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

Discovery of a New Prokaryotic Type I GTP Cyclohydrolase Family

GTP cyclohydrolase I (GCYH-I) is the first enzyme of the de novo tetrahydrofolate biosynthetic pathway present in bacteria, fungi, and plants, and encoded in Escherichia coli by the folE gene. It is also the first enzyme of the biopterin (BH4) pathway in Homo sapiens, where it is encoded by a homologous folE gene. A homology-based search of GCYH-I orthologs in all sequenced bacteria revealed a group of microbes, including several clinically important pathogens, that encoded all of the enzymes of the tetrahydrofolate biosynthesis pathway but GCYH-I, suggesting that an alternate family was present in these organisms. A prediction based on phylogenetic occurrence and physical clustering identified the COG1469 family as a potential candidate for this missing enzyme family. The GCYH-I activity of COG1469 family proteins from a variety of sources (Thermotoga maritima, Bacillus subtilis, Acinetobacter baylyi, and Neisseria gonorrhoeae) was experimentally verified in vivo and/or in vitro. Although there is no detectable sequence homology with the canonical GCYH-I, protein fold recognition based on sequence profiles, secondary structure, and solvation potential information suggests that, like GCYH-I proteins, COG1469 proteins are members of the tunnel-fold (T-fold) structural superfamily. This new GCYH-I family is found in approximately 20% of sequenced bacteria and is prevalent in Archaea, but the family is to this date absent in Eukarya.

Allosteric characteristics of GTP cyclohydrolase I from Escherichia coli

The kinetic and regulatory properties of GTP cyclohydrolase I were investigated using an improved enzyme assay and direct determination of the product, dihydroneopterin triphosphate. The enzyme was purified from Escherichia coli to absolute homogeneity as demonstrated by N-terminal sequencing of up to 50 amino acid residues. A 30-residue internal fragment showed 42% similarity with rat liver GTP cyclohydrolase I. The enzyme did not obey Michaelis-Menten kinetics or show a sigmoid reaction curve. The substrate saturation kinetics were found to be slow with low response to minor changes in GTP concentrations. GTP cyclohydrolase I has a relatively high apparent Km. The values are slightly different for enzyme purified by GTP-agarose (100 microM) and UTP-agarose (110 microM). Low turnover numbers of 12/min and 19/min were calculated for the respective enzyme preparations. GTP-cyclohydrolase-I activity was modulated in Vmax by K+, divalent cations, UTP and tetrahydrobiopterin. Divalent cations, such as Mg2+, had an activating effect with an optimum at 8 mM Mg2+. A different catalytic function and formation of a new, unidentified product by GTP cyclohydrolase I was observed in the presence of Ca2+. In the presence of 1 mM EDTA and Mg2+, GTP-cyclohydrolase-I activity was strongly inhibited by chelate complexes. UTP proved not to be a competitive inhibitor, but a positive modulator. The inhibition by chelate complexes was totally abolished by UTP. Tetrahydrobiopterin showed an inhibitory effect, with 50% inhibition at 100 microM tetrahydrobiopterin. UTP was able to reduce the inhibition by tetrahydrobiopterin. Using monoclonal antibody 1F11 (related to the GTP-binding site), and monoclonal antibody NS7 (mimicking tetrahydrobiopterin), different binding sites were demonstrated for GTP and tetrahydrobiopterin on each enzyme subunit. Western-blot competition analysis revealed a UTP-binding site different from the binding sites of GTP and tetrahydrobiopterin. Based on the kinetic behaviour and the kind of modulations observed we defined GTP cyclohydrolase I as an M-class allosteric enzyme.

1H-NMR and mass spectrometric studies of tetrahydropterins. Evidence for the structure of 6-pyruvoyl tetrahydropterin, an intermediate in the biosynthesis of tetrahydrobiopterin

The conversion of dihydroneopterin triphosphate in the presence of 6-pyruvoyl tetrahydropterin synthase was followed by 1H-NMR spectroscopy. The interpretation of the spectra of the product is unequivocal: they show formation of a tetrahydropterin system carrying a stereospecifically oriented substituent at the asymmetric C(6) atom. The spectra are compatible with formation of a (3')-CH3 function, and with complete removal of the 1' and 2' hydrogens of dihydroneopterin triphosphate. The fast-atom-bombardment/mass spectrometry study of the same product yields a [M + H]+ ion at m/z 238 compatible with the structure of 6-pyruvoyl tetrahydropterin. The data support the proposed structure of 6-pyruvoyl tetrahydropterin as a key intermediate in the biosynthesis of tetrahydrobiopterin.

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.

Identification of 5,6,7,8-tetrahydropterin and 5,6,7,8-tetrahydrobiopterin in Drosophila melanogaster

Using reversed-phase high-performance liquid chromatography with electrochemical detection we have demonstrated the occurrence of 5,6,7,8-tetrahydropterin and 5,6,7,8-tetrahydrobiopterin in Drosophila melanogaster. The former is the first time that has been detected in vivo. The identification has been based on the retention times, hydrodinamic voltagrams and the differential concentration in three strains of Drosophila melanogaster. Compared to the wild type, the Punch2 mutant has diminished levels of both pteridines, whereas Henna-recessive3 lacks completely tetrahydropterin and has increased levels of tetrahydrobiopterin, as expected according to their biochemical lesions.

Inhibition of GTP cyclohydrolase I by pterins

Pterins inhibit rat liver GTP cyclohydrolase I activity noncompetitively. Reduced pterins, such as 7,8-dihydro-D-neopterin, (6R,S)-5,6,7,8-tetrahydro-D-neopterin, 7,8-dihydro-L-biopterin, (6R)-5,6,7,8-tetrahydro-L-biopterin, L-sepiapterin, and DL-6-methyl-5,6,7,8-tetrahydropterin are approximately 12-times more potent as inhibitors than are oxidized pterins, such as D-neopterin, L-biopterin, and isoxanthopterin. They are also 12-times more potent than folates, such as folic acid, dihydrofolic acid, (+/-)-L-tetrahydrofolic acid, and aminopterin. The Ki values for 7,8-dihydro-D-neopterin, 7,8-dihydro-L-biopterin, and (6R)-5,6,7,8-tetrahydro-L-biopterin are 12.7 microM, 14.4 microM, and 15.7 microM, respectively. These results suggest that mammalian GTP cyclohydrolase I may be regulated by its metabolic end products.