A family of structural and functional models for the active site of a unique dioxygenase: Acireductone dioxygenase (ARD).
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ABSTRACT:
SUBMITTER: Blade GA
PROVIDER: S-EPMC7680382 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:Human indoleamine 2,3-dioxygenase catalyzes the oxidative cleavage of tryptophan to N-formyl kynurenine, the initial and rate-limiting step in the kynurenine pathway. Additionally, this enzyme has been identified as a possible target for cancer therapy. A 20-amino acid protein segment (the JK loop), which connects the J and K helices, was not resolved in the reported hIDO crystal structure. Previous studies have shown that this loop undergoes structural rearrangement upon substrate binding. In this work, we apply a combination of replica exchange molecular dynamics simulations and site-directed mutagenesis experiments to characterize the structure and dynamics of this protein region. Our simulations show that the JK loop can be divided into two regions: the first region (JK loop(C)) displays specific and well-defined conformations and is within hydrogen bonding distance of the substrate, while the second region (JK loop(N)) is highly disordered and exposed to the solvent. The peculiar flexible nature of JK loop(N) suggests that it may function as a target for post-translational modifications and/or a mediator for protein-protein interactions. In contrast, hydrogen bonding interactions are observed between the substrate and Thr379 in the highly conserved "GTGG" motif of JK loop(C), thereby anchoring JK loop(C) in a closed conformation, which secures the appropriate substrate binding mode for catalysis. Site-directed mutagenesis experiments confirm the key role of this residue, highlighting the importance of the JK loop(C) conformation in regulating the enzymatic activity. Furthermore, the existence of the partially and totally open conformations in the substrate-free form suggests a role of JK loop(C) in controlling substrate and product dynamics.
| S-EPMC5450165 | biostudies-literature
Project description:Acireductone dioxygenase (ARD) from the methionine salvage pathway of Klebsiella oxytoca is the only known naturally occurring metalloenzyme that catalyzes different reactions in vivo based solely on the identity of the divalent transition metal ion (Fe2+ or Ni2+) bound in the active site. The iron-containing isozyme catalyzes the cleavage of substrate 1,2-dihydroxy-3-keto-5-(thiomethyl)pent-1-ene (acireductone) by O2 to formate and the ketoacid precursor of methionine, whereas the nickel-containing isozyme uses the same substrates to catalyze an off-pathway shunt to form methylthiopropionate, carbon monoxide and formate. This dual chemistry was recently demonstrated in vitro by ARD from Mus musculus (MmARD), providing the first example of a mammalian ARD exhibiting metal-dependent catalysis. We now show that human ARD (HsARD) is also capable of metal-dependent dual chemistry. Recombinant HsARD was expressed and purified to obtain a homogeneous enzyme with a single transition metal ion bound. As with MmARD, the Fe2+-bound HsARD shows the highest activity and catalyzes on-pathway chemistry, whereas Ni2+, Co2+ or Mn2+ forms catalyze off-pathway chemistry. The thermal stability of the HsARD isozymes is a function of the metal ion identity, with Ni2+-bound HsARD being the most stable followed by Co2+ and Fe2+, and Mn2+-bound HsARD being the least stable. As with the bacterial ARD, solution NMR data suggest that HsARD isozymes can have significant structural differences depending upon the metal ion bound.
| S-EPMC5421613 | biostudies-literature
Project description:The two acireductone dioxygenase (ARD) isozymes from the methionine salvage pathway of Klebsiella oxytoca are the only known pair of naturally occurring metalloenzymes with distinct chemical and physical properties determined solely by the identity of the divalent transition metal ion (Fe(2+) or Ni(2+)) in the active site. We now show that this dual chemistry can also occur in mammals. ARD from Mus musculus (MmARD) was studied to relate the metal ion identity and three-dimensional structure to enzyme function. The iron-containing isozyme catalyzes the cleavage of 1,2-dihydroxy-3-keto-5-(thiomethyl)pent-1-ene (acireductone) by O2 to formate and the ketoacid precursor of methionine, which is the penultimate step in methionine salvage. The nickel-bound form of ARD catalyzes an off-pathway reaction resulting in formate, carbon monoxide (CO), and 3-(thiomethyl) propionate. Recombinant MmARD was expressed and purified to obtain a homogeneous enzyme with a single transition metal ion bound. The Fe(2+)-bound protein, which shows about 10-fold higher activity than that of others, catalyzes on-pathway chemistry, whereas the Ni(2+), Co(2+), or Mn(2+) forms exhibit off-pathway chemistry, as has been seen with ARD from Klebsiella. Thermal stability of the isozymes is strongly affected by the metal ion identity, with Ni(2+)-bound MmARD being the most stable, followed by Co(2+) and Fe(2+), and Mn(2+)-bound ARD being the least stable. Ni(2+)- and Co(2+)-bound MmARD were crystallized, and the structures of the two proteins found to be similar. Enzyme-ligand complexes provide insight into substrate binding, metal coordination, and the catalytic mechanism.
| S-EPMC5319410 | biostudies-literature
Project description:The two acireductone dioxygenase (ARD) isozymes from the methionine salvage pathway of Klebsiella ATCC 8724 present an unusual case in which two enzymes with different structures and distinct activities toward their common substrates (1,2-dihydroxy-3-oxo-5-(methylthio)pent-1-ene and dioxygen) are derived from the same polypeptide chain. Structural and functional differences between the two isozymes are determined by the type of M2+ metal ion bound in the active site. The Ni2+-bound NiARD catalyzes an off-pathway shunt from the methionine salvage pathway leading to the production of formate, methylthiopropionate, and carbon monoxide, while the Fe2+-bound FeARD' catalyzes the on-pathway formation of methionine precursor 2-keto-4-methylthiobutyrate and formate. Four potential protein-based metal ligands were identified by sequence homology and structural considerations. Based on the results of site-directed mutagenesis experiments, X-ray absorption spectroscopy (XAS), and isothermal calorimetry measurements, it is concluded that the same four residues, His96, His98, Glu102 and His140, provide the protein-based ligands for the metal in both the Ni- and Fe-containing forms of the enzyme, and subtle differences in the local backbone conformations trigger the observed structural and functional differences between the FeARD' and NiARD isozymes. Furthermore, both forms of the enzyme bind their respective metals with pseudo-octahedral geometry, and both may lose a histidine ligand upon binding of substrate under anaerobic conditions. However, mutations at two conserved nonligand acidic residues, Glu95 and Glu100, result in low metal contents for the mutant proteins as isolated, suggesting that some of the conserved charged residues may aid in transfer of metal from in vivo sources or prevent the loss of metal to stronger chelators. The Glu100 mutant reconstitutes readily but has low activity. Mutation of Asp101 results in an active enzyme that incorporates metal in vivo but shows evidence of mixed forms.
| S-EPMC2267756 | biostudies-literature
Project description:Bacterial pathogens must acquire host iron for survival and colonization. Because free iron is restricted in the host, numerous pathogens have evolved to overcome this limitation by using a family of monooxygenases that mediate the oxidative cleavage of heme into biliverdin, carbon monoxide, and iron. However, the etiological agent of tuberculosis, Mycobacterium tuberculosis, accomplishes this task without generating carbon monoxide, which potentially induces its latent state. Here we show that this unusual heme degradation reaction proceeds through sequential mono- and dioxygenation events within the single active center of MhuD, a mechanism unparalleled in enzyme catalysis. A key intermediate of the MhuD reaction is found to be meso-hydroxyheme, which reacts with O2 at an unusual position to completely suppress its monooxygenation but to allow ring cleavage through dioxygenation. This mechanistic change, possibly due to heavy steric deformation of hydroxyheme, rationally explains the unique heme catabolites of MhuD. Coexistence of mechanistically distinct functions is a previously unidentified strategy to expand the physiological outcome of enzymes, and may be applied to engineer unique biocatalysts.
| S-EPMC4833273 | biostudies-literature
Project description:Acireductone dioxygenase (ARD) catalyzes different reactions between O2 and 1,2-dihydroxy-3-oxo-5-(methylthio)pent-1-ene (acireductone) depending upon the metal bound in the active site. Ni2+ -ARD cleaves acireductone to formate, CO and methylthiopropionate. If Fe2+ is bound (ARD'), the same substrates yield methylthioketobutyrate and formate. The two forms differ in structure, and are chromatographically separable. Paramagnetism of Fe2+ renders the active site of ARD' inaccessible to standard NMR methods. The structure of ARD' has been determined using Fe2+ binding parameters determined by X-ray absorption spectroscopy and NMR restraints from H98S ARD, a metal-free diamagnetic protein that is isostructural with ARD'. ARD' retains the beta-sandwich fold of ARD, but a structural entropy switch increases order at one end of a two-helix system that bisects the beta-sandwich and decreases order at the other upon interconversion of ARD and ARD', causing loss of the C-terminal helix in ARD' and rearrangements of residues involved in substrate orientation in the active site.
| S-EPMC1808343 | biostudies-literature
Project description:Acireductone dioxygenase (ARD) from the methionine salvage pathway (MSP) is a unique enzyme that exhibits dual chemistry determined solely by the identity of the divalent transition-metal ion (Fe2+ or Ni2+) in the active site. The Fe2+-containing isozyme catalyzes the on-pathway reaction using substrates 1,2-dihydroxy-3-keto-5-methylthiopent-1-ene (acireductone) and dioxygen to generate formate and the ketoacid precursor of methionine, 2-keto-4-methylthiobutyrate, whereas the Ni2+-containing isozyme catalyzes an off-pathway shunt with the same substrates, generating methylthiopropionate, carbon monoxide, and formate. The dual chemistry of ARD was originally discovered in the bacterium Klebsiella oxytoca, but it has recently been shown that mammalian ARD enzymes (mouse and human) are also capable of catalyzing metal-dependent dual chemistry in vitro. This is particularly interesting, since carbon monoxide, one of the products of off-pathway reaction, has been identified as an antiapoptotic molecule in mammals. In addition, several biochemical and genetic studies have indicated an inhibitory role of human ARD in cancer. This comprehensive review describes the biochemical and structural characterization of the ARD family, the proposed experimental and theoretical approaches to establishing mechanisms for the dual chemistry, insights into the mechanism based on comparison with structurally and functionally similar enzymes, and the applications of this research to the field of artificial metalloenzymes and synthetic biology.
| S-EPMC5604235 | biostudies-literature
Project description:Acireductone dioxygenase (ARD) is an intriguing enzyme from the methionine salvage pathway that is capable of catalysing two different oxidation reactions with the same substrate depending on the type of the metal ion in the active site. To date, the structural information regarding the ARD-acireductone complex is limited and possible reaction mechanisms are still under debate. The results of joint experimental and computational studies undertaken to advance knowledge about ARD are reported. The crystal structure of an ARD from Homo sapiens was determined with selenomethionine. EPR spectroscopy suggested that binding acireductone triggers one protein residue to dissociate from Fe2+ , which allows NO (and presumably O2 ) to bind directly to the metal. Mössbauer spectroscopic data (interpreted with the aid of DFT calculations) was consistent with bidentate binding of acireductone to Fe2+ and concomitant dissociation of His88 from the metal. Major features of Fe vibrational spectra obtained for the native enzyme and upon addition of acireductone were reproduced by QM/MM calculations for the proposed models. A computational (QM/MM) study of the reaction mechanisms suggests that Fe2+ promotes O-O bond homolysis, which elicits cleavage of the C1-C2 bond of the substrate. Higher M3+ /M2+ redox potentials of other divalent metals do not support this pathway, and instead the reaction proceeds similarly to the key reaction step in the quercetin 2,3-dioxygenase mechanism.
| S-EPMC7883896 | biostudies-literature
Project description:Kinetic and spectroscopic studies have shown that the conserved active site residue His200 of the extradiol ring-cleaving homoprotocatechuate 2,3-dioxygenase (FeHPCD) from Brevibacterium fuscum is critical for efficient catalysis. The roles played by this residue are probed here by analysis of the steady-state kinetics, pH dependence, and X-ray crystal structures of the FeHPCD position 200 variants His200Asn, His200Gln, and His200Glu alone and in complex with three catecholic substrates (homoprotocatechuate, 4-sulfonylcatechol, and 4-nitrocatechol) possessing substituents with different inductive capacity. Structures determined at 1.35-1.75 Å resolution show that there is essentially no change in overall active site architecture or substrate binding mode for these variants when compared to the structures of the wild-type enzyme and its analogous complexes. This shows that the maximal 50-fold decrease in kcat for ring cleavage, the dramatic changes in pH dependence, and the switch from ring cleavage to ring oxidation of 4-nitrocatechol by the FeHPCD variants can be attributed specifically to the properties of the altered second-sphere residue and the substrate. The results suggest that proton transfer is necessary for catalysis, and that it occurs most efficiently when the substrate provides the proton and His200 serves as a catalyst. However, in the absence of an available substrate proton, a defined proton-transfer pathway in the protein can be utilized. Changes in the steric bulk and charge of the residue at position 200 appear to be capable of altering the rate-limiting step in catalysis and, perhaps, the nature of the reactive species.
| S-EPMC4558303 | biostudies-literature
Project description:Mononuclear non-heme iron complexes that serve as structural and functional mimics of the thiol dioxygenases (TDOs), cysteine dioxygenase (CDO) and cysteamine dioxygenase (ADO), have been prepared and characterized with crystallographic, spectroscopic, kinetic, and computational methods. The high-spin Fe(II) complexes feature the facially coordinating tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine (Ph2TIP) ligand that replicates the three histidine (3His) triad of the TDO active sites. Further coordination with bidentate l-cysteine ethyl ester (CysOEt) or cysteamine (CysAm) anions yielded five-coordinate (5C) complexes that resemble the substrate-bound forms of CDO and ADO, respectively. Detailed electronic-structure descriptions of the [Fe(Ph2TIP)(LS,N)]BPh4 complexes, where LS,N = CysOEt (1) or CysAm (2), were generated through a combination of spectroscopic techniques [electronic absorption, magnetic circular dichroism (MCD)] and density functional theory (DFT). Complexes 1 and 2 decompose in the presence of O2 to yield the corresponding sulfinic acid (RSO2H) products, thereby emulating the reactivity of the TDO enzymes and related complexes. Rate constants and activation parameters for the dioxygenation reactions were measured and interpreted with the aid of DFT calculations for O2-bound intermediates. Treatment of the TDO models with nitric oxide (NO)-a well-established surrogate of O2-led to a mixture of high-spin and low-spin {FeNO}7 species at low temperature (-70 °C), as indicated by electron paramagnetic resonance (EPR) spectroscopy. At room temperature, these Fe/NO adducts convert to a common species with EPR and infrared (IR) features typical of cationic dinitrosyl iron complexes (DNICs). To complement these results, parallel spectroscopic, computational, and O2/NO reactivity studies were carried out using previously reported TDO models that feature an anionic hydrotris(3-phenyl-5-methyl-pyrazolyl)borate (Ph,MeTp-) ligand. Though the O2 reactivities of the Ph2TIP- and Ph,MeTp-based complexes are quite similar, the supporting ligand perturbs the energies of Fe 3d-based molecular orbitals and modulates Fe-S bond covalency, suggesting possible rationales for the presence of neutral 3His coordination in CDO and ADO.
| S-EPMC5548292 | biostudies-literature