Bacterial (1H)-3-hydroxy-4-oxoquinaldine 2,4-dioxygenase (HOD) belongs to a class of oxygenases able to catalyze this energetically unfavorable reaction without any cofactor
Protein fold adaptation to novel enzymatic reactions is a fundamental evolutionary process. Cofactor-independent oxygenases degrading N-heteroaromatic substrates belong to the alpha/beta-hydrolase (ABH) fold superfamily that typically does not catalyze oxygenation reactions. Here, we have integrated crystallographic analyses at normoxic and hyperoxic conditions with molecular dynamics and quantum mechanical calculations to investigate its prototypic 1-H-3-hydroxy-4-oxoquinaldine 2,4-dioxygenase (HOD) member. O2 localization to the oxyanion hole, where catalysis occurs, is an unfavorable event and the direct competition between dioxygen and water for this site is modulated by the nucleophilic elbow residue. A hydrophobic pocket that overlaps with the organic substrate binding site can act as a proximal dioxygen reservoir. Freeze-trap pressurization allowed to determine the structure of the ternary complex with a substrate analogue and O2 bound at the oxyanion hole. Theoretical calculations reveal that O2 orientation is coupled to the charge of the bound organic ligand. When 1-H-3-hydroxy-4-oxoquinaldine is uncharged, O2 binds with its molecular axis along the ligands C2-C4 direction in full agreement with the crystal structure. Substrate activation triggered by deprotonation of its 3-OH group by the His-Asp dyad, rotates O2 by approximately 60 degrees. This geometry maximizes the charge-transfer between the substrate and O2 thus weakening the double bond of the latter. Electron density transfer to the O2(Pi*) orbital promotes the formation of the peroxide intermediate via intersystem crossing that is rate-determining. Our work provides a detailed picture of how evolution has repurposed the ABH-fold architecture and its simple catalytic machinery to accomplish metal-independent oxygenation.
Significance: Many of the current O2-dependent enzymes have evolved from classes that existed prior to the switch from a reducing to an oxidative atmosphere and whose original functions are unrelated to dioxygen chemistry. A group of bacterial dioxygenases belong to the alpha/beta-hydrolase (ABH) fold superfamily that typically does not catalyze oxygenation reactions. These enzymes degrade their N-heteroaromatic substrates in a cofactor-independent manner relying only on the simple nucleophile-histidine-acid ABH-fold catalytic toolbox. In this work we show how O2 localizes at the catalytic site by taking advantage of multiple strategies that minimize the strong competition by water, the co-substrate in the ancestral hydrolytic enzyme. We also show that substrate activation by the His-Asp catalytic dyad leads a ligand-O2 complex that maximizes the electron transfer from the organic substrate to O2, thus promoting intersystem crossing and circumventing the spin-forbiddeness of the reaction. Overall, our work explains how evolution has repurposed the ABH-fold architecture and its simple catalytic machinery to accomplish spin-restricted metal-independent oxygenation.
Stability, unfolding mechanism, spectroscopic, densimetric, and structural characteristics of the oxidatively stable C69S variant (HodC) of 1H-3-hydroxy-4-oxoquinaldine 2,4-dioxygenase (Hod) have been determined by classical and pressure modulation scanning calorimetry (DSC and PMDSC, respectively), circular dichroism (CD) spectroscopy, differential scanning densimetry (DSD), and dynamic light scattering measurements. At 25 degrees C, hexahistidine-tagged HodC has a hydrodynamic radius of 2.3 nm and is characterized by an unusually high degree of alpha-helical structure of approximately 60%, based on deconvolution of CD spectra. The percentage of beta-sheets and -turns is expected to be relatively low in view of its sequence similarity to proteins of the alpha/beta-hydrolase fold superfamily. His6HodC exhibits three-state unfolding (N <--> I <--> D) with an intermediate state I that exhibits at the transition temperature a volume larger than that of the native or denatured state. The intermediate state I is also associated with the highest isothermal expansion coefficient, alphaP, of the three states and exhibits a significantly lower percentage of alpha-helical structure than the native state. The stability difference between the native and intermediate state is rather small which makes I a potential candidate for reactions with various ligands, particularly those having a preference for the apparently preserved beta-type motifs.
        
Title: Dioxygenases without requirement for cofactors: identification of amino acid residues involved in substrate binding and catalysis, and testing for rate-limiting steps in the reaction of 1H-3-hydroxy-4-oxoquinaldine 2,4-dioxygenase Frerichs-Deeken U, Fetzner S Ref: Curr Microbiol, 51:344, 2005 : PubMed
1H-3-Hydroxy-4-oxoquinaldine 2,4-dioxygenase (Hod), catalyzing cleavage of its heteroaromatic substrate to form carbon monoxide and N-acetylanthranilate, belongs to the alpha/beta hydrolase fold family of enzymes. Analysis of protein variants suggested that Hod has adapted active-site residues of the alpha/beta hydrolase fold for the dioxygenolytic reaction. H251 was recently shown to act as a general base to abstract a proton from the organic substrate. Residue S101, which corresponds to the nucleophile of the catalytic triad of alpha/beta-hydrolases, presumably participates in binding the heteroaromatic substrate. H102 and residues located in the topological region of the triad's acidic residue appear to influence O2 binding and reactivity. A tyrosine residue might be involved in the turnover of the ternary complex [HodH+-3,4-dioxyquinaldine dianion-O2]. Absence of viscosity effects and kinetic solvent isotope effects suggests that turnover of the ternary complex, rather than substrate binding, product release, or proton movements, involves the rate-determining step in the reaction catalyzed by Hod.
Protein fold adaptation to novel enzymatic reactions is a fundamental evolutionary process. Cofactor-independent oxygenases degrading N-heteroaromatic substrates belong to the alpha/beta-hydrolase (ABH) fold superfamily that typically does not catalyze oxygenation reactions. Here, we have integrated crystallographic analyses at normoxic and hyperoxic conditions with molecular dynamics and quantum mechanical calculations to investigate its prototypic 1-H-3-hydroxy-4-oxoquinaldine 2,4-dioxygenase (HOD) member. O2 localization to the oxyanion hole, where catalysis occurs, is an unfavorable event and the direct competition between dioxygen and water for this site is modulated by the nucleophilic elbow residue. A hydrophobic pocket that overlaps with the organic substrate binding site can act as a proximal dioxygen reservoir. Freeze-trap pressurization allowed to determine the structure of the ternary complex with a substrate analogue and O2 bound at the oxyanion hole. Theoretical calculations reveal that O2 orientation is coupled to the charge of the bound organic ligand. When 1-H-3-hydroxy-4-oxoquinaldine is uncharged, O2 binds with its molecular axis along the ligands C2-C4 direction in full agreement with the crystal structure. Substrate activation triggered by deprotonation of its 3-OH group by the His-Asp dyad, rotates O2 by approximately 60 degrees. This geometry maximizes the charge-transfer between the substrate and O2 thus weakening the double bond of the latter. Electron density transfer to the O2(Pi*) orbital promotes the formation of the peroxide intermediate via intersystem crossing that is rate-determining. Our work provides a detailed picture of how evolution has repurposed the ABH-fold architecture and its simple catalytic machinery to accomplish metal-independent oxygenation.
Significance: Many of the current O2-dependent enzymes have evolved from classes that existed prior to the switch from a reducing to an oxidative atmosphere and whose original functions are unrelated to dioxygen chemistry. A group of bacterial dioxygenases belong to the alpha/beta-hydrolase (ABH) fold superfamily that typically does not catalyze oxygenation reactions. These enzymes degrade their N-heteroaromatic substrates in a cofactor-independent manner relying only on the simple nucleophile-histidine-acid ABH-fold catalytic toolbox. In this work we show how O2 localizes at the catalytic site by taking advantage of multiple strategies that minimize the strong competition by water, the co-substrate in the ancestral hydrolytic enzyme. We also show that substrate activation by the His-Asp catalytic dyad leads a ligand-O2 complex that maximizes the electron transfer from the organic substrate to O2, thus promoting intersystem crossing and circumventing the spin-forbiddeness of the reaction. Overall, our work explains how evolution has repurposed the ABH-fold architecture and its simple catalytic machinery to accomplish spin-restricted metal-independent oxygenation.
        
Title: Structural basis for recognition and ring-cleavage of the Pseudomonas quinolone signal (PQS) by AqdC, a mycobacterial dioxygenase of the alpha/beta-hydrolase fold family Wullich SC, Kobus S, Wienhold M, Hennecke U, Smits SHJ, Fetzner S Ref: J Struct Biol, 207:287, 2019 : PubMed
The cofactor-less dioxygenase AqdC of Mycobacteroides abscessus catalyzes the cleavage and thus inactivation of the Pseudomonas quinolone signal (PQS, 2-heptyl-3-hydroxy-4(1H)-quinolone), which plays a central role in the regulation of virulence factor production by Pseudomonas aeruginosa. We present here the crystal structures of AqdC in its native state and in complex with the PQS cleavage product N-octanoylanthranilic acid, and of mutant AqdC proteins in complex with PQS. AqdC possesses an alpha/beta-hydrolase fold core domain with additional helices forming a cap domain. The protein is traversed by a bipartite tunnel, with a funnel-like entry section leading to an elliptical substrate cavity where PQS positioning is mediated by a combination of hydrophobic interactions and hydrogen bonds, with the substrate's C4 carbonyl and C3 hydroxyl groups tethered by His97 and the catalytic His246, respectively. The side chain of the AqdC-bound product extends deeper into the "alkyl tail section" of the tunnel than PQS, tentatively suggesting product exit via this part of the tunnel. AqdC prefers PQS over congeners with shorter alkyl substituents at C2. Kinetic data confirmed the strict requirement of the active-site base His246 for catalysis, and suggested that evolution of the canonical nucleophile/His/Asp catalytic triad of the hydrolases to an Ala/His/Asp triad is favorable for catalyzing dioxygenolytic PQS ring cleavage.
Dioxygenases catalyze a diverse range of chemical reactions that involve the incorporation of oxygen into a substrate and typically use a transition metal or organic cofactor for reaction. Bacterial (1H)-3-hydroxy-4-oxoquinaldine 2,4-dioxygenase (HOD) belongs to a class of oxygenases able to catalyze this energetically unfavorable reaction without any cofactor. In the quinaldine metabolic pathway, HOD breaks down its natural N-heteroaromatic substrate using a mechanism that is still incompletely understood. Experimental and computational approaches were combined to study the initial step of the catalytic cycle. We have investigated the role of the active site His-251/Asp-126 dyad, proposed to be involved in substrate hydroxyl group deprotonation, a critical requirement for subsequent oxygen reaction. The pH profiles obtained under steady-state conditions for the H251A and D126A variants show a strong pH effect on their kcat and kcat/Km constants, with a decrease in kcat/Km of 5500- and 9-fold at pH 10.5, respectively. Substrate deprotonation studies under transient-state conditions show that this step is not rate-limiting and yield a pKa value of approximately 7.2 for WT HOD. A large solvent isotope effect was found, and the pKa value was shifted to approximately 8.3 in D2O. Crystallographic and computational studies reveal that the mutations have a minor effect on substrate positioning. Computational work shows that both His-251 and Asp-126 are essential for the proton transfer driving force of the initial reaction. This multidisciplinary study offers unambiguous support to the view that substrate deprotonation, driven by the His/Asp dyad, is an essential requirement for its activation.
        
Title: Structural basis for cofactor-independent dioxygenation of N-heteroaromatic compounds at the alpha/beta-hydrolase fold Steiner RA, Janssen HJ, Roversi P, Oakley AJ, Fetzner S Ref: Proc Natl Acad Sci U S A, 107:657, 2010 : PubMed
Enzymatic catalysis of oxygenation reactions in the absence of metal or organic cofactors is a considerable biochemical challenge. The CO-forming 1-H-3-hydroxy-4-oxoquinaldine 2,4-dioxygenase (HOD) from Arthrobacter nitroguajacolicus Ru61a and 1-H-3-hydroxy-4-oxoquinoline 2,4-dioxygenase (QDO) from Pseudomonas putida 33/1 are homologous cofactor-independent dioxygenases involved in the breakdown of N-heteroaromatic compounds. To date, they are the only dioxygenases suggested to belong to the alpha/beta-hydrolase fold superfamily. Members of this family typically catalyze hydrolytic processes rather than oxygenation reactions. We present here the crystal structures of both HOD and QDO in their native state as well as the structure of HOD in complex with its natural 1-H-3-hydroxy-4-oxoquinaldine substrate, its N-acetylanthranilate reaction product, and chloride as dioxygen mimic. HOD and QDO are structurally very similar. They possess a classical alpha/beta-hydrolase fold core domain additionally equipped with a cap domain. Organic substrates bind in a preorganized active site with an orientation ideally suited for selective deprotonation of their hydroxyl group by a His/Asp charge-relay system affording the generation of electron-donating species. The "oxyanion hole" of the alpha/beta-hydrolase fold, typically employed to stabilize the tetrahedral intermediate in ester hydrolysis reactions, is utilized here to host and control oxygen chemistry, which is proposed to involve a peroxide anion intermediate. Product release by proton back transfer from the catalytic histidine is driven by minimization of intramolecular charge repulsion. Structural and kinetic data suggest a nonnucleophilic general-base mechanism. Our analysis provides a framework to explain cofactor-independent dioxygenation within a protein architecture generally employed to catalyze hydrolytic reactions.
Stability, unfolding mechanism, spectroscopic, densimetric, and structural characteristics of the oxidatively stable C69S variant (HodC) of 1H-3-hydroxy-4-oxoquinaldine 2,4-dioxygenase (Hod) have been determined by classical and pressure modulation scanning calorimetry (DSC and PMDSC, respectively), circular dichroism (CD) spectroscopy, differential scanning densimetry (DSD), and dynamic light scattering measurements. At 25 degrees C, hexahistidine-tagged HodC has a hydrodynamic radius of 2.3 nm and is characterized by an unusually high degree of alpha-helical structure of approximately 60%, based on deconvolution of CD spectra. The percentage of beta-sheets and -turns is expected to be relatively low in view of its sequence similarity to proteins of the alpha/beta-hydrolase fold superfamily. His6HodC exhibits three-state unfolding (N <--> I <--> D) with an intermediate state I that exhibits at the transition temperature a volume larger than that of the native or denatured state. The intermediate state I is also associated with the highest isothermal expansion coefficient, alphaP, of the three states and exhibits a significantly lower percentage of alpha-helical structure than the native state. The stability difference between the native and intermediate state is rather small which makes I a potential candidate for reactions with various ligands, particularly those having a preference for the apparently preserved beta-type motifs.
        
Title: Dioxygenases without requirement for cofactors: identification of amino acid residues involved in substrate binding and catalysis, and testing for rate-limiting steps in the reaction of 1H-3-hydroxy-4-oxoquinaldine 2,4-dioxygenase Frerichs-Deeken U, Fetzner S Ref: Curr Microbiol, 51:344, 2005 : PubMed
1H-3-Hydroxy-4-oxoquinaldine 2,4-dioxygenase (Hod), catalyzing cleavage of its heteroaromatic substrate to form carbon monoxide and N-acetylanthranilate, belongs to the alpha/beta hydrolase fold family of enzymes. Analysis of protein variants suggested that Hod has adapted active-site residues of the alpha/beta hydrolase fold for the dioxygenolytic reaction. H251 was recently shown to act as a general base to abstract a proton from the organic substrate. Residue S101, which corresponds to the nucleophile of the catalytic triad of alpha/beta-hydrolases, presumably participates in binding the heteroaromatic substrate. H102 and residues located in the topological region of the triad's acidic residue appear to influence O2 binding and reactivity. A tyrosine residue might be involved in the turnover of the ternary complex [HodH+-3,4-dioxyquinaldine dianion-O2]. Absence of viscosity effects and kinetic solvent isotope effects suggests that turnover of the ternary complex, rather than substrate binding, product release, or proton movements, involves the rate-determining step in the reaction catalyzed by Hod.
        
Title: Dioxygenases without requirement for cofactors and their chemical model reaction: compulsory order ternary complex mechanism of 1H-3-hydroxy-4-oxoquinaldine 2,4-dioxygenase involving general base catalysis by histidine 251 and single-electron oxidation of the substrate dianion Frerichs-Deeken U, Ranguelova K, Kappl R, Huttermann J, Fetzner S Ref: Biochemistry, 43:14485, 2004 : PubMed
1H-3-Hydroxy-4-oxoquinaldine 2,4-dioxygenase (Hod) is a cofactor-less dioxygenase belonging to the alpha/beta hydrolase fold family, catalyzing the cleavage of 1H-3-hydroxy-4-oxoquinaldine (I) and 1H-3-hydroxy-4-oxoquinoline (II) to N-acetyl- and N-formylanthranilate, respectively, and carbon monoxide. Bisubstrate steady-state kinetics and product inhibition patterns of HodC, the C69A protein variant of Hod, suggested a compulsory-order ternary-complex mechanism, in which binding of the organic substrate precedes dioxygen binding, and carbon monoxide is released first. The specificity constants, k(cat)/K(m,A) and k(cat)/K(m,O)()2, were 1.4 x 10(8) and 3.0 x 10(5) M(-1) s(-1) with I and 1.2 x 10(5) and 0.41 x 10(5) M(-1) s(-1) with II, respectively. Whereas HodC catalyzes formation of the dianion of its organic substrate prior to dioxygen binding, HodC-H251A does not, suggesting that H251, which aligns with the histidine of the catalytic triad of the alpha/beta hydrolases, acts as general base in catalysis. Investigation of base-catalyzed dioxygenolysis of I by electron paramagnetic resonance (EPR) spectroscopy revealed formation of a resonance-stabilized radical upon exposure to dioxygen. Since in D(2)O spectral properties are not affected, exchangeable protons are not involved, confirming that the dianion is the reactive intermediate that undergoes single-electron oxidation. We suggest that in the ternary complex of the enzyme, direct single-electron transfer from the substrate dianion to dioxygen may occur, resulting in a radical pair. Based on the estimated spin distribution within the radical anion (observed in the model reaction of I), radical recombination may produce a C4- or C2-hydroperoxy(di)anion. Subsequent intramolecular attack would result in the 2,4-endoperoxy (di)anion that may collapse to the reaction products.
        
Title: 2,4-dioxygenases catalyzing N-heterocyclic-ring cleavage and formation of carbon monoxide. Purification and some properties of 1H-3-hydroxy-4-oxoquinaldine 2,4-dioxygenase from Arthrobacter sp. Ru61a and comparison with 1H-3-hydroxy-4-oxoquinoline 2,4-dioxygenase from Pseudomonas putida 33/1 Bauer I, Max N, Fetzner S, Lingens F Ref: European Journal of Biochemistry, 240:576, 1996 : PubMed
1H-3-Hydroxy-4-oxoquinaldine 2,4-dioxygenase (MeQDO) was purified from quinaldine-grown Arthrobacter sp. Ru61a. It was enriched 59-fold in a yield of 22%, and its properties were compared with 1H-3-hydroxy-4-oxoquinoline 2,4-dioxygenase (QDO) purified from Pseudomonas putida 33/1. The enzyme-catalyzed conversions were performed in an (18O)O2/(16O)O2 atmosphere. Two oxygen atoms of either (18O)O2 or (16O)O2 were incorporated at C2 and C4 of the respective substrates, indicating that these unusual enzymes, which catalyze the cleavage of two carbon-carbon bonds concomitant with CO formation, indeed are 2,4-dioxygenases. Both enzymes are small monomeric proteins of 32 kDa (MeQDO) and 30 kDa (QDO). The apparent K(m) values of MeQDO for 1H-3-hydroxy-4-oxoquinaldine and QDO for 1H-3-hydroxy-4-oxoquinoline were 30 microM and 24 microM, respectively. In both 2,4-dioxygenases, there was no spectral evidence for the presence of a chromophoric cofactor. EPR analyses of MeQDO did not reveal any signal that could be assigned to an organic radical species or to a metal, and X-ray fluorescence spectrometry of both enzymes did not show any metal present in stoichiometric amounts. Ethylxanthate, metal-chelating agents (tiron, alpha, alpha'-bipyridyl, 8-hydroxyquinoline, o-phenanthroline, EDTA, diphenylthiocarbazone, diethyldithiocarbamate), reagents that modify sulfhydryl groups (iodoacetamide, N-ethylmaleimide, p-hydroxymercuribenzoate), and reducing agents (sodium dithionite, dithiothreitol, mercaptoethanol) either did not affect 2,4-dioxygenolytic activities at all or inhibited at high concentrations only. With respect to the supposed lack of any cofactor and with respect to the inhibitors of dioxygenolytic activities, MeQDO and QDO resemble aci-reductone oxidase (CO-forming) from Klebsiella pneumoniae, which catalyzes 1,3-dioxygenolytic cleavage of 1,2-dihydroxy-3-keto-S-methylthiopentene anion (Wray, J. W. & Abeles, R. H. (1993) J. Biol. Chem. 268, 21466-21469; Wray, J. W. & Abeles, R. H. (1995) J. Biol. Chem. 270, 3147-3153). 1H-3-Hydroxy-4-oxoquinaldine and 1H-3-hydroxy-4-oxoquinoline were reactive towards molecular oxygen in the presence of the base catalyst potassium-tert.-butoxide in the aprotic solvent N,N-dimethylformamide. Base-catalyzed oxidation, yielding the same products as the enzyme-catalyzed conversions, provides a non-enzymic model reaction for 2,4-dioxygenolytic release of CO from 1H-3-hydroxy-4-oxoquinaldine and 1H-3-hydroxy-4-oxoquinoline.