Blue color, metal content, and substrate binding in 4-hydroxyphenylpyruvate dioxygenase from Pseudomonas sp. strain P. J. 874

Genetic improvement of tocotrienol content enhances the oxidative stability of canola oil

Background

Tocotrienols and tocopherols, which are synthesized in plastids of plant cells with similar functionalities, comprise vitamin E to serve as a potent lipid-soluble antioxidant in plants. The synthesis of tocopherols involves the condensation of homogentisic acid (HGA) and phytyl diphosphate (PDP) under the catalysis of homogentisate phytyltransferase (HPT). Tocotrienol synthesis is initiated by the condensation of HGA and geranylgeranyl diphosphate (GGDP) mediated by homogentisate geranylgeranyl transferase (HGGT). As one of the most important oil crops, canola seed is regarded as an ideal plant to efficiently improve the production of vitamin E tocochromanols through genetic engineering approaches. However, only a modest increase in tocopherol content has been achieved in canola seed to date.

Methods

In this study, we transformed barley HGGT (HvHGGT) into canola to improve total tocochromanol content in canola seeds.

Results and discussion

The results showed that the total tocochromanol content in the transgenic canola seeds could be maximally increased by fourfold relative to that in wild-type canola seeds. Notably, no negative impact on important agronomic traits was observed in transgenic canola plants, indicating great application potential of the HvHGGT gene in enhancing tocochromanol content in canola in the future. Moreover, the oil extracted from the transgenic canola seeds exhibited significantly enhanced oxidative stability under high temperature in addition to the increase in total tocochromanol content, demonstrating multiple desirable properties of HvHGGT.

Role of the N-terminus in human 4-hydroxyphenylpyruvate dioxygenase activity

4-Hydroxyphenylpyruvate dioxygenase (HPPD) is a key enzyme in tyrosine catabolism, catalysing the oxidation of 4-hydroxyphenylpyruvate to homogentisate. Genetic deficiency of this enzyme causes type III tyrosinaemia. The enzyme comprises two barrel-shaped domains formed by the N- and C-termini, with the active site located in the C-terminus. This study investigated the role of the N-terminus, located at the domain interface, in HPPD activity. We observed that the kcat/Km decreased ∼8-fold compared with wild type upon removal of the 12 N-terminal residues (ΔR13). Interestingly, the wild-type level of activity was retained in a mutant missing the 17 N-terminal residues, with a kcat/Km 11-fold higher than that of the ΔR13 mutant; however, the structural stability of this mutant was lower than that of wild type. A 2-fold decrease in catalytic efficiency was observed for the K10A and E12A mutants, indicating synergism between these residues in the enzyme catalytic function. A molecular dynamics simulation showed large RMS fluctuations in ΔR13 suggesting that conformational flexibility at the domain interface leads to lower activity in this mutant. These results demonstrate that the N-terminus maintains the stability of the domain interface to allow for catalysis at the active site of HPPD.

Crystal Structure of 4-Hydroxyphenylpyruvate Dioxygenase in Complex with Substrate Reveals a New Starting Point for Herbicide Discovery

4-Hydroxyphenylpyruvate dioxygenase (HPPD) is a promising target for drug and pesticide discovery. The unknown binding mode of substrate is still a big challenge for the understanding of enzymatic reaction mechanism and novel HPPD inhibitor design. Herein, we determined the first crystal structure of Arabidopsis thaliana HPPD (AtHPPD) in complex with its natural substrate (HPPA) at a resolution of 2.80 Å. Then, combination of hybrid quantum mechanics/molecular mechanics (QM/MM) calculations confirmed that HPPA takes keto rather than enol form inside the HPPD active pocket. Subsequent site-directed mutagenesis and kinetic analysis further showed that residues (Phe424, Asn423, Glu394, Gln307, Asn282, and Ser267) played important roles in substrate binding and catalytic cycle. Structural comparison between HPPA-AtHPPD and holo-AtHPPD revealed that Gln293 underwent a remarkable rotation upon the HPPA binding and formed H-bond network of Ser267-Asn282-Gln307-Gln293, resulting in the transformation of HPPD from an inactive state to active state. Finally, taking the conformation change of Gln293 as a target, we proposed a new strategy of blocking the transformation of HPPD from inactive state to active state to design a novel inhibitor with K_i value of 24.10 nM towards AtHPPD. The inhibitor has entered into industry development as the first selective herbicide used for the weed control in sorghum field. The crystal structure of AtHPPD in complex with the inhibitor (2.40 Å) confirmed the rationality of the design strategy. We believe that the present work provides a new starting point for the understanding of enzymatic reaction mechanism and the design of next generation HPPD inhibitors.

Effect of long-distance transportation on serum metabolic profiles of steer calves

Long-distance transportation is sometimes inevitable in the beef industry because of the geographic separation of major breeding and fattening areas. Long-distance transportation negatively impacts production and health of cattle, which may, at least partly, result from the disturbance of metabolism during and after transportation. However, alteration of metabolism remains elusive in transported cattle. We investigated the effects of transportation on the metabolomic profiles of Holstein steer calves. Non-targeted analysis of serum concentrations of low molecular weight metabolites was performed by gas chromatography mass spectrometry. Transportation affected 38 metabolites in the serum. A pathway analysis suggested that 26, 10, and 10 pathways were affected immediately after transportation, and 3 and 7 days after transportation, respectively. Some pathways were disturbed only immediately after transportation, likely because of feed and water withdrawal during transit. Nicotinate and nicotinamide metabolism, and citric acid cycle were affected for 3 days after transportation, whereas propionate metabolism, phenylalanine and tyrosine metabolism were affected throughout the experiment. Four pathways were not affected immediately after transportation, but were altered thereafter. These results suggested that many metabolic pathways had marked perturbations during transportation. Metabolites such as citric acid, propionate, tyrosine and niacin can be candidate supplements for mitigating transportation-induced adverse effects.

Substrate and Cofactor Range Differences of Two Cysteine Dioxygenases from Ralstonia eutropha H16

Cysteine dioxygenases (Cdos), which catalyze the sulfoxidation of cysteine to cysteine sulfinic acid (CSA), have been extensively studied in eukaryotes because of their roles in several diseases. In contrast, only a few prokaryotic enzymes of this type have been investigated. In Ralstonia eutropha H16, two Cdo homologues (CdoA and CdoB) have been identified previously. In vivo studies showed that Escherichia coli cells expressing CdoA could convert 3-mercaptopropionate (3MP) to 3-sulfinopropionate (3SP), whereas no 3SP could be detected in cells expressing CdoB. The objective of this study was to confirm these findings and to study both enzymes in detail by performing an in vitro characterization. The proteins were heterologously expressed and purified to apparent homogeneity by immobilized metal chelate affinity chromatography (IMAC). Subsequent analysis of the enzyme activities revealed striking differences with regard to their substrate ranges and their specificities for the transition metal cofactor, e.g., CdoA catalyzed the sulfoxidation of 3MP to a 3-fold-greater extent than the sulfoxidation of cysteine, whereas CdoB converted only cysteine. Moreover, the dependency of the activities of the Cdos from R. eutropha H16 on the metal cofactor in the active center could be demonstrated. The importance of CdoA for the metabolism of the sulfur compounds 3,3'-thiodipropionic acid (TDP) and 3,3'-dithiodipropionic acid (DTDP) by further converting their degradation product, 3MP, was confirmed. Since 3MP can also function as a precursor for polythioester (PTE) synthesis in R. eutropha H16, deletion of cdoA might enable increased synthesis of PTEs.

Genetic and biochemical basis for alternative routes of tocotrienol biosynthesis for enhanced vitamin E antioxidant production

Vitamin E tocotrienol synthesis in monocots requires homogentisate geranylgeranyl transferase (HGGT), which catalyzes the condensation of homogentisate and the unsaturated C20 isoprenoid geranylgeranyl diphosphate (GGDP). By contrast, vitamin E tocopherol synthesis is mediated by homogentisate phytyltransferase (HPT), which condenses homogentisate and the saturated C20 isoprenoid phytyl diphosphate (PDP). An HGGT-independent pathway for tocotrienol synthesis has also been shown to occur by de-regulation of homogentisate synthesis. In this paper, the basis for this pathway and its impact on vitamin E production when combined with HGGT are explored. An Arabidopsis line was initially developed that accumulates tocotrienols and homogentisate by co-expression of Arabidopsis hydroxyphenylpyruvate dioxygenase (HPPD) and Escherichia coli bi-functional chorismate mutase/prephenate dehydrogenase (TyrA). When crossed into the vte2-1 HPT null mutant, tocotrienol production was lost, indicating that HPT catalyzes tocotrienol synthesis in HPPD/TyrA-expressing plants by atypical use of GGDP as a substrate. Consistent with this, recombinant Arabidopsis HPT preferentially catalyzed in vitro production of the tocotrienol precursor geranylgeranyl benzoquinol only when presented with high molar ratios of GGDP:PDP. In addition, tocotrienol levels were highest in early growth stages in HPPD/TyrA lines, but decreased strongly relative to tocopherols during later growth stages when PDP is known to accumulate. Collectively, these results indicate that HPPD/TyrA-induced tocotrienol production requires HPT and occurs upon enrichment of GGDP relative to PDP in prenyl diphosphate pools. Finally, combined expression of HPPD/TyrA and HGGT in Arabidopsis leaves and seeds resulted in large additive increases in vitamin E production, indicating that homogentisate concentrations limit HGGT-catalyzed tocotrienol synthesis.

Unraveling the genetic basis of seed tocopherol content and composition in rapeseed (Brassica napus L.)

BACKGROUND

Tocopherols are important antioxidants in vegetable oils; when present as vitamin E, tocopherols are an essential nutrient for humans and livestock. Rapeseed (Brassica napus L, AACC, 2 n = 38) is one of the most important oil crops and a major source of tocopherols. Although the tocopherol biosynthetic pathway has been well elucidated in the model photosynthetic organisms Arabidopsis thaliana and Synechocystis sp. PCC6803, knowledge about the genetic basis of tocopherol biosynthesis in seeds of rapeseed is scant. This project was carried out to dissect the genetic basis of seed tocopherol content and composition in rapeseed through quantitative trait loci (QTL) detection, genome-wide association analysis, and homologous gene mapping.

METHODOLOGY/PRINCIPAL FINDINGS

We used a segregating Tapidor × Ningyou7 doubled haploid (TNDH) population, its reconstructed F(2) (RC-F(2)) population, and a panel of 142 rapeseed accessions (association panel). Genetic effects mainly contributed to phenotypic variations in tocopherol content and composition; environmental effects were also identified. Thirty-three unique QTL were detected for tocopherol content and composition in TNDH and RC-F(2) populations. Of these, seven QTL co-localized with candidate sequences associated with tocopherol biosynthesis through in silico and linkage mapping. Several near-isogenic lines carrying introgressions from the parent with higher tocopherol content showed highly increased tocopherol content compared with the recurrent parent. Genome-wide association analysis was performed with 142 B. napus accessions. Sixty-one loci were significantly associated with tocopherol content and composition, 11 of which were localized within the confidence intervals of tocopherol QTL.

CONCLUSIONS/SIGNIFICANCE

This joint QTL, candidate gene, and association mapping study sheds light on the genetic basis of seed tocopherol biosynthesis in rapeseed. The sequences presented here may be used for marker-assisted selection of oilseed rape lines with superior tocopherol content and composition.

Evidence for a dual role of an active site histidine in α-amino-β-carboxymuconate-ε-semialdehyde decarboxylase

The previously reported crystal structures of α-amino-β-carboxymuconate-ε-semialdehyde decarboxylase (ACMSD) show a five-coordinate Zn(II)(His)(3)(Asp)(OH(2)) active site. The water ligand is H-bonded to a conserved His228 residue adjacent to the metal center in ACMSD from Pseudomonas fluorescens (PfACMSD). Site-directed mutagenesis of His228 to tyrosine and glycine in this study results in a complete or significant loss of activity. Metal analysis shows that H228Y and H228G contain iron rather than zinc, indicating that this residue plays a role in the metal selectivity of the protein. As-isolated H228Y displays a blue color, which is not seen in wild-type ACMSD. Quinone staining and resonance Raman analyses indicate that the blue color originates from Fe(III)-tyrosinate ligand-to-metal charge transfer. Co(II)-substituted H228Y ACMSD is brown in color and exhibits an electron paramagnetic resonance spectrum showing a high-spin Co(II) center with a well-resolved (59)Co (I = 7/2) eight-line hyperfine splitting pattern. The X-ray crystal structures of as-isolated Fe-H228Y (2.8 Å) and Co-substituted (2.4 Å) and Zn-substituted H228Y (2.0 Å resolution) support the spectroscopic assignment of metal ligation of the Tyr228 residue. The crystal structure of Zn-H228G (2.6 Å) was also determined. These four structures show that the water ligand present in WT Zn-ACMSD is either missing (Fe-H228Y, Co-H228Y, and Zn-H228G) or disrupted (Zn-H228Y) in response to the His228 mutation. Together, these results highlight the importance of His228 for PfACMSD's metal specificity as well as maintaining a water molecule as a ligand of the metal center. His228 is thus proposed to play a role in activating the metal-bound water ligand for subsequent nucleophilic attack on the substrate.

Crystal structure of prolyl 4-hydroxylase from Bacillus anthracis

Prolyl 4-hydroxylases (P4H) catalyze the post-translational hydroxylation of proline residues and play a role in collagen production, hypoxia response, and cell wall development. P4Hs belong to the group of Fe(II)/alphaKG oxygenases and require Fe(II), alpha-ketoglutarate (alphaKG), and O(2) for activity. We report the 1.40 A structure of a P4H from Bacillus anthracis, the causative agent of anthrax, whose immunodominant exosporium protein BclA contains collagen-like repeat sequences. The structure reveals the double-stranded beta-helix core fold characteristic of Fe(II)/alphaKG oxygenases. This fold positions Fe-binding and alphaKG-binding residues in what is expected to be catalytically competent orientations and is consistent with proline peptide substrate binding at the active site mouth. Comparisons of the anthrax P4H structure with Cr P4H-1 structures reveal similarities in a peptide surface groove. However, sequence and structural comparisons suggest differences in conformation of adjacent loops may change the interaction with peptide substrates. These differences may be the basis of a substantial disparity between the K(M) values for the Cr P4H-1 compared to the anthrax and human P4H enzymes. Additionally, while previous structures of P4H enzymes are monomers, B. anthracis P4H forms an alpha(2) homodimer and suggests residues important for interactions between the alpha(2) subunits of alpha(2)beta(2) human collagen P4H. Thus, the anthrax P4H structure provides insight into the structure and function of the alpha-subunit of human P4H, which may aid in the development of selective inhibitors of the human P4H enzyme involved in fibrotic disease.

The diverse and pervasive chemistries of the alpha-keto acid dependent enzymes

The number of identified and confirmed alpha-keto acid dependent oxygenases is increasing rapidly. All of these enzymes have a relatively simple liganding arrangement for a single ferrous ion but collectively conduct a highly diverse set of chemistries. While hydroxylations and a variety of oxidation reactions have been most commonly observed, new reactions involving dealkylations, epimerizations and halogenations have recently been discovered. In this minireview we present what is known of the alpha-keto acid dependent enzymes and offer an argument that the chemistry that is unique to each enzyme occurs only after the production of a pivotal ferryl-oxo intermediate.

Post-translational self-hydroxylation: A probe for oxygen activation mechanisms in non-heme iron enzymes

Recent years have seen considerable evolution in our understanding of the mechanisms of oxygen activation by non-heme iron enzymes, with high-valent iron-oxo intermediates coming to the forefront as formidably potent oxidants. In the absence of substrate, the generation of vividly colored chromophores deriving from the self-hydroxylation of a nearby aromatic amino acid for a number of these enzymes has afforded an opportunity to discern the conditions under which O2 activation occurs to generate a high-valent iron intermediate, and has provided a basis for a rigorous mechanistic examination of the oxygenation process. Here, we summarize the current evidence for self-hydroxylation processes in both mononuclear non-heme iron enzymes and in mutant forms of ribonucleotide reductase, and place it within the context of our developing understanding of the oxidative transformations accomplished by non-heme iron centers.

Self-hydroxylation of taurine/alpha-ketoglutarate dioxygenase: evidence for more than one oxygen activation mechanism

2-Aminoethanesulfonic acid (taurine)/alpha-ketoglutarate (alphaKG) dioxygenase (TauD) is a mononuclear non-heme iron enzyme that catalyzes the hydroxylation of taurine to generate sulfite and aminoacetaldehyde in the presence of O2, alphaKG, and Fe(II). Fe(II)TauD complexed with alphaKG or succinate, the decarboxylated product of alphaKG, reacts with O2 in the absence of prime substrate to generate 550- and 720-nm chromophores, respectively, that are interconvertible by the addition or removal of bound bicarbonate and have resonance Raman features characteristic of an Fe(III)-catecholate complex. Mutagenesis studies suggest that both reactions result in the self-hydroxylation of the active-site residue Tyr73, and liquid chromatography nano-spray mass spectrometry/mass spectrometry evidence corroborates this result for the succinate reaction. Furthermore, isotope-labeling resonance Raman studies demonstrate that the oxygen atom incorporated into the tyrosyl residue derives from H2 18O and 18O2 for the alphaKG and succinate reactions, respectively, suggesting distinct mechanistic pathways. Whereas the alphaKG-dependent hydroxylation likely proceeds via an Fe(IV) = O intermediate that is known to be generated during substrate hydroxylation, we propose Fe(III)-OOH (or Fe(V) = O) as the oxygenating species in the succinate-dependent reaction. These results demonstrate the two oxygenating mechanisms available to enzymes with a 2-His-1-carboxylate triad, depending on whether the electron source donates one or two electrons.

4-Hydroxyphenylpyruvate dioxygenase

4-Hydroxyphenylpyruvate dioxygenase (HPPD) is an Fe(II)-dependent, non-heme oxygenase that catalyzes the conversion of 4-hydroxyphenylpyruvate to homogentisate. This reaction involves decarboxylation, substituent migration and aromatic oxygenation in a single catalytic cycle. HPPD is a member of the alpha-keto acid dependent oxygenases that typically require an alpha-keto acid (almost exclusively alpha-ketoglutarate) and molecular oxygen to either oxygenate or oxidize a third molecule. As an exception in this class of enzymes HPPD has only two substrates, does not use alpha-ketoglutarate, and incorporates both atoms of dioxygen into the aromatic product, homogentisate. The tertiary structure of the enzyme would suggest that its mechanism converged with that of other alpha-keto acid enzymes from an extradiol dioxygenase progenitor. The transformation catalyzed by HPPD has both agricultural and therapeutic significance. HPPD catalyzes the second step in the pathway for the catabolism of tyrosine, that is common to essentially all aerobic forms of life. In plants this pathway has an anabolic branch from homogentisate that forms essential isoprenoid redox cofactors such as plastoquinone and tocopherol. Naturally occurring multi-ketone molecules act as allelopathic agents by inhibiting HPPD and preventing the production of homogentisate and hence required redox cofactors. This has been the basis for the development of a range of very effective herbicides that are currently used commercially. In humans, deficiencies of specific enzymes of the tyrosine catabolism pathway give rise to a number of severe metabolic disorders. Interestingly, HPPD inhibitor/herbicide molecules act also as therapeutic agents for a number of debilitating and lethal inborn defects in tyrosine catabolism by preventing the accumulation of toxic metabolites.

4-Hydroxyphenylpyruvate Dioxygenase: A Hybrid Density Functional Study of the Catalytic Reaction Mechanism

Density functional calculations using the B3LYP functional has been used to study the reaction mechanism of 4-hydroxyphenylpyruvate dioxygenase. The first part of the catalytic reaction, dioxygen activation, is found to have the same mechanism as in alpha-ketoglutarate-dependent enzymes; the ternary enzyme-substrate-dioxygen complex is first decarboxylated to the iron(II)-peracid intermediate, followed by heterolytic cleavage of the O-O bond yielding an iron(IV)-oxo species. This highly reactive intermediate attacks the aromatic ring at the C1 position and forms a radical sigma complex, which can either form an arene oxide or undergo a C1-C2 side-chain migration. The arene oxide is found to have no catalytic relevance. The side-chain migration is a two-step process; the carbon-carbon bond cleavage first affords a biradical intermediate, followed by a decay of this species forming the new C-C bond. The ketone intermediate formed by a 1,2 shift of an acetic acid group rearomatizes either at the active site of the enzyme or in solution. The hypothetical oxidation of the aromatic ring at the C2 position was also studied to shed light on the 4-HPPD product specificity. In addition, the benzylic hydroxylation reaction, catalyzed by 4-hydroxymandelate synthase, was also studied. The results are in good agreement with the experimental findings.

Aberrant activity of the DNA repair enzyme AlkB

Escherichia coli AlkB is a DNA/RNA repair enzyme containing a mononuclear Fe(II) site that couples the oxidative decomposition of alpha-ketoglutarate (alphaKG) to the hydroxylation of 1-methyladenine or 3-methylcytosine lesions in DNA or RNA, resulting in release of formaldehyde and restoration of the normal bases. In the presence of Fe(II), alphaKG, and oxygen, but the absence of methylated DNA, AlkB was found to catalyze an aberrant reaction that generates a blue chromophore. The color is proposed to derive from Fe(III) coordinated by a hydroxytryptophan at position 178 as revealed by mass spectrometric analysis. Protein structural modeling confirms that Trp 178 is reasonably positioned to react with the Fe(IV)-oxo intermediate proposed to form at the active site.

The crystal structures of Zea mays and Arabidopsis 4-hydroxyphenylpyruvate dioxygenase

The transformation of 4-hydroxyphenylpyruvate to homogentisate, catalyzed by 4-hydroxyphenylpyruvate dioxygenase (HPPD), plays an important role in degrading aromatic amino acids. As the reaction product homogentisate serves as aromatic precursor for prenylquinone synthesis in plants, the enzyme is an interesting target for herbicides. In this study we report the first x-ray structures of the plant HPPDs of Zea mays and Arabidopsis in their substrate-free form at 2.0 A and 3.0 A resolution, respectively. Previous biochemical characterizations have demonstrated that eukaryotic enzymes behave as homodimers in contrast to prokaryotic HPPDs, which are homotetramers. Plant and bacterial enzymes share the overall fold but use orthogonal surfaces for oligomerization. In addition, comparison of both structures provides direct evidence that the C-terminal helix gates substrate access to the active site around a nonheme ferrous iron center. In the Z. mays HPPD structure this helix packs into the active site, sequestering it completely from the solvent. In contrast, in the Arabidopsis structure this helix tilted by about 60 degrees into the solvent and leaves the active site fully accessible. By elucidating the structure of plant HPPD enzymes we aim to provide a structural basis for the development of new herbicides.

Crystal structure of Pseudomonas fluorescens 4-hydroxyphenylpyruvate dioxygenase: an enzyme involved in the tyrosine degradation pathway

BACKGROUND

In plants and photosynthetic bacteria, the tyrosine degradation pathway is crucial because homogentisate, a tyrosine degradation product, is a precursor for the biosynthesis of photosynthetic pigments, such as quinones or tocophenols. Homogentisate biosynthesis includes a decarboxylation step, a dioxygenation and a rearrangement of the pyruvate sidechain. This complex reaction is carried out by a single enzyme, the 4-hydroxyphenylpyruvate dioxygenase (HPPD), a non-heme iron dependent enzyme that is active as a homotetramer in bacteria and as a homodimer in plants. Moreover, in humans, a HPPD deficiency is found to be related to tyrosinemia, a rare hereditary disorder of tyrosine catabolism.

RESULTS

We report here the crystal structure of Pseudomonas fluorescens HPPD refined to 2.4 A resolution (Rfree 27.6%; R factor 21.9%). The general topology of the protein comprises two barrel-shaped domains and is similar to the structures of Pseudomonas 2,3-dihydroxybiphenyl dioxygenase (DHBD) and Pseudomonas putida catechol 2,3-dioxygenase (MPC). Each structural domain contains two repeated betaalpha betabeta betaalpha modules. There is one non-heme iron atom per monomer liganded to the sidechains of His161, His240, Glu322 and one acetate molecule.

CONCLUSIONS

The analysis of the HPPD structure and its superposition with the structures of DHBD and MPC highlight some important differences in the active sites of these enzymes. These comparisons also suggest that the pyruvate part of the HPPD substrate (4-hydroxyphenylpyruvate) and the O2 molecule would occupy the three free coordination sites of the catalytic iron atom. This substrate-enzyme model will aid the design of new inhibitors of the homogentisate biosynthesis reaction.

Human 4-hydroxyphenylpyruvate dioxygenase. Primary structure and chromosomal localization of the gene

We report the primary structure of 4-hydroxyphenylpyruvate dioxygenase [4-hydroxyphenyl-pyruvate:oxygen oxidoreductase (hydroxylating, decarboxylating)]. The work is based on the isolation of cDNA clones from human liver lambda gt11 libraries. Several overlapping clones covering the coding sequence were characterized. In parallel, peptides from four different digests of the purified protein were analysed for their amino-acid sequence. These peptide sequences covered 86% of the cDNA-derived amino-acid sequence. This gives the sequence for a polypeptide of 392 amino acids with a calculated molecular mass of 44.8 kDa. There is more than 80% identity between the human and the pig enzymes and also between these enzymes and the F antigen from rat and the two allelic forms of this antigen from mouse. The enzyme has 53% conserved amino acids and 27% identical amino acids in common with 4-hydroxyphenylpyruvate dioxygenase from Pseudomonas sp. P.J. 874 and 52% conserved and 28% identical residues, with a protein from Shewanella colwelliana. At the C-terminus there is 61% identity between the seven proteins. These results indicate that these proteins are all 4-hydroxyphenylpyruvate dioxygenases. The identity of the C-terminus makes this part of the molecule a candidate for a functional role in the catalytic process. At conserved positions in all seven enzymes, there are two tyrosine residues and three histidine residues, i.e. amino acids which have been implicated as ligands for iron in 2-oxoacid-dependent dioxygenases. The gene encoding the enzyme was localized to chromosome 12q14-->qter by Southern-blot analysis of human-rodent somatic-cell hybrids.

Characterization of 4-hydroxyphenylpyruvate dioxygenase. Primary structure of the Pseudomonas enzyme

The primary structure of Pseudomonas 4-hydroxyphenylpyruvate dioxygenase was determined. Sequence degradation of the intact protein and of peptides from three different digests of the carboxymethylated protein established a 357-residue polypeptide chain with a free alpha-amino group. Hydroxylamine cleavage at a single Asn-Gly sequence was useful. Comparisons with known structures in data banks revealed no close relationship with other characterized proteins. The human enzyme has a related composition, suggesting that also the eukaryotic form belongs to this protein type, but with a blocked N-terminus like in many other eukaryotic intracellular proteins. Secondary structure predictions suggest an alpha/beta mixed structure, fairly typical of globular proteins, without long segments of hydrophobicity or charge, although a region in the middle of the C-terminal third of the subunit appears to have the most extreme properties. A ferric centre, correlating with enzyme activity and absorbance at 595 nm, has previously been assigned to tyrosinate coordination. The Tyr and His distributions, and the position of a single Cys residue, all suggest a few likely sites, outside the C-terminal segment, for this centre.