Crystal structure of PHYHD1A, a 2OG oxygenase related to phytanoyl-CoA hydroxylase

Phytanoyl-CoA hydroxylase (PAHX) catalyzes an important step in the metabolism of the fatty acid side chain of chlorophyll. PHYHD1 exists in three isoforms and is the closest human homologue of PAHX. We show that like PAHX, the PHYHD1A but likely not the PHYHD1B/C isoforms, is a functional Fe(II) and 2-oxoglutarate (2OG) dependent oxygenase. Crystallographic and biochemical analyses reveal that PHYHD1A has the double-stranded β-helix fold and Fe(II) and cosubstrate binding residues characteristic of the 2-oxoglutarate dependent oxygenases and catalyzes the conversion of 2-oxoglutarate to succinate and CO₂ in an iron-dependent manner. However, PHYHD1A did not couple 2OG turnover to the hydroxylation of acyl-coenzyme A derivatives that are substrates for PAHX, implying that it is not directly involved in phytanoyl coenzyme-A metabolism.     

Structure of human phytanoyl-CoA 2-hydroxylase identifies molecular mechanisms of Refsum disease

Refsum disease (RD), a neurological syndrome characterized by adult onset retinitis pigmentosa, anosmia, sensory neuropathy, and phytanic acidaemia, is caused by elevated levels of phytanic acid. Many cases of RD are associated with mutations in phytanoyl-CoA 2-hydroxylase (PAHX), an Fe(II) and 2-oxoglutarate (2OG)-dependent oxygenase that catalyzes the initial alpha-oxidation step in the degradation of phytenic acid in peroxisomes. We describe the x-ray crystallographic structure of PAHX to 2.5 A resolution complexed with Fe(II) and 2OG and predict the molecular consequences of mutations causing RD. Like other 2OG oxygenases, PAHX possesses a double-stranded beta-helix core, which supports three iron binding ligands (His(175), Asp(177), and His(264)); the 2-oxoacid group of 2OG binds to the Fe(II) in a bidentate manner. The manner in which PAHX binds to Fe(II) and 2OG together with the presence of a cysteine residue (Cys(191)) 6.7 A from the Fe(II) and two further histidine residues (His(155) and His(281)) at its active site distinguishes it from that of the other human 2OG oxygenase for which structures are available, factor inhibiting hypoxia-inducible factor. Of the 15 PAHX residues observed to be mutated in RD patients, 11 cluster in two distinct groups around the Fe(II) (Pro(173), His(175), Gln(176), Asp(177), and His(220)) and 2OG binding sites (Trp(193), Glu(197), Ile(199), Gly(204), Asn(269), and Arg(275)). PAHX may be the first of a new subfamily of coenzyme A-binding 2OG oxygenases.     

De novo expression of uncoupling protein 3 is associated to enhanced mitochondrial thioesterase-1 expression and fatty acid metabolism in liver of fenofibrate-treated rats

Clavaminate synthase (CAS), a 2-oxoglutarate (2OG) dependent dioxygenase, catalyses three steps in the biosynthesis of clavulanic acid. Crystals of CAS complexed with Fe(II), 2OG and deoxyguanidinoproclavaminate were exposed to nitric oxide (NO) acting as a dioxygen analogue. Prior to exposure with NO, the active site Fe(II) is octahedrally coordinated by a water molecule, the 2-oxo and 1-carboxylate groups of 2OG, and the side-chains of an aspartyl and two histidinyl residues. NO binds to the position previously occupied by the 2OG 1-carboxylate concomitant with rearrangement of the latter to the position previously occupied by the displaced water.     

Catalytic Mechanisms of Fe(II)- and 2-Oxoglutarate-dependent Oxygenases

Mononuclear non-heme Fe(II)- and 2-oxoglutarate (2OG)-dependent oxygenases comprise a large family of enzymes that utilize an Fe(IV)-oxo intermediate to initiate diverse oxidative transformations with important biological roles. Here, four of the major types of Fe(II)/2OG-dependent reactions are detailed: hydroxylation, halogenation, ring formation, and desaturation. In addition, an atypical epimerization reaction is described. Studies identifying several key intermediates in catalysis are concisely summarized, and the proposed mechanisms are explained. In addition, a variety of other transformations catalyzed by selected family members are briefly described to further highlight the chemical versatility of these enzymes.     

Integrative view of 2-oxoglutarate/Fe(II)-dependent oxygenase diversity and functions in bacteria

BackgroundThe 2-oxoglutarate/Fe(II)-dependent oxygenase (2OG oxygenase) superfamily is extremely diverse and includes enzymes responsible for protein modification, DNA and mRNA repair, and synthesis of secondary metabolites.MethodsTo investigate the evolutionary relationship and make functional inferences within this remarkably diverse superfamily in bacteria, we used a protein sequence similarity network and other bioinformatics tools to analyze the bacterial proteins in the superfamily.ResultsThe network based on experimentally characterized 2OG oxygenases reflects functional clustering. Networks based on all of the bacterial 2OG oxygenases from the Interpro database indicate that only few proteins in this superfamily are functionally defined. The uneven distribution of the enzymes supports the hypothesis that horizontal gene transfer plays an important role in 2OG oxygenase evolution. A hydrophobic tyrosine residue binding the primary substrates at the N-termini is conserved. At the C-termini, the iron-binding, oxoglutarate-binding, and hydrophobic motifs are conserved and coevolved. Considering the proteins in the family are largely unexplored, we annotated them by the Pfam database and hundreds of novel and multi-domain proteins are discovered. Among them, a two-domain protein containing an N-terminal peroxiredoxin domain and a C-terminal 2OG oxygenase domain was characterized enzymatically. The results show that the enzyme could catalyze the reduction of peroxide using 2-oxoglutarate as an electron donor.ConclusionsOur observations suggest relatively low evolutionary pressure on the bacterial 2OG oxygenases and a straightforward electron transfer pathway catalyzed by the two-domain 2OG oxygenase.General significanceThis work enables an expanded understanding of the diversity, evolution, and functions of bacterial 2OG oxygenases.     

The DNA-repair protein AlkB, EGL-9, and leprecan define new families of 2-oxoglutarate- and iron-dependent dioxygenases

Background  

Protein fold recognition using sequence profile searches frequently allows prediction of the structure and biochemical mechanisms of proteins with an important biological function but unknown biochemical activity. Here we describe such predictions resulting from an analysis of the 2-oxoglutarate (2OG) and Fe(II)-dependent oxygenases, a class of enzymes that are widespread in eukaryotes and bacteria and catalyze a variety of reactions typically involving the oxidation of an organic substrate using a dioxygen molecule.     

Alteration of the co-substrate selectivity of deacetoxycephalosporin C synthase. The role of arginine 258

Deacetoxycephalosporin C synthase is an iron(II) 2-oxoglutaratedependent oxygenase that catalyzes the oxidative ring-expansion of penicillin N to deacetoxycephalosporin C. The wild-type enzyme is only able to efficiently utilize 2-oxoglutarate and 2-oxoadipate as a 2-oxoacid co-substrate. Mutation of arginine 258, the side chain of which forms an electrostatic interaction with the 5-carboxylate of the 2-oxoglutarate co-substrate, to a glutamine residue reduced activity to about 5% of the wild-type enzyme with 2-oxoglutarate. However, other aliphatic 2-oxoacids, which were not co-substrates for the wild-type enzyme, were utilized by the R258Q mutant. These 2-oxoacids "rescued" catalytic activity to the level observed for the wild-type enzyme as judged by penicillin N and G conversion. These co-substrates underwent oxidative decarboxylation as observed for 2-oxoglutarate in the normal reaction with the wild-type enzyme. Crystal structures of the iron(II)- 2-oxo-3-methylbutanoate (1.5 A), and iron(II)-2-oxo-4-methylpentanoate (1.6 A) enzyme complexes were obtained, which reveal the molecular basis for this "chemical co-substrate rescue" and help to rationalize the co-substrate selectivity of 2-oxoglutaratedependent oxygenases.     

Evidence that two enzyme-derived histidine ligands are sufficient for iron binding and catalysis by factor inhibiting HIF (FIH)

A 2-His-1-carboxylate triad of iron binding residues is present in many non-heme iron oxygenases including the Fe(II) and 2-oxoglutarate (2OG)-dependent dioxygenases. Three variants (D201A, D201E, and D201G) of the iron binding Asp-201 residue of an asparaginyl hydroxylase, factor inhibiting HIF (FIH), were made and analyzed. FIH-D201A and FIH-D201E did not catalyze asparaginyl hydroxylation, but in the presence of a reducing agent, they displayed enhanced 2OG turnover when compared with wild-type FIH. Turnover of 2OG by FIH-D201A was significantly stimulated by the addition of HIF-1alpha(786-826) peptide. Like FIH-D201A and D201E, the D201G variant enhanced 2OG turnover but rather unexpectedly catalyzed asparaginyl hydroxylation. Crystal structures of the FIH-D201A and D201G variants in complex with Fe(II)/Zn(II), 2OG, and HIF-1alpha(786-826/788-806) implied that only two FIH-based residues (His-199 and His-279) are required for metal binding. The results indicate that variation of 2OG-dependent dioxygenase iron-ligating residues as a means of functional assignment should be treated with caution. The results are of mechanistic interest in the light of recent biochemical and structural analyses of non-heme iron and 2OG-dependent halogenases that are similar to the FIH-D201A/G variants in that they use only two His-residues to ligate iron.     

Studies on the activity of the hypoxia-inducible-factor hydroxylases using an oxygen consumption assay

The activity and levels of the metazoan HIF (hypoxia-inducible factor) are regulated by its hydroxylation, catalysed by 2OG (2-oxoglutarate)- and Fe(II)-dependent dioxygenases. An oxygen consumption assay was developed and used to study the relationship between HIF hydroxylase activity and oxygen concentration for recombinant forms of two human HIF hydroxylases, PHD2 (prolyl hydroxylase domain-containing protein 2) and FIH (factor inhibiting HIF), and compared with two other 2OG-dependent dioxygenases. Although there are caveats on the absolute values, the apparent K(m) (oxygen) values for PHD2 and FIH were within the range observed for other 2OG oxygenases. Recombinant protein substrates were found to have lower apparent K(m) (oxygen) values compared with shorter synthetic peptides of HIF. The analyses also suggest that human PHD2 is selective for fragments of the C-terminal over the N-terminal oxygen-dependent degradation domain of HIF-1alpha. The present results, albeit obtained under non-physiological conditions, imply that the apparent K(m) (oxygen) values of the HIF hydroxylases enable them to act as oxygen sensors providing their in vivo capacity is appropriately matched to a hydroxylation-sensitive signalling pathway.     

Incorporation of oxygen into the succinate co-product of iron(II) and 2-oxoglutarate dependent oxygenases from bacteria, plants and humans

The ferrous iron and 2-oxoglutarate (2OG) dependent oxygenases catalyse two electron oxidation reactions by coupling the oxidation of substrate to the oxidative decarboxylation of 2OG, giving succinate and carbon dioxide coproducts. The evidence available on the level of incorporation of one atom from dioxygen into succinate is inconclusive. Here, we demonstrate that five members of the 2OG oxygenase family, AlkB from Escherichia coli, anthocyanidin synthase and flavonol synthase from Arabidopsis thaliana, and prolyl hydroxylase domain enzyme 2 and factor inhibiting hypoxia-inducible factor-1 from Homo sapiens all incorporate a single oxygen atom, almost exclusively derived from dioxygen, into the succinate co-product.     

Protein Hydroxylation Catalyzed by 2-Oxoglutarate-dependent Oxygenases

The post-translational hydroxylation of prolyl and lysyl residues, as catalyzed by 2-oxoglutarate (2OG)-dependent oxygenases, was first identified in collagen biosynthesis. 2OG oxygenases also catalyze prolyl and asparaginyl hydroxylation of the hypoxia-inducible factors that play important roles in the adaptive response to hypoxia. Subsequently, they have been shown to catalyze N-demethylation (via hydroxylation) of N^ϵ-methylated histone lysyl residues, as well as hydroxylation of multiple other residues. Recent work has identified roles for 2OG oxygenases in the modification of translation-associated proteins, which in some cases appears to be conserved from microorganisms through to humans. Here we give an overview of protein hydroxylation catalyzed by 2OG oxygenases, focusing on recent discoveries.     

Structure of proline 3-hydroxylase. Evolution of the family of 2-oxoglutarate dependent oxygenases.

Iron (II)/2-oxoglutarate (2-OG)-dependent oxygenases catalyse oxidative reactions in a range of metabolic processes including the hydroxylation of proline and lysine residues during the post-translational modification of collagen. 2-OG oxygenases commonly require ascorbate for full activity. In the vitamin C deficient disease, scurvy, reduced activity of 2-OG oxygenases results in impaired formation of collagen. Here we report the crystal structure of bacterial proline 3-hydroxylase from Streptomyces sp., an enzyme which hydroxylates proline at position 3, the first of a 2-OG oxygenase catalysing oxidation of a free alpha-amino acid. Structures were obtained for the enzyme in the absence of iron (to 2.3A resolution, R=20.2%, Rfree=25.3%) and that complexed to iron (II) (to 2.4A resolution, R=19.8%, Rfree=22.6%). The structure contains conserved motifs present in other 2-OG oxygenases including a 'jelly roll' beta strand core and residues binding iron and 2-oxoglutarate, consistent with divergent evolution within the extended family. The structure differs significantly from many other 2-OG oxygenases in possessing a discrete C-terminal helical domain. Analysis of the structure suggests a model for proline binding and a mechanism for uncoupling of proline and 2-OG turnover.     

Kinetic rationale for selectivity toward N- and C-terminal oxygen-dependent degradation domain substrates mediated by a loop region of hypoxia-inducible factor prolyl hydroxylases

Hydroxylation of two conserved prolyl residues in the N- and C-terminal oxygen-dependent degradation domains (NODD and CODD) of the alpha-subunit of hypoxia-inducible factor (HIF) signals for its degradation via the ubiquitin-proteasome pathway. In human cells, three prolyl hydroxylases (PHDs 1-3) belonging to the Fe(II) and 2-oxoglutarate (2OG)-dependent oxygenase family catalyze prolyl hydroxylation with differing selectivity for CODD and NODD. Sequence analysis of the catalytic domains of the PHDs in the light of crystal structures for PHD2, and results for other 2OG oxygenases, suggested that either the C-terminal region or a loop linking two beta-strands (beta2 and beta3 in human PHD2) are important in determining substrate selectivity. Mutation analyses on PHD2 revealed that the beta2beta3 loop is a major determinant in conferring selectivity for CODD over NODD peptides. A chimeric PHD in which the beta2beta3 loop of PHD2 was replaced with that of PHD3 displayed an almost complete selectivity for CODD (in competition experiments), as observed for wild-type PHD3. CODD was observed to bind much more tightly to this chimeric protein than the wild type PHD2 catalytic domain.     

Spectroscopic and magnetic studies of wild-type and mutant forms of the Fe(II)- and 2-oxoglutarate-dependent decarboxylase ALKBH4

The Fe(II)/2OG (2-oxoglutarate)-dependent dioxygenase superfamily comprises proteins that couple substrate oxidation to decarboxylation of 2OG to succinate. A member of this class of mononuclear non-haem Fe proteins is the Escherichia coli DNA/RNA repair enzyme AlkB. In the present work, we describe the magnetic and optical properties of the yet uncharacterized human ALKBH4 (AlkB homologue). Through EPR and UV-visible spectroscopy studies, we address the Fe-binding environment of the proposed catalytic centre of wild-type ALKBH4 and an Fe(II)-binding mutant. We could observe a novel unusual Fe(III) high-spin EPR-active species in the presence of sulfide with a g(max) of 8.2. The Fe(II) site was probed with NO. An intact histidine-carboxylate site is necessary for productive Fe binding. We also report the presence of a unique cysteine-rich motif conserved in the N-terminus of ALKBH4 orthologues, and investigate its possible Fe-binding ability. Furthermore, we show that recombinant ALKBH4 mediates decarboxylation of 2OG in absence of primary substrate. This activity is dependent on Fe as well as on residues predicted to be involved in Fe(II) co-ordination. The present results demonstrate that ALKBH4 represents an active Fe(II)/2OG-dependent decarboxylase and suggest that the cysteine cluster is involved in processes other than Fe co-ordination.     

Small-molecule active pharmaceutical ingredients of approved cancer therapeutics inhibit human aspartate/asparagine-β-hydroxylase

Human aspartate/asparagine-β-hydroxylase (AspH) is a 2-oxoglutarate (2OG) dependent oxygenase that catalyses the hydroxylation of Asp/Asn-residues of epidermal growth factor-like domains (EGFDs). AspH is reported to be upregulated on the cell surface of invasive cancer cells in a manner distinguishing healthy from cancer cells. We report studies on the effect of small-molecule active pharmaceutical ingredients (APIs) of human cancer therapeutics on the catalytic activity of AspH using a high-throughput mass spectrometry (MS)-based inhibition assay. Human B-cell lymphoma-2 (Bcl-2)-protein inhibitors, including the (R)-enantiomer of the natural product gossypol, were observed to efficiently inhibit AspH, as does the antitumor antibiotic bleomycin A₂. The results may help in the design of AspH inhibitors with the potential of increased selectivity compared to the previously identified Fe(II)-chelating or 2OG-competitive inhibitors. With regard to the clinical use of bleomycin A₂ and of the Bcl-2 inhibitor venetoclax, the results suggest that possible side-effects mediated through the inhibition of AspH and other 2OG oxygenases should be considered.     

The C-4 stereochemistry of leucocyanidin substrates for anthocyanidin synthase affects product selectivity

Anthocyanidin synthase (ANS), an iron(II) and 2-oxoglutarate (2OG) dependent oxygenase, catalyses the penultimate step in anthocyanin biosynthesis by oxidation of the 2R,3S,4S-cis-leucoanthocyanidins. It has been believed that in vivo the products of ANS are the anthocyanidins. However, in vitro studies on ANS using optically active cis- and trans-leucocyanidin substrates identified cyanidin as only a minor product; instead both quercetin and dihydroquercetin are products with the distribution being dependent on the C-4 stereochemistry of the leucocyanidin substrates.     

Structure and mechanism of anthocyanidin synthase from Arabidopsis thaliana.

Flavonoids are common colorants in plants and have long-established biomedicinal properties. Anthocyanidin synthase (ANS), a 2-oxoglutarate iron-dependent oxygenase, catalyzes the penultimate step in the biosynthesis of the anthocyanin class of flavonoids. The crystal structure of ANS reveals a multicomponent active site containing metal, cosubstrate, and two molecules of a substrate analog (dihydroquercetin). An additional structure obtained after 30 min exposure to dioxygen is consistent with the oxidation of the dihydroquercetin to quercetin and the concomitant decarboxylation of 2-oxoglutarate to succinate. Together with in vitro studies, the crystal structures suggest a mechanism for ANS-catalyzed anthocyanidin formation from the natural leucoanthocyanidin substrates involving stereoselective C-3 hydroxylation. The structure of ANS provides a template for the ubiquitous family of plant nonhaem oxygenases for future engineering and inhibition studies.