Characterization of an Fe(2+)-dependent archaeal-specific GTP cyclohydrolase, MptA, from Methanocaldococcus jannaschii

The first step in the biosynthesis of pterins in bacteria and plants is the conversion of GTP to 7,8-dihydro-d-neopterin triphosphate catalyzed by GTP cyclohydrolase I (GTPCHI). Although GTP has been shown to be a precursor of pterins in archaea, homologues of GTPCHI have not been identified in most archaeal genomes. Here we report the identification of a new GTP cyclohydrolase that converts GTP to 7,8-dihydro-d-neopterin 2',3'-cyclic phosphate, the first intermediate in methanopterin biosynthesis in methanogenic archaea. The enzyme from Methanocaldococcus jannaschii is designated MptA to indicate that it catalyzes the first step in the biosynthesis of methanopterin. MptA is the archetype of a new class of GTP cyclohydrolases that catalyzes a series of reactions most similar to that seen with GTPCHI but unique in that the cyclic phosphate is the product. MptA was found to require Fe2+ for activity. Mutation of conserved histidine residues H200N, H293N, and H295N, expected to be involved in Fe2+ binding, resulted in reduced enzymatic activity but no reduction in the amount of bound iron.

Interaction of human GTP cyclohydrolase I with its splice variants

Tetrahydrobiopterin is an essential cofactor for aromatic amino acid hydroxylases, ether lipid oxidase and nitric oxide synthases. Its biosynthesis in mammals is regulated by the activity of the homodecameric enzyme GCH (GTP cyclohydrolase I; EC 3.5.4.16). In previous work, catalytically inactive human GCH splice variants differing from the wild-type enzyme within the last 20 C-terminal amino acids were identified. In the present study, we searched for a possible role of these splice variants. Gel filtration profiles of purified recombinant proteins showed that variant GCHs form high-molecular-mass oligomers similar to the wild-type enzyme. Co-expression of splice variants together with wild-type GCH in mammalian cells revealed that GCH levels were reduced in the presence of splice variants. Commensurate with these findings, the GCH activity obtained for wild-type enzyme was reduced 2.5-fold through co-expression with GCH splice variants. Western blots of native gels suggest that splice variants form decamers despite C-terminal truncation. Therefore one possible explanation for the effect of GCH splice variants could be that inactive variants are incorporated into GCH heterodecamers, decreasing the enzyme stability and activity.

Cloning, expression, purification, and characterization of Nocardia sp. GTP cyclohydrolase I

The sequence of the gene from Nocardia sp. NRRL 5646 encoding GTP cyclohydrolase I (GCH), gch, and its adjacent regions was determined. The open reading frame of Nocardia gch contains 684 nucleotides, and the deduced amino acid sequence represents a protein of 227 amino acid residues with a calculated molecular mass of 24,563Da. The uncommon start codon TTG was identified by matching the N-terminal amino acid sequence of purified Nocardia GCH with the deduced amino acid sequence. A likely ribosomal binding site was identified 9bp upstream of the translational start site. The 3' end flank region encodes a peptide that shares high homology with dihydropteroate synthases. Nocardia GCH has 73 and 60% identity to the proteins encoded by the putative gch of Mycobacterium tuberculosis and Streptomyces coelicolor, respectively. Nocardia GCH was highly expressed in Escherichia coli cells carrying a pHAT10 based expression vector, and moderately expressed in Mycobacterium smegmatis cells carrying a pSMT3 based expression vector. Enterokinase digestion of recombinant Nocardia GCH, and in-gel digestion of Nocardia GCH and recombinant GCH followed by MALDI-TOF-MS analysis, confirmed that the actual subunit size of the enzyme was 24.5kDa. Thus, we conclude that the active form of native Nocardia GCH is a decamer. Our earlier incorrect conclusion was that the native enzyme was an octamer derived from the anomalous SDS-PAGE migration of the subunit.

GTP cyclohydrolase I: purification, characterization, and effects of inhibition on nitric oxide synthase in nocardia species

GTP cyclohydrolase I (GTPCH) catalyzes the first step in pteridine biosynthesis in Nocardia sp. strain NRRL 5646. This enzyme is important in the biosynthesis of tetrahydrobiopterin (BH4), a reducing cofactor required for nitric oxide synthase (NOS) and other enzyme systems in this organism. GTPCH was purified more than 5,000-fold to apparent homogeneity by a combination of ammonium sulfate fractionation, GTP-agarose, DEAE Sepharose, and Ultragel AcA 34 chromatography. The purified enzyme gave a single band for a protein estimated to be 32 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The molecular mass of the native enzyme was estimated to be 253 kDa by gel filtration, indicating that the active enzyme is a homo-octamer. The enzyme follows Michaelis-Menten kinetics, with a Km for GTP of 6.5 micromoles. Nocardia GTPCH possessed a unique N-terminal amino acid sequence. The pH and temperature optima for the enzyme were 7.8 and 56 degrees C, respectively. The enzyme was heat stable and slightly activated by potassium ion but was inhibited by calcium, copper, zinc, and mercury, but not magnesium. BH4 inhibited enzyme activity by 25% at a concentration of 100 micromoles. 2,4-Diamino-6-hydroxypyrimidine (DAHP) appeared to competitively inhibit the enzyme, with a Ki of 0.23 mM. With Nocardia cultures, DAHP decreased medium levels of NO2- plus NO3-. Results suggest that in Nocardia cells, NOS synthesis of nitric oxide is indirectly decreased by reducing the biosynthesis of an essential reducing cofactor, BH4.

A member of a new class of GTP cyclohydrolases produces formylaminopyrimidine nucleotide monophosphates

The hyperthermophilic euryarchaeon Methanococcus jannaschii has no recognizable homologues of the canonical GTP cyclohydrolase enzymes that are required for riboflavin and pteridine biosyntheses. Instead, it uses a new type of thermostable GTP cyclohydrolase enzyme that produces 2-amino-5-formylamino-6-ribofuranosylamino-4(3H)-pyrimidinone ribonucleotide monophosphate and inorganic phosphate. Whereas canonical GTP cyclohydrolases produce this formylamino-pyrimidine nucleotide as a reaction intermediate, this compound is shown to be an end product of the purified recombinant M.jannaschii enzyme. Unlike other enzymes that hydrolyze the alpha-beta phosphate anhydride bond of GTP, this new enzyme completely hydrolyzes pyrophosphate to inorganic phosphate. As a result, the enzyme has a steady-state turnover of 21 min(-)(1), which is much faster than those of canonical GTP cyclohydrolase enzymes. The effects of substrate analogues and inhibitors suggest that the GTP cyclohydrolase and pyrophosphate phosphohydrolase activities occur at independent sites, although both activities depend on Mg(2+).

Biosynthesis of vitamin b2 (riboflavin)

The biosynthesis of one riboflavin molecule requires one molecule of GTP and two molecules of ribulose 5-phosphate as substrates. The imidazole ring of GTP is hydrolytically opened, yielding a 4, 5-diaminopyrimidine which is converted to 5-amino-6-ribitylamino-2, 4(1H,3H)-pyrimidinedione by a sequence of deamination, side chain reduction and dephosphorylation. Condensation of 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione with 3, 4-dihydroxy-2-butanone 4-phosphate obtained from ribulose 5-phosphate affords 6,7-dimethyl-8-ribityllumazine. Dismutation of the lumazine derivative yields riboflavin and 5-amino-6-ribitylamino-2,4(1H,3H)-pyrimidinedione, which is recycled in the biosynthetic pathway. The structure of the biosynthetic enzyme, 6,7-dimethyl-8-ribityllumazine synthase, has been studied in considerable detail.

Purification and characterization of GTP cyclohydrolase I from Streptomyces tubercidicus, a producer of tubercidin

GTP cyclohydrolase I catalyzing the first reaction in the biosynthesis of pterin moiety of folic acid in bacteria, was purified from Streptomyces tubercidicus by at least 203-fold with a yield of 32% to apparent homogeneity, using ammonium sulfate fractionation, DEAE-cellulose, Sepharose CL-6B, and hydroxylapatite column chromatography. The molecular weight of the native enzyme was estimated to be 230,000 daltons by gel permeation chromatography. The purified enzyme gave a single band on sodium dodesyl sulfate-polyacrylamide gel electrophoresis and its molecular weight was apparently 58,000 daltons. These results indicate that the enzyme consists of four subunits with the same molecular weight. The K(m) and Vmax values for GTP of the purified enzyme were determined to be 80 microM and 90 nmol/min (mg protein), respectively. The optimum pH and temperature for the enzyme reaction were pH 7.5-8.5 and 40-42 degrees C, respectively. Coenzyme or metal ion was not required for the enzyme activity. The enzyme activity was inhibited by most divalent cations, while it was slightly activated by potassium ion. In case of nucleotides, CTP, GMP, GDP, and UTP inhibited enzyme activity, among which GDP exhibited the strongest inhibitory effect.

Potential of Escherichia coli GTP cyclohydrolase II for hydrolyzing 8-oxo-dGTP, a mutagenic substrate for DNA synthesis

MutT protein of Escherichia coli prevents the occurrence of A:T --> C:G transversion by hydrolyzing an oxidized form of dGTP, 8-oxo-7, 8-dihydro-2'-deoxyguanosine 5'-triphosphate (8-oxo-dGTP), which is produced by active oxygen species. In a search for mutT-related genes, we found that the ribA gene, encoding GTP cyclohydrolase II, is able to reduce the increased level of mutation frequency of the mutT strain to almost the normal level, provided that the gene product is overproduced. Purified preparations of Escherichia coli GTP cyclohydrolase II protein as well as the histidine hexamer-tagged recombinant GTP cyclohydrolase II protein efficiently hydrolyze 8-oxo-dGTP and 8-oxo-GTP, producing 8-oxo-dGMP and 8-oxo-GMP, respectively. dGTP was not hydrolyzed by these preparations. GTP cyclohydrolase II catalyzes conversion of GTP to 2, 5-diamino-6-hydroxy-4-(5-phosphoribosylamino)-pyrimidine, which constitutes the first step for riboflavin synthesis. The Km values for the three types of guanine nucleotides, GTP, 8-oxo-GTP, and 8-oxo-dGTP, were almost the same. In the mutT- background, ribA- cells showed higher spontaneous mutation frequencies as compared with that of ribA+ cells. Thus, GTP cyclohydrolase II, the ribA gene product, has a potential to protect genetic material from the untoward effects of endogenous oxygen radicals.

Purification and cloning of the GTP cyclohydrolase I feedback regulatory protein, GFRP

The activity of GTP cyclohydrolase I, the initial enzyme of the de novo pathway for biosynthesis of tetrahydrobiopterin, the cofactor required for aromatic amino acid hydroxylations and nitric oxide synthesis, is sensitive to end-product feedback inhibition by tetrahydrobiopterin. This inhibition by tetrahydrobiopterin is mediated by the GTP cyclohydrolase I feedback regulatory protein GFRP, previously named p35 (Harada, T., Kagamiyama, H., and Hatakeyama, K. (1993) Science 260, 1507-1510), and -phenylalanine specifically reverses the tetrahydrobiopterin-dependent inhibition. As a first step in the investigation of the physiological role of this unique mechanism of regulation, a convenient procedure has been developed to co-purify to homogeneity both GTP cyclohydrolase I and GFRP from rat liver. GTP cyclohydrolase I and GFRP exist in a complex which can be bound to a GTP-affinity column from which GTP cyclohydrolase I and GFRP are separately and selectively eluted. GFRP is dissociated from the GTP agarose-bound complex with 0.2 NaCl, a concentration of salt which also effectively blocks the tetrahydrobiopterin-dependent inhibitory activity of GFRP. GTP cyclohydrolase I is then eluted from the GTP-agarose column with GTP. Both GFRP and GTP cyclohydrolase I were then purified separately to near homogeneity by sequential high performance anion exchange and gel filtration chromatography. GFRP was found to have a native molecular mass of 20 kDa and consist of a homodimer of 9.5-kDa subunits. Based on peptide sequences obtained from purified GFRP, oligonucleotides were synthesized and used to clone a cDNA from a rat liver cDNA library by polymerase chain reaction-based methods. The cDNA contained an open reading frame that encoded a novel protein of 84 amino acids (calculated molecular mass 9665 daltons). This protein when expressed in Escherichia coli as a thioredoxin fusion protein had tetrahydrobiopterin-dependent GTP cyclohydrolase I inhibitory activity. Northern blot analysis indicated the presence of an 0.8-kilobase GFRP mRNA in most rat tissues, the amounts generally correlating with levels of GTP cyclohydrolase I and tetrahydrobiopterin. Thus, mRNA levels were relatively high in liver and kidney and somewhat lower in testis, heart, brain, and lung. These results suggest that GFRP is widely expressed and may play a role in regulating not only phenylalanine metabolism in the liver, but also the production of biogenic amine neurotransmitters as well as nitric oxide synthesis.

Active site topology and reaction mechanism of GTP cyclohydrolase I

GTP cyclohydrolase I of Escherichia coli is a torus-shaped homodecamer with D5 symmetry and catalyzes a complex ring expansion reaction conducive to the formation of dihydroneopterin triphosphate from GTP. The x-ray structure of a complex of the enzyme with the substrate analog, dGTP, bound at the active site was determined at a resolution of 3 A. In the decamer, 10 equivalent active sites are present, each of which contains a 10-A deep pocket formed by surface areas of 3 adjacent subunits. The substrate forms a complex hydrogen bond network with the protein. Active site residues were modified by site-directed mutagenesis, and enzyme activities of the mutant proteins were measured. On this basis, a mechanism of the enzyme-catalyzed reaction is proposed. Cleavage of the imidazole ring is initiated by protonation of N7 by His-179 followed by the attack of water at C8 of the purine system. Cystine Cys-110 Cys-181 may be involved in this reaction step. Opening of the imidazole ring may be in concert with cleavage of the furanose ring to generate a Schiff's base from the glycoside. The gamma-phosphate of GTP may be involved in the subsequent Amadori rearrangement of the carbohydrate side chain by activating the hydroxyl group of Ser-135.

Isolation of a full-length cDNA clone for human GTP cyclohydrolase I type 1 from pheochromocytoma

Although the existence of three different cDNA forms of human GTP cyclohydrolase I (GCH I) have been reported (Togari et al., 1992), the full-length sequence of any human GCH I cDNA involving poly (A) tail has not yet been documented. In the present study, we first isolated a full-length cDNA clone encoding human GCH I type 1 from human pheochromocytoma cDNA library. The length of the cDNA insert was 2,921 base pairs including poly (A) tail. RNA blot analysis showed a single mRNA species of 4.0 kb in human pheochromocytoma tissue.

Identification of a rabphilin-3A-interacting protein as GTP cyclohydrolase I in PC12 cells

Rabphilin-3A is a putative target protein for Rab3A small GTP-binding protein which is implicated in neurotransmitter release. To identify a Rabphilin-3A-interacting protein, proteins were immunoprecipitated by an anti-Rabphilin-3A polyclonal antibody from the lysate of PC12 cells and subjected to sodium dodecyl sulfate polyacrylamide gel electrophoresis followed by protein staining. Several proteins were coimmunoprecipitated with Rabphilin-3A and one of these proteins with a M(r) of about 30 KDa was phosphorylated in intact PC12 cells stimulated by high KCl. The amino acid sequence analysis of this 30 KDa protein revealed that it is GTP cyclohydrolase I.

Crystallization and preliminary crystallographic characterization of GTP cyclohydrolase I from Escherichia coli

GTP cyclohydrolase I of Escherichia coli has been purified from a recombinant bacterial strain. The enzyme was crystallized from 0.6 M-sodium citrate and from 0.8 M-sodium/potassium phosphate, respectively. Crystals grown in citrate showed X-ray diffraction extending to a resolution better than 3 A. The space group was P2(1) with cell dimensions a = 204.8 A, b = 210.1 A, c = 72.2 A, alpha = gamma = 90 degrees and beta = 95.8 degrees.

[Purification and properties of GTP-cyclohydrolase from Bacillus subtilis]

Highly purified GTP-cyclohydrolase was obtained by fractionation of cell extracts with ammonium sulfate, ion-exchange and hydrophobic chromatography. The N-terminal amino acid sequence and amino acid composition of the protein were determined. According to SDS-PAGE data, the molecular weight of the enzyme is 45 kDa. The active enzyme has several isoforms separable by native electrophoresis. The maximal enzyme activity is determined at 1.5 mM Mn2+; 70% of enzymatic activity is detected with Mg2+. The enzyme is inhibited by heavy metal ions and chelators and is inactive in the absence of thiol-reducing agents. The enzyme activity is detected in a broad range of pH with a maximum at pH 8.2. The pyrimidine product of the GTP-cyclohydrolase reaction. 2.5-diamino-6-hydroxy-4-ribosylaminopyrimidine-5'-phosphate was purified and identified. Another product of this reaction is pyrophosphate.

Isolation and characterization of GTP cyclohydrolase I from mouse liver. Comparison of normal and the hph-1 mutant

GTP cyclohydrolase I, an enzyme that catalyzes the first reaction in the pathway for the biosynthesis of pterin compounds, was purified from of C3H mouse liver by 192-fold to apparent homogeneity, using Ultrogel AcA34, DEAE-Trisacryl, and GTP-agarose gels. Its native molecular weight was estimated at 362,000. When the enzyme was subjected to electrophoresis on a denaturing polyacrylamide gel, only one protein band was evident, and its molecular weight was estimated at 55,700. The NH2-terminal amino acid of this enzyme was serine. These results indicate the enzyme consists of six to eight subunits. No coenzyme or metal ion was required for activity. This enzyme activity was inhibited by most of divalent cations and was slightly activated by potassium ion. The Km value for GTP was determined to be 17.3 microM. The temperature and pH optima for the activity were 60 degrees C and pH 8.0-8.5, respectively. The expected products, a dihydroneopterin compound and formic acid, were found in a molar ratio of 1.01. A polyclonal antiserum generated against the purified enzyme was used to compare GTP cyclohydrolase I from the hph-1 mutant and normal mouse. The hph-1 mutant liver contained only 8% of normal specific activity, but a normal amount of GTP cyclohydrolase I antigen as compared with the C3H mouse. Subunit molecular weight and electrophoretic behavior of GTP cyclohydrolase I from hph-1 mutant were not different from those of the enzyme from C3H mouse. These results suggest that the hph-1 mutation may involve alteration of the catalytic site but does not detectably alter the whole enzyme structure.

Purification and characterization of rat liver GTP cyclohydrolase I. Cooperative binding of GTP to the enzyme

GTP cyclohydrolase I, an enzyme that catalyzes the first step in the biosynthetic pathway of tetrahydrobiopterin, has been purified about 38,000-fold to apparent homogeneity from rat liver extract with a yield of 5%. The molecular weight of the enzyme was estimated to be 300,000 by gel filtration on Ultrogel AcA 34. The purified enzyme gave a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis at a position corresponding to a molecular weight of 30,000. N-terminal amino acid sequence analysis gave a single amino acid at every step of the Edman degradation up to residue 10. These results suggest that the enzyme is probably a homopolymer. The enzyme showed positive cooperativity with a Hill coefficient of 2.4 at a substrate (GTP) concentration of 10-50 microM. The Vmax value of the enzyme was 45 nmol/min.mg protein. The GTP concentration producing half-maximal velocity was 30 microM at a KCl concentration of 0.1 M. This value increased as the KCl concentration rose, without any change in Vmax or Hill number. Biosynthesis of tetrahydrobiopterin may be controlled by the intracellular level of GTP.

Human liver GTP cyclohydrolase I: purification and some properties

Human liver guanosine triphosphate (GTP) cyclohydrolase I has been purified more than 1,700-fold to what appears to be homogeneity. The active enzyme complex has an estimated molecular weight of 453,000 +/- 11,500 by gel filtration chromatography. It consists of a polypeptide of 149,000 +/- 4,000 mol wt by SDS-polyacrylamide gel electrophoresis. The activity of the enzyme is heat stable and is inhibited by di- and trivalent cations. The enzyme has an optimum pH of 7.7 in sodium phosphate buffer. It uses GTP as a sole substrate, with a Km of 116 microM.

Purification of GTP cyclohydrolase I from human liver and production of specific monoclonal antibodies

GTP cyclohydrolase I, the first enzyme in the de novo biosynthesis of tetrahydrobiopterin, was enriched more than 13,000-fold from human liver by preparative isoelectric focusing using Sephadex G-200 SF gels. The pI of the active enzyme was determined as 5.6 by analytical isoelectric focusing in the same matrix. The native enzyme has an apparent molecular mass of 440 kDa and appears to be composed of eight 50-kDa subunits as estimated from SDS/PAGE. The enriched enzyme preparation was used to produce specific monoclonal antibodies. From 11 monoclonal antibodies obtained, one was extensively characterized for further applications. This monoclonal antibody belongs to the IgM class and shows immunoreactivity with GTP cyclohydrolase I both from man and from Escherichia coli. It is capable of highly sensitive detection of GTP cyclohydrolase I by ELISA and by Western blot analysis. The monoclonal antibody was used for the immunoenzymatic localisation of GTP cyclohydrolase I in human peripheral blood mononuclear cells. Furthermore, it was possible to demonstrate the absence of immunoreactivity in cells with GTP cyclohydrolase I deficiency. The antibody's use as a tool either for differential diagnosis of atypical phenylketonuria due to GTP cyclohydrolase I deficiency or prenatal diagnosis of this severe inherited metabolic disease is now under investigation.

Repetitive recycling of guanosine triphosphate cyclohydrolase I for synthesis of dihydroneopterin triphosphate

A procedure for enzymatic production of dihydroneopterin triphosphate is described that allows GTP cyclohydrolase I to be reused repetitively. The reaction takes place in an ultrafiltration cell, and the product is collected in the filtrate, whereas the enzyme remains in the cell to be reused with additional substrate. This is repeated until the enzyme activity drops below a desirable level. The purity of the dihydroneopterin triphosphate is satisfactory for utilization of this compound for studies on enzymes involved in the synthesis of tetrahydrobiopterin and drosopterin. A procedure for purification of dihydroneopterin triphosphate is described that uses C18-silica and silica cartridges.

Purification of guanosine triphosphate cyclohydrolase I from Escherichia coli. The use of competitive inhibitors versus substrate as ligands in affinity chromatography

Different affinity chromatography ligands have been compared for the purification of guanosine triphosphate (GTP) cyclohydrolase I, an enzyme that catalyses the transformation of GTP into formate and dihydroneopterin triphosphate, the first metabolite in the biosynthetic pathway of the pterins. When this enzyme is purified by affinity chromatography on GTP-Sepharose a major fraction of the activity is lost and the yield of enzyme decreases as the amount of enzyme applied to the column decreases. The use of nucleotide competitive inhibitors (UTP and ATP) as ligands in the affinity column has shown that the extent of inactivation of the enzyme is related to the affinity of the enzyme for the ligand. Further, the extent of inactivation was reduced by reducing the length of the columns when using the same volume of GTP-Sepharose. Dihydrofolate-Sepharose gave consistently higher yields of GTP cyclohydrolase I regardless of the amount of enzyme applied, but several other proteins were also obtained. For a high purification of GTP cyclohydrolase I the best yield may be obtained with UTP as the affinity ligand and with the shortest length possible of the affinity column, and the purity of enzyme is comparable with that obtained with GTP-Sepharose.

Purification and characterization of GTP cyclohydrolase I from Drosophila melanogaster

The enzyme GTP cyclohydrolase I, which catalyzes the first step in the pteridine biosynthetic pathway, has been purified by at least 4400-fold from Drosophila melanogaster. The active complex has an apparent molecular mass of 575,000 daltons, as estimated from gel filtration. This high molecular mass complex appears to be composed of a number of 39,000-dalton subunits. A polyspecific antiserum generated against the active complex has been used to identify this polypeptide as being severely affected by mutations in Punch, the structural gene for GTP cyclohydrolase. Enzyme activity is inhibited by divalent cations and high ionic strength buffers. No cofactors have been demonstrated to be required for enzyme activity. The enzyme displays positive cooperativity in phosphate buffer, a Hill number of 2.1, but only slight cooperativity in Tris buffer, a Hill number of 1.2.

The application of 8-aminoguanosine triphosphate, a new inhibitor of GTP cyclohydrolase I, to the purification of the enzyme from human liver

GTP cyclohydrolase I from human liver and Escherichia coli is competitively inhibited by 8-aminoguanosine triphosphate with a dissociation constant (Ki) of 0.25 mumol/l. 8-Aminoguanosine triphosphate, prepared from GTP and hydroxylamine-O-sulfonic acid, was coupled to Sepharose 4B and used as an affinity adsorbent for a 309-fold purification of GTP cyclohydrolase I from human liver. GTP cyclohydrolase I from human liver is a relatively heat-stable enzyme with a half-life of 2 min at 80 degrees C, an isoelectric point (pI) of about 5.6, and a Km for GTP of 31 mumol/l. Addition of KCl (0.3 mol/l) increased the Km to 153 mumol/l. No cofactors were required for activity. L-erythro-5,6,7,8-Tetrahydrobiopterin, L-erythro-7,8-dihydrobiopterin, L-sepiapterin and 8-aminoguanosine triphosphate were strong inhibitors.

Guanosine triphosphate cyclohydrolase in Plasmodium falciparum and other Plasmodium species

GTP cyclohydrolase (EC 3.5.4.16), the first enzyme in the pteridine pathway leading to the de novo formation of folic acid, has been identified and isolated from the human malaria parasite, Plasmodium falciparum. The enzyme was purified 200-fold by high performance size-exclusion chromatography on a TSK-G-3000 SW protein column. The molecular weight was estimated at 300 000. Optimal enzyme activity was observed at pH 8.0 and 42 degrees C. The Km for GTP was 54.6 microM. Products of the enzyme reaction were identified as the carbon-8 of GTP and D-erythro-dihydroneopterin triphosphate. ATP was a competitive inhibitor (Ki = 600 microM) of the enzyme. Activity of the enzyme was Mg2+-independent, whereas Mn2+, Cu2+ and Hg2+ (5 mM) were inhibitory. GTP cyclohydrolase activity was also identified in a murine parasite, Plasmodium berghei, and a simian parasite, Plasmodium knowlesi. Activity of the enzyme in P. knowlesi, an intrinsically synchronous quotidian parasite, was found to be dependent on the stage of parasite development.

GTP-cyclohydrolases: a review

The occurrence, properties and functions of GTP-cyclohydrolases in mammalian and non-mammalian systems is reviewed. GTP-cyclohydrolases catalyse the removal of C-8 atom from GTP as formic acid. GTP-cyclohydrolase I (EC 3.5.4.16) converts GTP into D-erythro-7, 8-dihydroneopterin triphosphate, whereas GTP-cyclohydrolase II forms 2,5-diamino-4-oxo-6-(5'-phosphoribosyl) -amino pyrimidine, a possible intermediate in the biosynthesis of riboflavin. GTP-cyclohydrolase I is the first enzyme in the biosynthesis of tetrahydrobiopterin, a cofactor of the monooxygenases of the aromatic amino acids. It is the rate limiting enzyme in many mammals, but not in man. Recently, patients with GTP -cyclohydrolase I deficiency were described, a variant form of tetrahydrobiopterin-deficient hyperphenylalaninaemia.

[Purification and properties of GTP-cyclohydrolase of the yeast Pichia guilliermondii]

GTP-cyclohydrolase was isolated from the Fe-deficient cells of Pichia guilliermondii and purified 440-fold by treatment of extracts with streptomycin sulfate as well as by protein fractionation with (NH4)2SO4 at 25-45% saturation, gel filtration through Sephadex G-200 and DEAE-cellulose chromatography. The curves for the dependence of specific activity of GTP-cyclohydrolase on substrate and cofactor concentrations are non-hyperbolic; the values of [S]0.5 for GTP and Mg2+ are 2.2 X 10(-5) and 2 X 10(-4) M, respectively. The enzyme activity is inhibited by pyrophosphate ([I]0.5 = 5.8 X 10(-4) M), orthophosphate ([I]0.5 = 4.5 X 10(-3) M), heavy metal ions and chelating agents. The temperature optimum for the enzyme activity lies at 42-45 degrees C. The enzyme is labile at 4 degrees C but can well be stored at -15 degrees C. The pyrimidine product of the cyclohydrolase reaction, 2.5-diamino-6-oxy-4-ribosyl-aminopyrimidine-5'-phosphate, as well as pyrophosphate were purified from the reaction medium and identified.

Occurrence of GTP cyclohydrolase I in Bacillus stearothermophilus

A GTP cyclohydrolase which catalyzes the removal of carbon 8 of GTP as formic acid to yield a single pteridine compound occurs in an obligate thermophile Bacillus stearothermophilus ATCC 8005. The enzyme was purified 5.5-fold. Its molecular weight and Stoke's radius were estimated as 105,000 and 45.3 A, respectively. The Km for GTP was 0.98 microM. The temperature and pH optima for activity were 60-65 degrees C and 8.0-8.4, respectively. No divalent cation was required for the reaction. The pteridine product was 3'-triphosphate of 2-amino-4-hydroxy-6-(D-erythro-1',2',3'-trihydroxypropyl)-7,8-dihydropteridine (dihydroneopterin triphosphate), identified by isolating its immediate derivative, 2',3'-cyclic phosphate of 2-amino-4-hydroxy-6-(D-erythro-1',2',3'-trihydroxypropyl)pteridine (neopterin cyclic phosphate). The radioactive product from [8-14C]GTP agreed with 14C-formate. Molar ratio of formate release to pteridine formation was 1.0.

Biopterin. III. Purification and characterization of enzymes involved in the cerebral synthesis of 7,8-dihydrobiopterin

Three specific enzymes are involved in the cerebral synthesis of 7,8-dihydrobiopterin from GTP. These were isolated, purified, and characterized. The first enzyme, also catalyzing the rate-limiting step, is GTP-cyclohydrolase A-I or Mg2+-dependent A-II, which hydrolyze the GTP to the specific product 2-amino-6-(5-triphosphoribosyl)-amino-5-or-6-formamido-6-hydroxypyrimidine (FPyd-P3). FPyd-P3 is cyclized by a synthetase to D-erythro-7,8-dihydroneopterintriphosphate (NPTH2-P3). The new enzyme, D-erythro-7,8-dihydroneopterintriphosphate synthetase (enzyme B) is a basic protein of 9177 daltons containing three free SH groups, isoleucyl-seryl- as N- and valyl-glutamyl- as C-terminals. This enzyme of 69 amino acid residues from rat and 68 residues (one less aspartic acid) from guinea pig brain contains no hydroxyproline, methionine, or tryptophan. The enzyme from rat brain will gradually convert its product NPTH2-P3 to BH2, wherease the enzyme from guinea pig brain lacks this property. 2,4-amino-6-hydroxypyrimidine and dFPyd-P3 are effective inhibitors of this enzyme. The synthesis of BH2 from NPTH2-P3, but not from 7,8-dihydroneopterin, is catalyzed by L-erythro-7,8-dihydrobiopterin synthetase (enzyme C), which was purified to electrophoretic purity. This enzyme does not require pyridine nucleotides of Mc2+ for its catalysis.

Partial purification of 6-(D-erythro-1',2',3'-trihydroxypropyl)-7,8-dihydropterin triphosphate synthetase from chicken liver

An enzyme that catalyzes the formation of 6-(D-erythro-1',2',3'-trihydroxypropyl)-7,8-dihydropterin triphosphate (D-erythrodihydroneopterin triphosphate) and formic acid from GTP has been purified about 3700-fold from homogenates of chicken liver. The molecular weight of the enzyme, D-erythrodihydroneopterin triphosphate synthetase (GTP cyclohydrolase), has been estimated to be 125,000 by gel filtration on Ultrogel AcA-34. The enzyme functions optimally between pH 8.0 and 9.2 and is considerably heat-stable. No cofactors or metal ions have been demonstrated to be required for activity; however, the reaction is strongly inhibited by Cu2+ and Hg2+. GTP is the most efficient substrate, with GDP being 1/17 as active and guanosine, GMP, and ATP being inactive. The Km for GTP has been found to be 14 micrometer. Although the overall reaction catalyzed by D-erythrodihydroneopterin triphosphate synthetase from chicken liver is identical with that from Escherichia coli GTP cyclohydrolase, immunological studies show no apparent homology between the two enzymes.

Characteristics of guanosine triphosphate cyclohydrolase I purified from Escherichia coli

GTP cyclohydrolase I, an enzyme that catalyzes the first reaction in the pathway for the biosynthesis of the pteridine portion of folic acid, was purified from Escherichia coli by 3,900-fold to apparent homogeneity. Its molecular weight is estimated at 210,000. At relatively high concentrations of salt (e.g. 0.3 M KCl) the enzyme can be dissociated into seemingly identical subunits of 51,000 molecular weight. Removal of the salt allows reassociation. GTP, ATP, and inorganic orthophosphate at concentration of 5 muM, 100muM, and 0.2 mM, respectively, promote the reassociation of the subunits even in the presence of 0.3 M salt. The subunits have little or no catalytic activity. When the enzyme was subjected to electrophoresis on polyacrylamide gel under denaturing conditions (in the presence of sodium dodecyl sulfate) only one protein band was evident; its molecular weight was estimated at 25,500. Proline was determined as the only NH2-terminal amino acid residue of the enzyme. These observations suggest that the enzyme consists of four identical subunits and that each subunit contains two identical polypeptide chains. Enough GTP was bound to the enzyme to suggest that each polypeptide contains one GTP binding site. The Km value for GTP IS 0.02 MuM. ATP, dGTP, and guanosine 5'-tetraphosphate are competitive inhibitors with Ki values of 0.25 muM, 0.24 muM, and 0.13 muM, respectively. Orthophosphate is an uncompetitive inhibitor. The enzyme is relatively heat-stable; its half-life at 82 degrees is 7 min. Salt (NaCl, KCl, NH4Cl) at a concentration of 0.1 M activates the enzyme by 4- to 5-fold. The only products of the action of the enzyme are formate and the triphosphoester of 2-amino-4-hydroxy-6-(D-erythro-1',2',3'-trihydroxypropyl)-7,8-dihydropteridine (H2-neopterin-PPP). The evidence strongly suggests that this single enzyme catalyzes 4 independent chemical reactions in the conversion of GTP to H2-neopterin-PPP.

Partial purification and properties of guanosine triphosphate cyclohydrolase from Drosophila melanogaster

The first enzyme (named GTP cyclohydrolase) in the pathway for the biosynthesis of pteridines has been partially purified from extracts of late pupae and young adults of Drosophila melanogaster. This enzyme catalyzes the hydrolytic removal from GTP of carbon 8 as formate and the synthesis of 2-amino-4-hydroxy-6-(D-erythro-1',2',3'-trihydroxypropyl)-7,8-dihydropteridine triphosphate (dihydroneopterin triphosphate). Some of the properties of the enzyme are as follows: it functions optimally at pH 7.8 and at 42 C; activity is unaffected by KCl and NaCl, but divalent cations (Mg2+, Mn2+, Zn2+, and Ca2+) are inhibitory; the Km for GTP is 22 muM; and the molecular weight is estimated at 345,000 from gel filtration experiments. Of a number of nucleotides tested, only GDP and dGTP were used to any extent as substrate in place of GTP, and these respective compounds were used only 1.8% and 1.5% as well as GTP.

Purification and properties of guanosine triphosphate cyclohydrolase II from Escherichia coli

An enzyme that uses GTP as substrate for the formation in stoichiometric quantities of formate, inorganic pyrophosphate, and 2,5-diamino-6-hydroxy-4-(ribosylamino)pyrimidine-5'-phosphate has been purified 2200-fold from extracts of Escherichia coli B. This enzyme is named GTP cyclohydrolase II to distinguish it from a previously studied E. coli enzyme, named GTP cyclohydrolase (and called GTP cyclohydrolase I in this paper), that catalyzes the first of a series of enzymatic reactions leading to the biosynthesis of the pteridine portion of folic acid (Burg, A. W., and Brown, G. M. (1968) J. Biol. Chem. 243, 2349-2358). Some of the properties of GTP cyclohydrolase II are: (a) divalent cations are required for activity (Mg2+ is most effective); (b) its molecular weight, estimated by filtration on Sephadex G-200, is 44,000; (c) the K-m for GTP is 41 mum; (d) its pH optimum is 8.5; and (e) its activity is inhibited by inorganic pyrophosphate, one of the products of the reaction. Compounds not used as substrate are: GDP, GMP, guanosine, dGTP, ATP, ITP, and XTP. Properties a, b, c, and e (above), as well as the nature of the products, distinguish this enzyme from GTP cyclohydrolase I. Since GTP cyclohydrolase II apparently is not concerned with the biosynthesis of folic acid, the possible physiological role of this enzyme in the biosynthesis of riboflavin is considered in the light of the present investigations and the previously published work on riboflavin biosynthesis by other investigators.