Pterin-dependent hydroxylation

What pterin-dependent hydroxylation reactions are important to the human body Point out similarities and differences between flavin and pterin hydroxylase mechanisms. 

Tryptophan hydroxylase (EC 1.14.16.4) is the rate-limiting enzyme that catalyses the pterin-dependent hydroxylation of tryptophan to form 5-hydroxytryptophan. Northern analysis of human pineal gland revealed the presence of two mRNA species (Austin and O Donnell 1999). The cellular concentration of tryptophan hydroxylase in pinealocytes was extremely high throughout the pineal gland. 

Among the mononuclear non-haem iron enzymes catalysing hydroxylation reactions (Table 2.3) we can distinguish between intramolecular dioxygenases and external mononoxygenases. The former can be divided into those which are pterin-dependent and those which use a-ketoacids such as a-keto glutarate as obligatory... 

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 least understood aspect of NO synthases is the requirement for tetrahydrobiopterin, BH4, the same coenzyme required by the other pterin-dependent monooxygenases (Eq. 18-44). The presence of this coenzyme in the reduced BH4 form is essential for step a of Eq. 18-65 but not for step b. This suggests that in step a an organic peroxide might be generated by BH4 and used to form an oxo-iron hydroxylating reagent. 

A combination of decarboxylation and hydroxyla-tion of the ring of tyrosine produces derivatives of o-dihydroxybenzene (catechol), which play important roles as neurotransmitters and are also precursors to melanin, the black pigment of skin and hair. Catecholamines may be formed by decarboxylation of tyrosine into tyramine (step e, Fig. 25-5) and subsequent oxidation. However, the quantitatively more important route is hydroxylation by the reduced pterin-dependent tyrosine hydroxylase (Chapter 18) to 3,4-dihydroxyphenylalanine, better known as dopa. The latter is decarboxylated to dopamine.1313 Hydroxylation of dopamine by an ascorbic acid and... 

The cofactor appears to include a novel pterin.996-998 The properties of the pterin depend upon the nature of the side-chain in the 6-position. The structure shown in Figure 39 has been proposed997 on the basis that molybdopterin is related to urothione, oxidized to pterin-6-carboxylic acid, and contains in the side-chain two sulfur groups, a double bond, a hydroxyl function and a terminal phosphate group. Two stable fluorescent derivatives of molybdopterin have been characterized,999 which may be of value in view of the extreme instability of the native molybdoprotein when released from the enzyme. 

The important metaboUsm of the neurotransmitters norepinephrine, epinephrine, dopa, and serotonin involves pterin-dependent monooxygenases. The direct biocatalytic hydroxylation of the aromatic amino acids phenylalanine, tyrosine, and tryptophane requires tetrahydrobiopterin and Fe as the cofactors [60]. The cleavage of unsaturated glyceryl ethers by glyceryl ether monooxygenase also requires tetrahydrobioterin as the cofactor [61]. 

The pterin-dependent oxygenases, typified by the aryl amino acid hydroxylases, are a small family of closely related enzymes, which are essential to mammalian physiology. This class of metalloenzymes employs tetrahydrobiopterin (BH4) as a two-electron donating cofactor for the activation of O2. Members of this class include phenylalanine (PheH), tyrosine (TyrH) and tryptophan (TrpH) hydroxylases, which effect regiospecific aromatic hydroxylations of the namesake amino acids. 

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. 

Tetrahydrobiopterin (BPH4) is the natural cofactor required for the mammalian aromatic amino acid monooxygenases phenylalanine, tyrosine and tryptophan hydroxylase [4,89]. During the course of the reaction catalyzed by these enzymes, a molecule of oxygen is cleaved in order to hydroxylate the respective amino acid substrate. The remaining atom of oxygen is reduced to water at the expense of the cofactor, which is oxidized to the quinonoid form. Despite the many studies on the pterin-dependent hydroxylases, their precise mechanism of action is not well understood. This discussion will focus on mammalian phenylalanine hydroxylase (PAH), which has been favored for investigation due to its relative stability and ease of... 

In summary, we may add that bacterial utilization of quinoline and its derivatives as a rule depends on the availability of traces of molybdate in the culture medium [363], In contrast, growth of the bacterial strains on the first intermediate of each catabolic pathway, namely, the lH-2-oxo or 1 II-4-oxo derivatives of the quinoline compound was not affected by the availability of molybdate. This observation indicated a possible role of the trace element molybdenum in the initial hydroxylation at C2. In enzymes, Mo occurs as part of the redox-active co-factor, and all the Mo-enzymes involved in N-heteroatomic compound metabolism, contain a pterin Mo co-factor. The catalyzed reaction involves the transfer of an oxygen atom to or from a substrate molecule in a two-electron redox reaction. The oxygen is supplied by the aqueous solvent. Certainly, the Mo-enzymes play an important role in the initial steps of N-containing heterocycles degradation. 

Another subgroup of the 2His-lcarboxylate family is dependent on a reduced pterin cofactor (5). They catalyze hydrox-ylations at the aromatic positions of amino acids in phenylalanine catabolism and hormone biosynthesis (Fig. 2). Unlike the a-KG-dependent enzymes, the pterin co-substrate does not ligate to the iron directly. In the reaction cycle, the pterin cosubstrate supplies two electrons for the heteiolysis of O2 to give a yet to be characterized iron-oxygen hydroxylating species. 

Fig. 20.2 Pterin-cofactor-dependence of phenylalanine hydroxylase. Tetrahydrobiopterin, synthesized from guanosine triphosphate, donates the electron required to convert molecular oxygen to H O and hydroxylate the ring of phenylalanine to produce tyrosine. The resulting dihydrobiopterin is recycled by conversion to tetrahydrobiopterin using NADPH. (From McGUvery RW. Biochemistry A Functional Approach, 2nd edn. WB Saunders, Philadelphia, 1979)...

---