GTP cyclohydrolase

In plants and several microorganisms the conversion of GTP to dihydroneopterin triphosphate provides the first committed intermediate in the biosynthesis of tetrahydrofolate [263]. In animals, dihydroneopterin triphosphate is converted to tetrahydrobiopterin which fimctions as an essential cofactor to NO synthases, and monooxygenases [264]. The active site Zn(II) is postulated, as in other Zn-containing enzymes, to polarize its bound water ligand by increasing its acidity, subsequently generating an hydroxyl nucleophile in close proximity of the imidazole ring of the bound substrate. 

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In E. coli GTP cyclohydrolase catalyzes the conversion of GTP (33) into 7,8-dihydroneoptetin triphosphate (34) via a three-step sequence. Hydrolysis of the triphosphate group of (34) is achieved by a nonspecific pyrophosphatase to afford dihydroneopterin (35) (65). The free alcohol (36) is obtained by the removal of residual phosphate by an unknown phosphomonoesterase. The dihydroneoptetin undergoes a retro-aldol reaction with the elimination of a hydroxy acetaldehyde moiety. Addition of a pyrophosphate group affords hydroxymethyl-7,8-dihydroptetin pyrophosphate (37). Dihydropteroate synthase catalyzes the condensation of hydroxymethyl-7,8-dihydropteroate pyrophosphate with PABA to furnish 7,8-dihydropteroate (38). Finally, L-glutamic acid is condensed with 7,8-dihydropteroate in the presence of dihydrofolate synthetase. 

This interesting conversion of a five- into a six-membered heterocyclic ring was proven by the isolation of the enzyme GTP-cyclohydrolase from E. coli (71MI21600) and a similar one from Lactobacillus platarum (B-71MI21601) which catalyzes the reaction (300)(303). Dephosphorylation leads to 7,8-dihydro-D-neopterin (304), which is then cleaved in the side-chain to 6-hydroxymethyl-7,8-dihydropterin (305), the direct precursor of 7,8-dihy-dropteroic acid and 7,8-dihydrofolic acid (224). The alcohol (305) requires ATP and Mg " for the condensation with p-aminobenzoic and p-aminobenzoylglutamic acid, indicating pyrophosphate formation to (306) prior to the substitution step. 

Determination of GTP cyclohydrolase and D-erythro-7,8-dihydroneopterin triphosphate synthetase... 

FIGURE 40-2 The phenylalanine hydroxylase (PAH) pathway. Phenylketonuria usually is caused by a congenital deficiency of PAH (reaction 1), but it also can result from defects in the metabolism of biopterin, which is a cofactor for the hydroxylase. Enzymes (1) Phenylalanine hydroxylase (2) Dihydropteridine reductase (3) GTP cyclohydrolase (4) 6-pyruvoyltetrahydrobiopterin synthase. BH4, tetrahydrobiopterin DEDT, o-erythro-dihydroneopterin triphosphate QH2, dihydrobiopterin. 

Other causes of PKU secondary to defective tetrahydrobiopterin synthesis include GTP cyclohydrolase deficiency and 6-pyravoyltetrahydrobiopterin synthase deficiency. Patients with either defect have psychomotor retardation, truncal hypotonia with limb hypertonia, seizures and a tendency to hyperthermia. The intravenous administration of BH4 may lower blood phenylalanine levels but this cofactor may not readily cross the blood-brain barrier. Treatment with synthetic pterin analogs or supplementation with tryptophan and carbidopa may prove more efficacious, particularly if treatment is started early in life. 

These patients suffer from a genetic defect of dopamine synthesis, caused by reduced GTP cyclohydrolase activity. This enzyme is rate-limiting in the biosynthesis of tetra-hydrobiopterin, a cofactor of the dopamine-synthesizing enzyme tyrosine hydroxylase (see Fig. 40-2). 

GTP CYCLOHYDROLASES GUANYLATE CYCASE HEXOSE-1-PHOSPHATE GUANYLYL-TRANSFERASE... 

CMP SYNTHETASE GTP CYCLOHYDROLASE II GUANYLATE CYCLASE HEXOSE-1-PHOSPHATE CUANYLYL-TRANSEERASE... 

The enzymes in the zebra fish pathway are presumably very similar to those of other vertebrates. However a completely different type of GTP cyclohydrolase has been identified in the hyperthermophilic euryarchaeon, Methanococcus jannashii <2002B15074>. This enzyme, in purified recombinant form, produced as a stable end product 2-amino-5-formylamino-6-ribofuranosylamino-4(3//)-pyrimidinone monophosphate, a compound that is an intermediate in the action of normal GTP cyclohydrolases. The biosynthesis of the incorporation of the pterin into methanopterin in Methanobacterium thermoautotrophicum has been proposed to occur via substitution of 7,8-dihydro-6-hydroxymethylpterin diphosphate 227 (Scheme 44) <1998BBA257>. 

An important discovery is that procaryotes contain a different GTP cyclohydrolase 1 family distinct from the well studied canonical eukaryotic enzyme. Potentially, this enzyme is a target for new antibacterial drugs <2006JBC37586>. 

DHPR Dihydropteridine reductase, DRD dopa-responsive dystonia, GTPCH GTP cyclohydrolase I, n normal, PCD pterin-4a-carbinolamine dehydratase, PTPS 6-pyruvoyltetrahydrobiopterin synthase, SR sepiapterin... 

To prepare lysates from (nonstimulated) fibroblasts, cells from one confluent 78-cm2 plate are suspended in 0.15 ml lysis buffer (see below) and lysed by freezing and thawing six times and subsequent centrifugation at 13,000 x for 5 min. An aliquot of 0.05 ml of the supernatant is directly used for the enzyme assay. The preparation of tissue homogenate is described in section 6.1.4.1. GTP cyclohydrolase I, subheading Specimen . 

Ichinose H, Ohye T, Takahashi E, Seki N, Hori T, Segawa M, Nomura Y, Endo K, Tanaka H, Tsuji S, Fujita K, Nagatsu T (1994) Hereditary progressive dystonia with marked diurnal fluctuation caused by mutation in the GTP cyclohydrolase I gene. Nature Genet 8 236-241... 

Viveros OH, Lee CL, Abou-Donia MM, Nixon JC, Nichol CA (1981) Biopterin cofactor biosynthesis independent regulation of GTP cyclohydrolase in adrenal medulla and cortex. Science 213 349-350... 

Hatakeyama K, Yoneyama T (1998) A sensitive assay for the enzymatic activity of GTP cyclohydrolase I. Methods Mol Biol 100 265-272... 

BH4 is an obligatory cofactor for both tyrosine and tryptophan hydroxylase. Consequently, the inborn errors of BH4 metabolism are associated with impaired dopamine and serotonin turnover, which is reflected by decreased concentrations of HVA and 5HIAA in the CSF. Whilst such a pattern is particularly true for the autosomal recessive disorders of BH4 metabolism, an autosomal dominant disorder of BH4 metabolism, (autosomal dominant GTP cyclohydrolase deficiency) is not always associated with marked decreases in the CSF concentration of HVA and 5HIAA [1]. 

Both the fungus Eremothecium (Box 15-B) and mutants of Saccharomyces have been used to deduce the pathways of riboflavin synthesis outlined in Figure 25-20. The first reaction (step a) is identical to step a of Fig. 25-19 but is catalyzed by a different GTP cyclohydrolase.362 Instead of an Amadori rearrangement it catalyzes the hydrolytic deamination and dephosphorylation (step b) to give the flavin precursor... 

Various hormones and cytokines are known to induce the expression of the GCH gene in neural, lymphocytic and endothelial cells, and in different cell lines, resulting in an increased BH4 content [140-144], At the post-transcriptional level, BH4 was shown to inhibit, and phenylalanine to stimulate, GCH activity through interaction with GFRP, a GTP cyclohydrolase I feedback regulatory protein [145]. GCH, which is a homodecameric protein,... 

Most of hyperphenylalaninemia are caused by a mutation in the PAH gene. About 5% of hyperphenylalaninemia is caused by genetic defects in the BH4-metabolizing enzymes, and called malignant-type or atypical hyperphenylalaninemia. Patients with malignant-type hyperphenylalaninemia develop neurological symptoms due to deficiency of catecholamines and serotonin, as well as hyperphenylalaninemia. For example, patients with GTP cyclohydrolase deficiency show severe retardation of development, severe muscular hypotonia of the trunk and hypertonia of extremities, convulsions, and frequent episodes of hyperthermis without infection [160,161]. 

Neopterin. Neopterin is a low-molecular-weight substance derived from guanosine triphosphate (GTP) via the enzyme GTP-cyclohydrolase 1. Numerous investigators have demonstrated that neopterin, a product of human macrophages stimulated by y interferon and other cytokines, is a useful in vivo marker of the activation of cellular immunity (U4, Wl). Increased values of neopterin in body fluids have been reported in patients with malignancy, infections, and several inflammatory states. 

Mandel, R. J. et al. (1998). Characterization of intrastriatal recombinant adeno-associated virus-mediated gene transfer of human tyrosine hydroxylase and human GTP-cyclohydrolase I in a rat model of Parkinson s disease. J. Neurosci. 18(11), 4271-4284. 

Shen, Y. et al. (2000). Triple transduction with adeno-associated virus vectors expressing tyrosine hydroxylase, aromatic-L-amino-acid decarboxylase, and GTP cyclohydrolase I for gene therapy of Parkinson s disease. Hum. Gene Ther. 11(11), 1509-1519. 

Glucose was first converted enzymatically into ribose 5 -phosphate from which GMP was subsequently obtained by the action of phosphoribosylpyrophosphate synthetase and guanosine phos-phoribosyl transferase. A two-step phosphorylation to GTP followed by treatment with recombinant GTP-cyclohydrolase I from E. coli gave (51) <90Ml 718-12). This method also allows l4C-labeling in the 6- and 7-position as well as the carbon sidechain. 

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