Tryptophan 5-monooxygenase

One of the best characterized physiological functions of (6R)-tetrahydrobio-pterin (BH4, 43) is the action as a cofactor for aromatic amino acid hydroxylases (Scheme 28). There are three types of aromatic amino acid hydroxylases phenylalanine hydroxylase [PAH phenylalanine monooxygenase (EC 1.14.16.1)], tyrosine hydroxylase [TH tyrosine monooxygenase (EC 1.14.16.2)] and tryptophan hydroxylase [TPH tryptophan monooxygenase (EC 1.14.16.4)]. PAH converts L-phenylalanine (125) to L-tyrosine (126), a reaction important for the catabolism of excess phenylalanine taken from the diet. TH and TPH catalyze the first step in the biosyntheses of catecholamines and serotonin, respectively. Catecholamines, i.e., dopamine, noradrenaline and adrenaline, and serotonin, are important neurotransmitters and hormones. TH hydroxylates L-tyrosine (126) to form l-DOPA (3,4-dihydroxyphenylalanine, 127), and TPH catalyzes the hydroxylation of L-tryptophan (128) to 5-hydroxytryptophan (129). The hydroxylated products, 127 and 129, are decarboxylated by the action of aromatic amino acid decarboxylase to dopamine (130) and serotonin (131), respectively. 

A careful study of indolic compounds produced by R. phaseoli showed that tryptophan could be converted into lAA, indole-3-ethanol, and indole-3-methanol, but indole-3-acetamide was not formed [189]. Bradyrhizobium, however, were shown to contain indole-3-acetamide as well as the indoleacetamide hydrolase activity, suggesting the presence of the tryptophan monooxygenase/indoleacetamide hydrolase pathway [190,191] in these organisms. 

Fig. 2. The pathway for indoleacetic acid synthesis in IP. savastanoi. Control of lAA synthesis occurs in part by inhibition of tryptophan monooxygenase by indoleacetamide. Because the inhibition is competitively reversed by the substrate, tryptophan, tryptophan pool size is important in modulating the inhibition by indoleacetamide.

Serotonergic neurons contain the enzyme L-tryptophan-5-monooxygenase (EC 1.14.16.4), more commonly termed tryptophan hydroxylase, which converts tryptophan to 5-hydroxytryptophan (5-HTP) (Fig. 13-5). Tryptophan hydroxylase contains 444 amino acids, corresponding to a molecular weight of about 51 Da. This enzyme is synthesized in serotonergic cell bodies of the raphe nuclei and is found only in cells that synthesize 5-HT. Therefore its distribution in brain is similar to that of 5-HT itself. The Km of tryptophan hydroxylase for tryptophan is approximately 30-60 pmol/1, a concentration comparable to that of tryptophan in brain. If the concentration of tryptophan in serotonergic neurons is assumed to be comparable to that in whole brain, the enzyme would not be saturated with substrate, and the formation of 5-HT in brain would be expected to rise as the brain concentration of tryptophan increases. This has been found to occur in response to raising the dietary intake of tryptophan specifically. 

The following is review on the molecular and physical properties of this class of monooxygenases, which are also known as hydroxylases. A typical monooxygenase reaction is the hydroxylation of an alkane to an alcohol which involves a reduced cosubstrate that reduces a second atom within the O2 molecule to form water. Flavin-containing monooxygenases include lysine oxygenase and 4-hydroxybenzoate hydroxylase. Reduced pteri-dines are involved in the phenylalanine hydroxylase and tryptophan hydroxylase reactions. See also Cytochrome P-450... 

Tire tetrahydrobiopterin formed in this reaction is similar in structure to a reduced flavin. The mechanism of its interaction with 02 could reasonably be the same as that of 4-hydroxybenzoate hydroxylase. However, phenylalanine hydroxylase, which catalyzes the formation of tyrosine (Eq. 18-45), a dimer of 451-residue subunits, contains one Fe per subunit,113 313i whereas flavin monooxygenases are devoid of iron. Tyrosine hydroxylase416 193 and tryptophan hydroxylase420 have very similar properties. All three enzymes contain regulatory, catalytic, and tetramerization domains as well as a common Fe-binding motif in their active sites.413 421 4213... 

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]. 

---