Cyclohydrolase and

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

Pomper BK, Vorholt JA, Chistoserdova L, et al. 1999. A methenyl tetrahydromethanopterin cyclohydrolase and a methenyl tetrahydrofolate cyclohydrolase in Methylobacterium extorquens AMI. Eur J Biochem 261 475-80. 

All methylene-H4MPT dehydrogenases (both types) purified so far are monofunctional. In the H4folate system, the analogous enzyme can be monofunctional, or may have methenyl-H4folate cyclohydrolase and/or formyl-H4folate synthetase activity [362-371]. 

Tetrahydrobiopterin is synthesized starting from GTP and requires at least three enzymes. The first committed step is GTP-cyclohydrolase, which converts GTP to dihy-droneopterin triphosphate. 6-Pyruvoyltetrahydrobiopterin synthase transforms dihydroneopterin triphosphate into 6-pyruvoyltetrahydrobiopterin. The latter is reduced to tetrahydrobiopterin by NADPH-dependent sepi-apterin reductase. Deficiency of GTP-cyclohydrolase and... 

A bifunctional enzyme, which contains the activities of AIR carboxylase and SAICAR synthetase, catalyzes reactions 6 and 7 of the purine pathway (AIR CAIR SAICAR Fig. 14-18). A second bifunctional enzyme, IMP synthase, contains the activities of AICAR transformylase and IMP cyclohydrolase, and it catalyzes reactions 9 and 10 of the pathway (AICAR FAICAR —> IMP). 

Human IMP synthase has a subunit molecular weight of 62.1 kDa and associates as a dimer. A trifunctional enzyme, Cj-THF synthase, containing (V °-methenyl tetrahydrofolate (5,10-CH-THF) cyclohydrolase and A °-formyl tetrahydrofolate (lO-CHO-THF) synthetase, catalyzes the reactions 5,10-CH2-THF 5,10-CH-THF and THF —> lO-CHO-THF. The A °-CH-THF produced is a substrate for GAR and AICAR transformylases catalyzing reactions 3 and 9 of the pathway. In higher eukaryotes, the dehydrogenase and cyclohydrolase activities are located in one domain of the protein, which is fused to a larger synthetase domain, forming a trifunctional enzyme. 

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. 

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. 

Vorholt JA, Chistoserdova L, Stolyar SM, et al. 1999. Distribution of tetrahy-dromethanopterin-dependent enzymes in methylotrophic bacteria and phylogeny of methenyl tetrahydromethanopterin cyclohydrolases. J Bacteriol 181 5750-7. 

This group includes the coenzyme forms of water-soluble vitamin B2 or riboflavin. Synthesis occurs by initial cyclohydrolase action on the guanine ring of GTP and subsequent steps lead to the synthesis of the isoalloxazine ring structure (see structures below). 

This enzyme [EC 1.1.1.23] catalyzes the reaction of l-histidinol with two NAD+ to produce L-histidine and two NADH. L-Histidinal will also serve as a substrate for this protein. The Neurospora crassa enzyme will also catalyze the reactions of phosphoribosyl-AMP cyclohydrolase [EC 3.5.4.19] and phosphoribosyl-ATP pyrophosphatase [EC 3.6.1.31]. 

This enzyme [EC 3.6.1.31] catalyzes the hydrolysis of 5-phosphoribosyl-ATP to produce 5-phosphoribosyl-AMP and pyrophosphate (or, diphosphate). The Neurospora crassa enzyme also catalyzes the reactions of histidinol dehydrogenase and phosphoribosyl-AMP cyclohydrolase. 

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 . 

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... 

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... 

The tetrahydrofolates do not function as tightly enzyme-bound coenzymes. Rather, they function as cosubstrates for a variety of enzymes associated with one-carbon metabolism. /Vl0-formyltetrahydrofolate is produced enzymatically from tetrahydrofolate and formate in an ATP-linked process in which formate is activated by phosphorylation to formyl phosphate the formyl group of formyl phosphate is then transferred to A10 of tetrahydrofolate. A10-Formyltetrahydrofolate is a formyl donor substrate for some enzymes and is interconvertible with A5,A10-methenyltetrahydrofolate by the action of cyclohydrolase. 

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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