Tetrahydrobiopterin electron transfer

Figure 11.4. Reaction catalyzedby Nitric oxide synthase (a), and arrangement of the coenzymes in the dimer (b). Electrons flow from NADPH and flavins of one snbnnit to the heme of the other. Arg denotes the snbstrate binding site. Tetrahydrobiopterin (BH4) also participates in electron transfer.

Hurshman, A.R. and M.A. Marietta (2002). Reactions catalyzed by the heme domain of inducible nitric oxide synthase Evidence for the involvement of tetrahydrobiopterin in electron transfer. Biochemistry 41, 3439-3456. 

On account of the requirement of NOS for tetrahy-drobiopterin, two general approaches toward a modulation of NOS activity seem feasible, manipulation of intracellular tetrahydrobiopterin levels and pterin-binding site antagonists. However, recombinant tetrahydrobiopterin-free NOS II catalysed the oxidation of four N-hydroxyguanidines tested by NADPH and O2, with formation of N02 and NOs" at rates between 20 and 80 nmol min" (mg of protein)" (Moali et al. 2001). In the case of N-(4-chlorophenyO-hT-hydroxyguanidine, formation of the corresponding urea and cyanamide was also detected besides that of N02" and NO3". These tetrahydrobiopterin-free NOS Il-dependent reactions were inhibited by modulators of electron transfer in NOS such as thiocitrulline or imidazole. 

Previous studies had shown that tetrahydrobiopterin could be a cofactor in mitochondrial electron transfer. The tetrahydropterin enters the chain at the level of cytochrome c and is able to catalyze oxygen reduction, but without the generation of ATP. The pool of tetrahydropterins can be continuously regenerated through the action of an NADPH dependent dihydropterin reductase, which catalyzes the reduction of the quinoid dihydropterin. It is not known if this enzyme is also present in the mitochondria. However, it has been shown that tetrahydropterin can easily cross the mitochondrial membrane. The physiological function of this system, which apparently couples NADPH/NADH pools directly through tetrahydropterin, cytochrome c, cytochrome oxidase to oxygen, but without simultaneous oxidative phosphorylation, is still not known. 

Rarely, phenylketonuria results from a defect in the metabolism of biopterin, a cofactor for the phenylalanine hydroxylase pathway. The electron donor for phenylalanine hydroxylase is tetrahydrobiopterin (BH4), which transfers electrons to molecular oxygen to form tyrosine and dihydrobiopterin (QH2 Fig. 40-2 reaction 2). BH4 is regenerated from QH2 in an NADH-dependent reaction that is catalyzed by dihydropteridine reductase (DHPR), which is widely distributed. In the brain, this... 

The NOSs are best characterized as cytochrome P-450-like hemeprot-eins (Bredt et al., 1991 Stuehr and Ikeda, 1992 White and Marietta, 1992). They can be broadly divided into a reductase domain at the COOH terminus and an oxidative domain at the NH2 terminus (Fig. 1). The primary amino acid sequences of NOS isoforms share common consensus sequence binding sites for calmodulin, NADPH, flavin-adenine dinucleotide (FAD), and flavin mononucleotide (FMN) (Bredt et al., 1991 Marsden et al., 1992 Sessa et al., 1992 Xie et al., 1992 Lyons et al., 1992 Lowenstein et al., 1992). Each enzyme functions as a dimeric protein in catalyzing the NADPH-dependent five-electron oxidation of L-arginine to generate NO. L-Citrulline is a by-product (Back et al., 1993 Abu and Stuehr, 1993). Electrons are supplied by NADPH, transferred along the flavins and calmodulin, and presented to the catalytic heme center (Stuehr and Ikeda, 1992 White and Marietta, 1992). The NOS apoenzyme requires tetrahydrobiopterin, prosthetic heme (ferroprotoporphyrin IX), calmodulin, FMN, and FAD as cofactors for monomer assembly and/or catalytic activity (Abu and Stuehr, 1993 Mayer and Werner, 1994 Kwon etal., 1989 Stuehr and Ikeda, 1992 Stuehr and Griffith, 1992 White and Marietta, 1992 McMillan etal., 1992 Klatt... 

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