Cyclohydrolase synthesis

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). 

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). 

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... 

D-e/yf/iro-7,8-Dihydroneopterin triphosphate synthetase, or GTP cyclohydrolase I (EC 3.5.4.16), catalyzes the formation of D-eryr/iro-dihydroneopterin triphosphate (NH2TP) from GTP. This activity is required for the synthesis of tetrahydrobiopterin. The HPLC assay developed for this activity involves the direct measurement of neopterin phosphates after separation from GTP and its other hydrolytic products. 

The precursors for riboflavin biosynthesis in plants and microorganisms are guanosine triphosphate and ribulose 5-phosphate. As shown in Figure 7.3, the first step is hydrolytic opening of the imidazole ring of GTP, with release of carbon-8 as formate, and concomitant release of pyrophosphate. This is the same as the first reaction in the synthesis ofpterins (Section 10.2.4), but utilizes a different isoenzyme of GTP cyclohydrolase (Bacher et al., 2000, 2001). 

Patients with a variety of cancers and some viral diseases excrete relatively large amounts of neopterin, formed by dephosphorylation and oxidation of dihydroneopterin triphosphate, an intermediate in biopterin synthesis. This reflects the induction of GTP cyclohydrolase by interferon-y and tumor necrosis factor-a in response to the increased requirement for tetrahydrobiopterin for nitric oxide synthesis (Section 10.4.2). It is thus a marker of ceU-mediated immune reactions and permits monitoring of disease progression (Werner et al., 1993,1998 Berdowska and Zwirska-Korczala, 2001). 

Guanosine triphosphate and ribulose-5-phosphate are recruited in a 1 2 stoichiometric ratio by GTP cyclohydrolase II and DHBP synthase, respectively, for riboflavin biosynthesis. Since at substrate saturation the activity of B. subtilis DHBP is twice the activity of B. suhtilis cyclohydrolase II (DSM, unpublished observations) and since both enzymatic activities are associated with the same bifunctional protein encoded by rihA, the balanced formation of the pyrimidinedione and the dihydroxybutanone intermediates is ensured. However, the ifg.s constant of DHBP synthase ( 1 mmol is about 100-fold higher than the ifg.s constant of GTP cyclohydrolase II imposing the risk of excessive synthesis of the pyrimidinone and pyrimidinedione intermediates in case of reduced intracellular concentrations of pentose phosphate pathway intermediates. This can be expected, for instance, in glucose-limited fed-batch fermentations, which are frequentiy used in industrial applications. The pyrimidinone and pyrimidinedione intermediates are highly reactive, oxidative compounds, which can do serious damage on the bacteria. 

In contrast to PAH, requirement for the BH4 cofactor is much lower for the NOS enzyme. The values for BH4 for PAH and NOS are 25-30 pmolP andO.2-0.3 pmol respectively. Pastor questioned the importance of BH4 competition between these two hepatic enzymes. They showed that basal BH4 synthesis appears to be adequate to support iNOS activity, whereas BH4 is increased to support PAH activity. Phenylalanine markedly increased BH4 biosynthesis (through GTP cyclohydrolase feedback regulatory protein, GFRP), whereas arginine had no effect. The... 

Fig. 12.1 Tetrahydrobiopterin synthesis and recirculation left side) and enzyme reactions requiting BH4 as a cofactor right side). Each enzyme that has the potential to create a deficiency is noted with a corresponding number (i) GTP cyclohydrolase 1 (GTPCH) (2) 6-pyruvoyltetra-...

Gibson CL, Paulini K, Suckling CJ (1997) Synthesis of potential inhibitors of GTP-cyclohydrolase I an efficient synthesis of 8-substituted 7-deazaguanines. Chem Commun 371-372... 

Fig. 3. The pathway of de novo purine ribonucleotide biosynthesis. The pathway includes the synthesis of PRPP, which is also used in the synthesis of pyrimidines, pyridine nucleotides, histidine, and tryptophan in plants. The enzymes catalyzing the numbered reactions are (1) PRPP synthetase, (2) PRPP amidotransferase, (3) GAR synthetase, (4) GAR transformylase, (5) FGAR amidotransferase, (6) AIR synthetase, (7) AIR carboxylase, (8) succino-AICAR synthetase, (9) adenylosuccinase, (10) AICAR transformylase, and (11) IMP cyclohydrolase.

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