyclic Ligands

We report the macrocyclic ring size-electronic structure-electrophilic reactivity correlation of mononuclear nonheme iron(III)-peroxo complexes bearing N-tetramethylated cyclam analogues (n-TMC), [FeIII(O2)(12-TMC)]+ (1), [FeIII(O2)(13-TMC)]+ (2), and [FeIII(O2)(14-TMC)]+ (3), as a model study of Rieske oxygenases. The Fe(III)-peroxo complexes show the same δ and pseudo-σ bonds between iron and the peroxo ligand. However, the strength of these interactions varies depending on the ring size of the n-TMC ligands; the overall Fe-O bond strength and the strength of the Fe-O2 δ bond increase gradually as the ring size of the n-TMC ligands becomes smaller, such as from 14-TMC to 13-TMC to 12-TMC. MCD spectroscopy plays a key role in assigning the characteristic low-energy δ → δ* LMCT band, which provides direct insight into the strength of the Fe-O2 δ bond and which, in turn, is correlated with the superoxo character of the iron-peroxo group. In oxidation reactions, reactivities of 1-3 toward hydrocarbon C-H bond activation are compared, revealing the reactivity order of 1 > 2 > 3; the [FeIII(O2)(n-TMC)]+ complex with a smaller n-TMC ring size, 12-TMC, is much more reactive than that with a larger n-TMC ring size, 14-TMC. DFT analysis shows that the Fe(III)-peroxo complex is not reactive toward C-H bonds, but it is the end-on Fe(II)-superoxo valence tautomer that is responsible for the observed reactivity. The hydrogen atom abstraction (HAA) reactivity of these intermediates is correlated with the overall donicity of the n-TMC ligand, which modulates the energy of the singly occupied π* superoxo frontier orbital that serves as the electron acceptor in the HAA reaction. The implications of these results for the mechanism of Rieske oxygenases are further discussed.

Deciphering Deoxynybomycin Biosynthesis Reveals Fe(II)/α-Ketoglutarate-Dependent Dioxygenase-Catalyzed Oxazoline Ring Formation and Decomposition

The antibacterial agents deoxynybomycin (DNM) and nybomycin (NM) have a unique tetracyclic structure featuring an angularly fused 4-oxazoline ring. Here, we report the identification of key enzymes responsible for forming the 4-oxazoline ring in Embleya hyalina NBRC 13850 by comparative bioinformatics analysis of the biosynthetic gene clusters encoding structurally similar natural products DNM, deoxynyboquinone (DNQ), and diazaquinomycins (DAQs). The N-methyltransferase DnmS plays a crucial role in catalyzing the N-dimethylation of a tricyclic precursor prenybomycin to generate NM D; subsequently, the Fe(II)/α-ketoglutarate-dependent dioxygenase (Fe/αKGD) DnmT catalyzes the formation of a 4-oxazoline ring from NM D to produce DNM; finally, a second Fe/αKGD DnmU catalyzes the C-12 hydroxylation of DNM to yield NM. Strikingly, DnmT is shown to display unexpected functions to also catalyze the decomposition of the 4-oxazoline ring and the N-demethylation, thereby converting DNM back to prenybomycin, to putatively serve as a manner to control the intracellular yield of DNM. Structure modeling, site-directed mutagenesis, and quantum mechanics calculations provide mechanistic insights into the DnmT-catalyzed reactions. This work expands our understanding of the functional diversity of Fe/αKGDs in natural product biosynthesis.

LcSAO1, an Unconventional DOXB Clade 2OGD Enzyme from Ligusticum chuanxiong Catalyzes the Biosynthesis of Plant-Derived Natural Medicine Butylphthalide

Butylphthalide, a prescription medicine recognized for its efficacy in treating ischemic strokes approved by the State Food and Drug Administration of China in 2005, is sourced from the traditional botanical remedy Ligusticum chuanxiong. While chemical synthesis offers a viable route, limitations in the production of isomeric variants with compromised bioactivity necessitate alternative strategies. Addressing this issue, biosynthesis offers a promising solution. However, the intricate in vivo pathway for butylphthalide biosynthesis remains elusive. In this study, we examined the distribution of butylphthalide across various tissues of L. chuanxiong and found a significant accumulation in the rhizome. By searching transcriptome data from different tissues of L. chuanxiong, we identified four rhizome-specific genes annotated as 2-oxoglutarate-dependent dioxygenase (2-OGDs) that emerged as promising candidates involved in butylphthalide biosynthesis. Among them, LcSAO1 demonstrates the ability to catalyze the desaturation of senkyunolide A at the C-4 and C-5 positions, yielding the production of butylphthalide. Experimental validation through transient expression assays in Nicotiana benthamiana corroborates this transformative enzymatic activity. Notably, phylogenetic analysis of LcSAO1 revealed that it belongs to the DOXB clade, which typically encompasses genes with hydroxylation activity, rather than desaturation. Further structure modelling and site-directed mutagenesis highlighted the critical roles of three amino acid residues, T98, S176, and T178, in substrate binding and enzyme activity. By unraveling the intricacies of the senkyunolide A desaturase, the penultimate step in the butylphthalide biosynthesis cascade, our findings illuminate novel avenues for advancing synthetic biology research in the realm of medicinal natural products.

A chlorophyll c synthase widely co-opted by phytoplankton

Marine and terrestrial photosynthesis exhibit a schism in the accessory chlorophyll (Chl) that complements the function of Chl a: Chl b for green plants versus Chl c for most eukaryotic phytoplankton. The enzymes that mediate Chl c biosynthesis have long remained elusive. In this work, we identified the CHLC dioxygenase (Phatr3_J43737) from the marine diatom Phaeodactylum tricornutum as the Chl c synthase. The chlc mutants lacked Chl c, instead accumulating its precursors, and exhibited growth defects. In vitro, recombinant CHLC protein converted these precursors into Chl c, thereby confirming its identity. Phylogenetic evidence demonstrates conserved use of CHLC across phyla but also the existence of distinct Chl c synthases in different algal groups. Our study addresses a long-outstanding question with implications for both contemporary and ancient marine photosynthesis.

Interaction of Vanadium Complexes with Proteins: Revisiting the Reported Structures in the Protein Data Bank (PDB) since 2015

The structural determination and characterization of molecules, namely proteins and enzymes, is crucial to gaining a better understanding of their role in different chemical and biological processes. The continuous technical developments in the experimental and computational resources of X-ray diffraction (XRD) and, more recently, cryogenic Electron Microscopy (cryo-EM) led to an enormous growth in the number of structures deposited in the Protein Data Bank (PDB). Bioinorganic chemistry arose as a relevant discipline in biology and therapeutics, with a massive number of studies reporting the effects of metal complexes on biological systems, with vanadium complexes being one of the relevant systems addressed. In this review, we focus on the interactions of vanadium compounds (VCs) with proteins. Several types of binding are established between VCs and proteins/enzymes. Considering that the V-species that bind may differ from those initially added, the mentioned structural techniques are pivotal to clarifying the nature and variety of interactions of VCs with proteins and to proposing the mechanisms involved either in enzymatic inhibition or catalysis. As such, we provide an account of the available structural information of VCs bound to proteins obtained by both XRD and/or cryo-EM, mainly exploring the more recent structures, particularly those containing organic-based vanadium complexes.

Synergistic Binding of the Halide and Cationic Prime Substrate of l-Lysine 4-Chlorinase, BesD, in Both Ferrous and Ferryl States

An aliphatic halogenase requires four substrates: 2-oxoglutarate (2OG), halide (Cl- or Br-), the halogenation target ("prime substrate"), and dioxygen. In well-studied cases, the three nongaseous substrates must bind to activate the enzyme's Fe(II) cofactor for efficient capture of O2. Halide, 2OG, and (lastly) O2 all coordinate directly to the cofactor to initiate its conversion to a cis-halo-oxo-iron(IV) (haloferryl) complex, which abstracts hydrogen (H•) from the non-coordinating prime substrate to enable radicaloid carbon-halogen coupling. We dissected the kinetic pathway and thermodynamic linkage in binding of the first three substrates of the l-lysine 4-chlorinase, BesD. After addition of 2OG, subsequent coordination of the halide to the cofactor and binding of cationic l-Lys near the cofactor are associated with strong heterotropic cooperativity. Progression to the haloferryl intermediate upon the addition of O2 does not trap the substrates in the active site and, in fact, markedly diminishes cooperativity between halide and l-Lys. The surprising lability of the BesD•[Fe(IV)=O]•Cl•succinate•l-Lys complex engenders pathways for decay of the haloferryl intermediate that do not result in l-Lys chlorination, especially at low chloride concentrations; one identified pathway involves oxidation of glycerol. The mechanistic data imply (i) that BesD may have evolved from a hydroxylase ancestor either relatively recently or under weak selective pressure for efficient chlorination and (ii) that acquisition of its activity may have involved the emergence of linkage between l-Lys binding and chloride coordination following the loss of the anionic protein-carboxylate iron ligand present in extant hydroxylases.

Synergistic Binding of the Halide and Cationic Prime Substrate of the l-Lysine 4-Chlorinase, BesD, in Both Ferrous and Ferryl States

An aliphatic halogenase requires four substrates: 2-oxoglutarate (2OG), halide (Cl - or Br - ), the halogenation target ("prime substrate"), and dioxygen. In well-studied cases, the three non-gaseous substrates must bind to activate the enzyme's Fe(II) cofactor for efficient capture of O 2 . Halide, 2OG, and (lastly) O 2 all coordinate directly to the cofactor to initiate its conversion to a cis -halo-oxo-iron(IV) (haloferryl) complex, which abstracts hydrogen (H•) from the non-coordinating prime substrate to enable radicaloid carbon-halogen coupling. We dissected the kinetic pathway and thermodynamic linkage in binding of the first three substrates of the l -lysine 4-chlorinase, BesD. After 2OG adds, subsequent coordination of the halide to the cofactor and binding of cationic l -Lys near the cofactor are associated with strong heterotropic cooperativity. Progression to the haloferryl intermediate upon addition of O 2 does not trap the substrates in the active site and, in fact, markedly diminishes cooperativity between halide and l -Lys. The surprising lability of the BesD•[Fe(IV)=O]•Cl•succinate• l -Lys complex engenders pathways for decay of the haloferryl intermediate that do not result in l -Lys chlorination, especially at low chloride concentrations; one identified pathway involves oxidation of glycerol. The mechanistic data imply that (i) BesD may have evolved from a hydroxylase ancestor either relatively recently or under weak selective pressure for efficient chlorination and (ii) that acquisition of its activity may have involved the emergence of linkage between l -Lys binding and chloride coordination following loss of the anionic protein-carboxylate iron ligand present in extant hydroxylases.

Mechanistic Study of Chemoselectivity for Carbon Radical Hydroxylation versus Chlorination with FeIII (OH)(Cl) Complexes

The Fe^III (OH)(Cl) complex resembles the key intermediate proposed for the non-heme iron halogenases. Goldberg and co-workers reported that the Fe^III (OH)(Cl) RC reacts with triphenylmethyl radical 1 to give an exclusive hydroxylation product. To understand the chemoselectivity of the reaction of RC with 1, density functional theory (DFT) calculations have been conducted. From RC, the competing pathways were identified as the OH-transfer, Cl-transfer, and isomerization pathways. The direct Cl-transfer is more favorable than direct OH-transfer by 2.8 kcal/mol. The hydrogen bonding interactions between the hydroxyl group and the pendent amine ligand impede the direct OH-transfer from RC. Compared with the direct Cl-transfer pathway, the isomerization pathways require lower barriers. In isomer RC_iso2 , the equatorial hydroxyl group, which has smaller diabatic bond dissociation energy, prefers to transfer to form the hydroxylation product. In Fe^III (Cl)₂ RC2 and RC2_iso , the equatorial chloride group also prefers to transfer to give the chlorination product.

Engineering Fe(II)/α-Ketoglutarate-Dependent Halogenases and Desaturases

Fe(II)/α-ketoglutarate-dependent dioxygenases (α-KGDs) are widespread enzymes in aerobic biology and serve a remarkable array of biological functions, including roles in collagen biosynthesis, plant and animal development, transcriptional regulation, nucleic acid modification, and secondary metabolite biosynthesis. This functional diversity is reflected in the enzymes' catalytic flexibility as α-KGDs can catalyze an intriguing set of synthetically valuable reactions, such as hydroxylations, halogenations, and desaturations, capturing the interest of scientists across disciplines. Mechanistically, all α-KGDs are understood to follow a similar activation pathway to generate a substrate radical, yet how individual members of the enzyme family direct this key intermediate toward the different reaction outcomes remains elusive, triggering structural, computational, spectroscopic, kinetic, and enzyme engineering studies. In this Perspective, we will highlight how first enzyme and substrate engineering examples suggest that the chemical reaction pathway within α-KGDs can be intentionally tailored using rational design principles. We will delineate the structural and mechanistic investigations of the reprogrammed enzymes and how they begin to inform about the enzymes' structure-function relationships that determine chemoselectivity. Application of this knowledge in future enzyme and substrate engineering campaigns will lead to the development of powerful C-H activation catalysts for chemical synthesis.

Crystal structure of the α-ketoglutarate-dependent non-heme iron oxygenase CmnC in capreomycin biosynthesis and its engineering to catalyze hydroxylation of the substrate enantiomer

CmnC is an α-ketoglutarate (α-KG)-dependent non-heme iron oxygenase involved in the formation of the l-capreomycidine (l-Cap) moiety in capreomycin (CMN) biosynthesis. CmnC and its homologues, VioC in viomycin (VIO) biosynthesis and OrfP in streptothricin (STT) biosynthesis, catalyze hydroxylation of l-Arg to form β-hydroxy l-Arg (CmnC and VioC) or β,γ-dihydroxy l-Arg (OrfP). In this study, a combination of biochemical characterization and structural determination was performed to understand the substrate binding environment and substrate specificity of CmnC. Interestingly, despite having a high conservation of the substrate binding environment among CmnC, VioC, and OrfP, only OrfP can hydroxylate the substrate enantiomer d-Arg. Superposition of the structures of CmnC, VioC, and OrfP revealed a similar folds and overall structures. The active site residues of CmnC, VioC, and OrfP are almost conserved; however Leu136, Ser138, and Asp249 around the substrate binding pocket in CmnC are replaced by Gln, Gly, and Tyr in OrfP, respectively. These residues may play important roles for the substrate binding. The mutagenesis analysis revealed that the triple mutant CmnCL136Q,S138G,D249Y switches the substrate stereoselectivity from l-Arg to d-Arg with ∼6% relative activity. The crystal structure of CmnCL136Q,S138G,D249Y in complex with d-Arg revealed that the substrate loses partial interactions and adopts a different orientation in the binding site. This study provides insights into the enzyme engineering to α-KG non-heme iron oxygenases for adjustment to the substrate stereoselectivity and development of biocatalysts.

Elucidation of divergent desaturation pathways in the formation of vinyl isonitrile and isocyanoacrylate

Two different types of desaturations are employed by iron- and 2-oxoglutarate-dependent (Fe/2OG) enzymes to construct vinyl isonitrile and isocyanoacrylate moieties found in isonitrile-containing natural products. A substrate-bound protein structure reveals a plausible strategy to affect desaturation and hints at substrate promiscuity of these enzymes. Analogs are synthesized and used as mechanistic probes to validate structural observations. Instead of proceeding through hydroxylated intermediate as previously proposed, a plausible carbocation species is utilized to trigger C=C bond installation. These Fe/2OG enzymes can also accommodate analogs with opposite chirality and different functional groups including isonitrile-(D)-tyrosine, N-formyl tyrosine, and phloretic acid, while maintaining the reaction selectivity.

While canonical oxygen-rebound pathway has been utilized by several Fe/2OG enzymes, insights into the desaturation process remain to be elucidated. Here, the authors suggest that a carbocation might be deployed to enable chemically divergent desaturations through decarboxylation and deprotonation.

An artificial pathway for trans-4-hydroxy-L-pipecolic acid production from L-lysine in Escherichia coli

Trans-4-hydroxy-L-pipecolic acid (Trans-4-HyPip) is a hydroxylated product of L-pipecolic acid, and which is widely used in pharmaceutical and chemical industry. Here, a trans-4-HyPip biosynthesis module was designed and constructed in Escherichia coli by overexpressing lysine α-oxidase, Δ1-piperideine-2-carboxylase reductase, glucose dehydrogenase, lysine permease, catalase and L-pipecolic acid trans-4-hydroxylase for expanding the lysine catabolism pathway. 4.89 g/L of trans-4-HyPip was generated in shake flasks from 8 g/L of L-pipecolic acid. By this approach, 14.86 g/L of trans-4-HyPip was produced from lysine after 48 h in a 5-L bioreactor. As far as we know, this is the first multi-enzyme cascade catalytic system for the production of trans-4-HyPip using Escherichia coli from L-lysine. Therefore, it can be considered as a potential candidate for industrial production of trans-4-HyPip in microorganisms.

Peroxy Intermediate Drives Carbon Bond Activation in the Dioxygenase AsqJ

Dioxygenases catalyze stereoselective oxygen atom transfer in metabolic pathways of biological, industrial, and pharmaceutical importance, but their precise chemical principles remain controversial. The α-ketoglutarate (αKG)-dependent dioxygenase AsqJ synthesizes biomedically active quinolone alkaloids via desaturation and subsequent epoxidation of a carbon-carbon bond in the cyclopeptin substrate. Here, we combine high-resolution X-ray crystallography with enzyme engineering, quantum-classical (QM/MM) simulations, and biochemical assays to describe a peroxidic intermediate that bridges the substrate and active site metal ion in AsqJ. Homolytic cleavage of this moiety during substrate epoxidation generates an activated high-valent ferryl (Fe^IV = O) species that mediates the next catalytic cycle, possibly without the consumption of the metabolically valuable αKG cosubstrate. Our combined findings provide an important understanding of chemical bond activation principles in complex enzymatic reaction networks and molecular mechanisms of dioxygenases.

Synthesis of 6,6- and 7,7-difluoro-1-acetamidopyrrolizidines and their oxidation catalyzed by the nonheme Fe oxygenase LolO

LolO, a 2-oxoglutarate-dependent nonheme Fe oxygenase, catalyzes both the hydroxylation and cycloetherification of 1- exo -acetamidopyrrolizidine (AcAP), a pathway intermediate in the biosynthesis of the loline alkaloids. We have prepared fluorinated AcAP analogs to aid in continued mechanistic investigation of the unusual LolO-catalyzed cycloetherification step. LolO was able to first hydroxylate and then cycloetherify 6,6-difluoro-AcAP (prepared from N , O -protected 4-oxoproline), forming a difluorinated analog of N -acetylnorloline (NANL) and providing evidence for a cycloetherification mechanism involving a C(7) radical as opposed to a C(7) carbocation. By contrast, LolO was able to hydroxylate 7,7-difluoro-AcAP (prepared from 3-oxoproline) but failed to cycloetherify it, forming (1 R , 2 R , 8 S )-7,7-difluoro-2-hydroxy-AcAP as the sole product. Because it completely blocks the cycloetherification step, 7,7-difluoro-AcAP has the potential to become an important tool for accumulating and characterizing the LolO intermediate responsible for catalyzing cycloetherification of 2-hydroxy-AcAP.

Mechanism of Methyldehydrofosmidomycin Maturation: Use Olefination to Enable Chain Elongation

Utilization of mononuclear iron- and 2-oxoglutarate-dependent (Fe/2OG) enzymes to enable C-H bond functionalization is a widely used strategy to diversify the structural complexity of natural products. Besides those well-studied reactions including hydroxylation, epoxidation, and halogenation, in the biosynthetic pathway of dehydrofosmidomycin, an Fe/2OG enzyme is reported to catalyze desaturation, alkyl chain elongation, along with demethylation in which trimethyl-2-aminoethylphosphonate is converted into methyldehydrofosmidomycin. How this transformation takes place is largely unknown. Herein, we characterized the reactive species, revealed the structure of the reaction intermediate, and used mechanistic probes to investigate the reaction pathway and mechanism. These results led to the elucidation of a two-step process in which the first reaction employs a long-lived Fe(IV)-oxo species to trigger C═C bond installation. During the second reaction, the olefin installed in situ enables C-C bond formation that is accompanied with a C-N bond cleavage and hydroxylation to furnish the alkyl chain elongation and demethylation. This work expands the reaction repertoire of Fe/2OG enzymes by introducing a new pathway to the known C-C bond formation mechanisms utilized by metalloenzymes.

Properties of the Reactants and Their Interactions within and with the Enzyme Binding Cavity Determine Reaction Selectivities. The Case of Fe(II)/2-Oxoglutarate Dependent Enzymes

Fe(II)/2-oxoglutarate dependent dioxygenases (ODDs) share a double stranded beta helix (DSBH) fold and utilise a common reactive intermediate, ferryl species, to catalyse oxidative transformations of substrates. Despite the structural similarities, ODDs accept a variety of substrates and facilitate a wide range of reactions, that is hydroxylations, desaturations, (oxa)cyclisations and ring rearrangements. In this review we present and discuss the factors contributing to the observed (regio)selectivities of ODDs. They span from inherent properties of the reactants, that is, substrate molecule and iron cofactor, to the interactions between the substrate and the enzyme's binding cavity; the latter can counterbalance the effect of the former. Based on results of both experimental and computational studies dedicated to ODDs, we also line out the properties of the reactants which promote reaction outcomes other than the "default" hydroxylation. It turns out that the reaction selectivity depends on a delicate balance of interactions between the components of the investigated system.

Stereochemical Assignment and Absolute Abundance of Nonproteinogenic Amino Acid Homoarginine in Marine Sponges

Together with arginine, the nonproteinogenic amino acid homoarginine is a substrate for the production of vasodilator nitric oxide in the human body. In marine sponges, homoarginine has been postulated to serve as a precursor for the biosynthesis of pyrrole-imidazole alkaloid and bromotyrosine alkaloid classes of natural products. The absolute abundance of homoarginine, its abundance relative to arginine, and its stereochemical assignment in marine sponges are not known. Here, using stable isotope dilution mass spectrometry, we quantify the absolute abundances of homoarginine and arginine in marine sponges. We find that the abundance of homoarginine is highly variable and can far exceed the concentration of arginine, even in sponges where incorporation of homoarginine in natural products cannot be rationalized. The [homoarginine]/[arginine] ratio in marine sponges is greater than that in human analytes. By derivatization of sponge extracts with Marfey's reagent and comparison with authentic standards, we determine the l-isomer of homoarginine to be exclusively present in sponges. Our results shed light on the presence of the high abundance of homoarginine in marine sponge metabolomes and provide the foundation to investigate the biosynthetic routes and physiological roles of this nonproteinogenic amino acid in sponge physiology.

Mechanisms of O2 Activation by Mononuclear Non-Heme Iron Enzymes

Two major subclasses of mononuclear non-heme ferrous enzymes use two electron-donating organic cofactors (α-ketoglutarate or pterin) to activate O₂ to form Fe^IV═O intermediates that further react with their substrates through hydrogen atom abstraction or electrophilic aromatic substitution. New spectroscopic methodologies have been developed, enabling the study of the active sites in these enzymes and their oxygen intermediates. Coupled to electronic structure calculations, the results of these spectroscopies provide fundamental insight into mechanism. This Perspective summarizes the results of these studies in elucidating the mechanism of dioxygen activation to form the Fe^IV═O intermediate and the geometric and electronic structure of this intermediate that enables its high reactivity and selectivity in product formation.

C-H Bond Cleavage by Bioinspired Nonheme Metal Complexes

The functionalization of C-H bonds is one of the most challenging transformations in synthetic chemistry. In biology, these processes are well-known and are achieved with a variety of metalloenzymes, many of which contain a single metal center within their active sites. The most well studied are those with Fe centers, and the emerging experimental data show that high-valent iron oxido species are the intermediates responsible for cleaving the C-H bond. This Forum Article describes the state of this field with an emphasis on nonheme Fe enzymes and current experimental results that provide insights into the properties that make these species capable of C-H bond cleavage. These parameters are also briefly considered in regard to manganese oxido complexes and Cu-containing metalloenzymes. Synthetic iron oxido complexes are discussed to highlight their utility as spectroscopic and mechanistic probes and reagents for C-H bond functionalization. Avenues for future research are also examined.

Activation modes in biocatalytic radical cyclization reactions

Radical cyclizations are essential reactions in the biosynthesis of secondary metabolites and the chemical synthesis of societally valuable molecules. In this review, we highlight the general mechanisms utilized in biocatalytic radical cyclizations. We specifically highlight cytochrome P450 monooxygenases (P450s) involved in the biosynthesis of mycocyclosin and vancomycin, nonheme iron- and α-ketoglutarate-dependent dioxygenases (Fe/αKGDs) used in the biosynthesis of kainic acid, scopolamine, and isopenicillin N, and radical S-adenosylmethionine (SAM) enzymes that facilitate the biosynthesis of oxetanocin A, menaquinone, and F420. Beyond natural mechanisms, we also examine repurposed flavin-dependent “ene”-reductases (ERED) for non-natural radical cyclization. Overall, these general mechanisms underscore the opportunity for enzymes to augment and enhance the synthesis of complex molecules using radical mechanisms.

Synthesis of Imidazole and Histidine-Derived Cross-Linkers as Analogues of GOLD and Desmosine

Amino acid derivatives with a central cationic heterocyclic core (e.g., imidazolium) are biologically relevant cross-linkers of proteins and advanced glycation end (AGE) products. Here, imidazolium-containing cross-linkers were synthesized from imidazole or histidine by N-alkylation employing aspartate- and glutamate-derived mesylates as key step. Biological investigations were carried out to probe the biocompatibility of these compounds.

Density Functional Theory Study into the Reaction Mechanism of Isonitrile Biosynthesis by the Nonheme Iron Enzyme ScoE

The nonheme iron enzyme ScoE catalyzes the biosynthesis of an isonitrile substituent in a peptide chain. To understand details of the reaction mechanism we created a large active site cluster model of 212 atoms that contains substrate, the active oxidant and the first- and second-coordination sphere of the protein and solvent. Several possible reaction mechanisms were tested and it is shown that isonitrile can only be formed through two consecutive catalytic cycles that both use one molecule of dioxygen and α-ketoglutarate. In both cycles the active species is an iron(IV)-oxo species that in the first reaction cycle reacts through two consecutive hydrogen atom abstraction steps: first from the N–H group and thereafter from the C–H group to desaturate the NH-CH2 bond. The alternative ordering of hydrogen atom abstraction steps was also tested but found to be higher in energy. Moreover, the electronic configurations along that pathway implicate an initial hydride transfer followed by proton transfer. We highlight an active site Lys residue that is shown to donate charge in the transition states and influences the relative barrier heights and bifurcation pathways. A second catalytic cycle of the reaction of iron(IV)-oxo with desaturated substrate starts with hydrogen atom abstraction followed by decarboxylation to give isonitrile directly. The catalytic cycle is completed with a proton transfer to iron(II)-hydroxo to generate the iron(II)-water resting state. The work is compared with experimental observation and previous computational studies on this system and put in a larger perspective of nonheme iron chemistry.

A metabolic regulon reveals early and late acting enzymes in neuroactive Lycopodium alkaloid biosynthesis

Plants synthesize many diverse small molecules that affect function of the mammalian central nervous system, making them crucial sources of therapeutics for neurological disorders. A notable portion of neuroactive phytochemicals are lysine-derived alkaloids, but the mechanisms by which plants produce these compounds have remained largely unexplored. To better understand how plants synthesize these metabolites, we focused on biosynthesis of the Lycopodium alkaloids that are produced by club mosses, a clade of plants used traditionally as herbal medicines. Hundreds of Lycopodium alkaloids have been described, including huperzine A (HupA), an acetylcholine esterase inhibitor that has generated interest as a treatment for the symptoms of Alzheimer's disease. Through combined metabolomic profiling and transcriptomics, we have identified a developmentally controlled set of biosynthetic genes, or potential regulon, for the Lycopodium alkaloids. The discovery of this putative regulon facilitated the biosynthetic reconstitution and functional characterization of six enzymes that act in the initiation and conclusion of HupA biosynthesis. This includes a type III polyketide synthase that catalyzes a crucial imine-polyketide condensation, as well as three Fe(II)/2-oxoglutarate-dependent dioxygenase (2OGD) enzymes that catalyze transformations (pyridone ring-forming desaturation, piperidine ring cleavage, and redox-neutral isomerization) within downstream HupA biosynthesis. Our results expand the diversity of known chemical transformations catalyzed by 2OGDs and provide mechanistic insight into the function of noncanonical type III PKS enzymes that generate plant alkaloid scaffolds. These data offer insight into the chemical logic of Lys-derived alkaloid biosynthesis and demonstrate the tightly coordinated coexpression of secondary metabolic genes for the biosynthesis of medicinal alkaloids.

N-Nitrosation Mechanism Catalyzed by Non-heme Iron-Containing Enzyme SznF Involving Intramolecular Oxidative Rearrangement

The non-heme iron-dependent enzyme SznF catalyzes a critical N-nitrosation step during the N-nitrosourea pharmacophore biosynthesis in streptozotocin. The intramolecular oxidative rearrangement process is known to proceed at the Fe^II-containing active site in the cupin domain of SznF, but its mechanism has not been elucidated to date. In this study, based on the density functional theory calculations, a unique mechanism was proposed for the N-nitrosation reaction catalyzed by SznF in which a four-electron oxidation process is accomplished through a series of complicated electron transferring between the iron center and substrate to bypass the high-valent Fe^IV═O species. In the catalytic reaction pathway, the O₂ binds to the iron center and attacks on the substrate to form the peroxo bridge intermediate by obtaining two electrons from the substrate exclusively. Then, instead of cleaving the peroxo bridge, the C^ε-N^ω bond of the substrate is homolytically cleaved first to form a carbocation intermediate, which polarizes the peroxo bridge and promotes its heterolysis. After O-O bond cleavage, the following reaction steps proceed effortlessly so that the N-nitrosation is accomplished without NO exchange among reaction species.

Electrostatic Perturbations from the Protein Affect C-H Bond Strengths of the Substrate and Enable Negative Catalysis in the TmpA Biosynthesis Enzyme

The nonheme iron dioxygenase 2-(trimethylammonio)-ethylphosphonate dioxygenase (TmpA) is an enzyme involved in the regio- and chemoselective hydroxylation at the C¹ -position of the substrate as part of the biosynthesis of glycine betaine in bacteria and carnitine in humans. To understand how the enzyme avoids breaking the weak C² -H bond in favor of C¹ -hydroxylation, we set up a cluster model of 242 atoms representing the first and second coordination sphere of the metal center and substrate binding pocket, and investigated possible reaction mechanisms of substrate activation by an iron(IV)-oxo species by density functional theory methods. In agreement with experimental product distributions, the calculations predict a favorable C¹ -hydroxylation pathway. The calculations show that the selectivity is guided through electrostatic perturbations inside the protein from charged residues, external electric fields and electric dipole moments. In particular, charged residues influence and perturb the homolytic bond strength of the C¹ -H and C² -H bonds of the substrate, and strongly strengthens the C² -H bond in the substrate-bound orientation.

Reinvigorating the Chiral Pool: Chemoenzymatic Approaches to Complex Peptides and Terpenoids

Biocatalytic transformations that leverage the selectivity and efficiency of enzymes represent powerful tools for the construction of complex natural products. Enabled by innovations in genome mining, bioinformatics, and enzyme engineering, synthetic chemists are now more than ever able to develop and employ enzymes to solve outstanding chemical problems, one of which is the reliable and facile generation of stereochemistry within natural product scaffolds. In recognition of this unmet need, our group has sought to advance novel chemoenzymatic strategies to both expand and reinvigorate the chiral pool. Broadly defined, the chiral pool comprises cheap, enantiopure feedstock chemicals that serve as popular foundations for asymmetric total synthesis. Among these building blocks, amino acids and enantiopure terpenes, whose core structures can be mapped onto several classes of structurally and pharmaceutically intriguing natural products, are of particular interest to the synthetic community.In this Account, we summarize recent efforts from our group in leveraging biocatalytic transformations to expand the chiral pool, as well as efforts toward the efficient application of these transformations in natural products total synthesis, the ultimate testing ground for any novel methodology. First, we describe several examples of enzymatic generation of noncanonical amino acids as means to simplify the synthesis of peptide natural products. By extracting amino acid hydroxylases from native biosynthetic pathways, we obtain efficient access to hydroxylated variants of proline, lysine, arginine, and their derivatives. The newly installed hydroxyl moiety then becomes a chemical handle that can facilitate additional complexity generation, thereby expanding the pool of amino acid-derived building blocks available for peptide synthesis. Next, we present our efforts in enzymatic C-H oxidations of diverse terpene scaffolds, in which traditional chemistry can be combined with strategic applications of biocatalysis to selectively and efficiently derivatize several commercial terpenoid skeletons. The synergistic logic of this approach enables a small handful of synthetic intermediates to provide access to a plethora of terpenoid natural product families. Taken together, these findings demonstrate the advantages of applying enzymes in total synthesis in conjunction with established methodologies, as well as toward the expansion of the chiral pool to enable facile incorporation of stereochemistry during synthetic campaigns.

An Iron(IV)-Oxo Intermediate Initiating l-Arginine Oxidation but Not Ethylene Production by the 2-Oxoglutarate-Dependent Oxygenase, Ethylene-Forming Enzyme

Ethylene-forming enzyme (EFE) is an ambifunctional iron(II)- and 2-oxoglutarate-dependent (Fe/2OG) oxygenase. In its major (EF) reaction, it converts carbons 1, 2, and 5 of 2OG to CO₂ and carbons 3 and 4 to ethylene, a four-electron oxidation drastically different from the simpler decarboxylation of 2OG to succinate mediated by all other Fe/2OG enzymes. EFE also catalyzes a minor reaction, in which the normal decarboxylation is coupled to oxidation of l-arginine (a required activator for the EF pathway), resulting in its conversion to l-glutamate semialdehyde and guanidine. Here we show that, consistent with precedent, the l-Arg-oxidation (RO) pathway proceeds via an iron(IV)-oxo (ferryl) intermediate. Use of 5,5-[²H₂]-l-Arg slows decay of the ferryl complex by >16-fold, implying that RO is initiated by hydrogen-atom transfer (HAT) from C5. That this large substrate deuterium kinetic isotope effect has no impact on the EF:RO partition ratio implies that the same ferryl intermediate cannot be on the EF pathway; the pathways must diverge earlier. Consistent with this conclusion, the variant enzyme bearing the Asp191Glu ligand substitution accumulates ∼4 times as much of the ferryl complex as the wild-type enzyme and exhibits a ∼40-fold diminished EF:RO partition ratio. The selective detriment of this nearly conservative substitution to the EF pathway implies that it has unusually stringent stereoelectronic requirements. An active-site, like-charge guanidinium pair, which involves the l-Arg substrate/activator and is unique to EFE among four crystallographically characterized l-Arg-modifying Fe/2OG oxygenases, may serve to selectively stabilize the transition state leading to the unique EF branch.

Fermentative Production of l-2-Hydroxyglutarate by Engineered Corynebacterium glutamicum via Pathway Extension of l-Lysine Biosynthesis

l-2-hydroxyglutarate (l-2HG) is a trifunctional building block and highly attractive for the chemical and pharmaceutical industries. The natural l-lysine biosynthesis pathway of the amino acid producer Corynebacterium glutamicum was extended for the fermentative production of l-2HG. Since l-2HG is not native to the metabolism of C. glutamicum metabolic engineering of a genome-streamlined l-lysine overproducing strain was required to enable the conversion of l-lysine to l-2HG in a six-step synthetic pathway. To this end, l-lysine decarboxylase was cascaded with two transamination reactions, two NAD(P)-dependent oxidation reactions and the terminal 2-oxoglutarate-dependent glutarate hydroxylase. Of three sources for glutarate hydroxylase the metalloenzyme CsiD from Pseudomonas putida supported l-2HG production to the highest titers. Genetic experiments suggested a role of succinate exporter SucE for export of l-2HG and improving expression of its gene by chromosomal exchange of its native promoter improved l-2HG production. The availability of Fe²⁺ as cofactor of CsiD was identified as a major bottleneck in the conversion of glutarate to l-2HG. As consequence of strain engineering and media adaptation product titers of 34 ± 0 mM were obtained in a microcultivation system. The glucose-based process was stable in 2 L bioreactor cultivations and a l-2HG titer of 3.5 g L⁻¹ was obtained at the higher of two tested aeration levels. Production of l-2HG from a sidestream of the starch industry as renewable substrate was demonstrated. To the best of our knowledge, this study is the first description of fermentative production of l-2HG, a monomeric precursor used in electrochromic polyamides, to cross-link polyamides or to increase their biodegradability.

What Determines the Selectivity of Arginine Dihydroxylation by the Nonheme Iron Enzyme OrfP?

The nonheme iron enzyme OrfP reacts with l-Arg selectively to form the 3R,4R-dihydroxyarginine product, which in mammals can inhibit the nitric oxide synthase enzymes involved in blood pressure control. To understand the mechanisms of dioxygen activation of l-Arg by OrfP and how it enables two sequential oxidation cycles on the same substrate, we performed a density functional theory study on a large active site cluster model. We show that substrate binding and positioning in the active site guides a highly selective reaction through C³ -H hydrogen atom abstraction. This happens despite the fact that the C³ -H and C⁴ -H bond strengths of l-Arg are very similar. Electronic differences in the two hydrogen atom abstraction pathways drive the reaction with an initial C³ -H activation to a low-energy ⁵ σ-pathway, while substrate positioning destabilizes the C⁴ -H abstraction and sends it over the higher-lying ⁵ π-pathway. We show that substrate and monohydroxylated products are strongly bound in the substrate binding pocket and hence product release is difficult and consequently its lifetime will be long enough to trigger a second oxygenation cycle.