Biosynthesis of Pterins

The formation of biopterin involves dephosphorylation and reduction of the side chain of dihydroneopterin triphosphate, followed by inversion of the conformation of the two hydroxyl groups, by way of intermediate oxidation to (symmetrical) oxo-groups, catalyzed by sepiapterin reductase. 

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). 

Hgiue 10.2. Biosynthesis of folic acid and tetrahydrobiopterin. GTP cyclohydrolase I, EC 3.5.4.16 dihydropteroate synthase, EC 2.5.1.15 pyruvoyl-tetrahydrobiopterin synthase, EC 4.6.1.10 and sepiapterin reductase, EC 1.1.1.153. 

dihydrofolate reductase is an importemt teuget for chemotherapy of cemcer, bacterial infections, and malaria. 

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There are a number of studies on the biosynthesis of various pteridines, i.e., xanthopterin (65), isoxanthopterin (67), erythropterin (73), leucopterin (68), and pterin (62) (509-511). The most important intermediate of the proposed biosynthetic pathway from guanosine triphosphate (GTP) (604) seems to be di-hydroneopterin triphosphate (H2-NTP) (605), however, because evidence has recently been accumulated indicating that pteridines such as biopterin (70), sepiapterin (81), and drosopterins (87) are synthesized from GTP (604) by way of H2-NTP (605) (Scheme 76) (5/2). 

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>. 

The aromatic amino add hydroxylases (AAHs) are a family of pterin-dependent enzymes comprising phenylalanine hydroxylase (PAH), tyrosine hydroxylase (TH), and tryptophan hydroxylase (TPH, with two gene products TPH1 and TPH2). The AAHs perform the hydroxylation of aromatic amino adds and play an important role in mammalian metabolism and in the biosynthesis of... 

The hereditary absence of phenylalanine hydroxylase, which is found principally in the liver, is the cause of the biochemical defect phenylketonuria (Chapter 25, Section B).430 4308 Especially important in the metabolism of the brain are tyrosine hydroxylase, which converts tyrosine to 3,4-dihydroxyphenylalanine, the rate-limiting step in biosynthesis of the catecholamines (Chapter 25), and tryptophan hydroxylase, which catalyzes formation of 5-hydroxytryptophan, the first step in synthesis of the neurotransmitter 5-hydroxytryptamine (Chapter 25). All three of the pterin-dependent hydroxylases are under complex regulatory control.431 432 For example, tyrosine hydroxylase is acted on by at least four kinases with phosphorylation occurring at several sites.431 433 4338 The kinases are responsive to nerve growth factor and epidermal growth factor,434 cAMP,435 Ca2+ + calmodulin, and Ca2+ + phospholipid (protein kinase C).436 The hydroxylase is inhibited by its endproducts, the catecholamines,435 and its activity is also affected by the availability of tetrahydrobiopterin.436... 

Figure 25-19 The biosynthesis of folic acid and other pterins.

The fifth chapter, Tetrahydrobiopterin and Related Biologically Important Pterins by Shizuaki Murata, Hiroshi Ichinose and Fumi Urano, describes a modern aspect of pteridine chemistry and biochemistry. Pteridine derivatives play a very important role in the biosynthesis of amino acids, nucleic acids, neurotransmitters and nitrogenmonooxides, and metabolism of purine and aromatic amino acids. Some pteridines are used in chemotherapy and for the diagnosis of various diseases. From these points of view, this article will attract considerable attention from medicinal and pharmaceutical chemists, and also heterocyclic chemists and biochemists. 

The final topic addressed in this chapter is the biosynthesis of the dithiolene cofactor ligand and its coordination to molybdenum and tungsten in the enzymes. Nature has clearly devised a synthetic process to overcome the twin difficulties of building a reactive dithiolene unit bearing a complicated and equally reactive pterin substituent. Molecular biology has been the tool to elucidate the steps in this complex process. Although the dithiolene formation step remains mainly a subject of conjecture, definitive information about the reagent molecule that will eventually be converted to a dithiolene is known. 

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

The metabolism of folic acid involves reduction of the pterin ting to different forms of tetrahydrofolylglutamate. The reduction is catalyzed by dihydtofolate reductase and NADPH functions as a hydrogen donor. The metabolic roles of the folate coenzymes are to serve as acceptors or donors of one-carbon units in a variety of reactions. These one-carbon units exist in different oxidation states and include methanol, formaldehyde, and formate. The resulting tetrahydrofolylglutamate is an enzyme cofactor in amino acid metabolism and in the biosynthesis of purine and pyrimidines (10,96). The one-carbon unit is attached at either the N-5 or N-10 position. The activated one-carbon unit of 5,10-methylene-H folate (5) is a substrate of T-synthase, an important enzyme of growing cells. 5-10-Methylene-H folate (5) is reduced to 5-methyl-H,j folate (4) and is used in methionine biosynthesis. Alternatively, it can be oxidized to 10-formyl-H folate (7) for use in the purine biosynthetic pathway. 

The fourth class, the pterin-dependent hydroxylases, includes the aromatic amino acid hydroxylases, which use tetrahydrobiopterin as cofactor for the hydroxylation of Phe, Tyr, and Trp. The latter two hydroxylases catalyse the rate-limiting steps in the biosynthesis of the neurotransmitters/hormones dopamine/noradreanalme/ adrenaline and serotonin, respectively. 

Even though E. coli is a very well-studied bacterium, many interesting mechanistic problems in cofactor biosynthesis in this organism remain unsolved. The mechanisms for the formation of the nicotinamide ring of NAD, the pyridine ring of pyridoxal, the pterin system of molybdopterin, and the thiazole and pyrimidine rings of thiamin are unknown. The sulfur transfer chemistry involved in the biosynthesis of lipoic acid, biotin, thiamin and molybdopterin is not yet understood. The formation of the isopentenylpyrophosphate precursor to the prenyl side chain of ubiquinone and menaquinone does not occur by the mevalonate pathway. None of the enzymes involved in this alternative terpene biosynthetic pathway have been characterized. The aim of this review is to focus attention on these unsolved mechanistic problems. 

In eubacteria, the queuosine pathway can be conceptualized as occurring at two levels the free heterocyclic base and the nucleoside attached to RNA (Figure 33). For many years, it has been known that the key intermediate, prefix, is derived from GTP. Owing to lack of evidence, it has long been thought that preQi derives from GTP in a process analogous to that of pterin/folate biosynthesis and/or that for the antibiotic toyacamycin. PreQ is the substrate for the eubacterial TGT which incorporates it into the substrate tRNA. Two subsequent reactions occur at the level of the tRNA in which a cyclopentyl moiety is added and then converted into the cyclopentene. 

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