7«-Hydroxylase, reaction catalyzed

CYP17 is the 17 alpha-hydroxylase and 17-20 lyase, two different reactions catalyzed by one enzyme and required for production of testosterone and estrogen, respectively. Defects in this enzyme affect development at puberty. 

Figure28-10. The phenylalanine hydroxylase reaction. Two distinct enzymatic activities are involved. Activity II catalyzes reduction of dihydrobiopterin by NADPH, and activity I the reduction of O2 to HjO and of phenylalanine to tyrosine. This reaction is associated with several defects of phenylalanine metabolism discussed in Chapter 30.

Figure 28-11. The prolyl hydroxylase reaction. The substrate is a proline-rich peptide. During the course of the reaction, molecular oxygen is incorporated into both succinate and proline. Lysyl hydroxylase catalyzes an analogous reaction.

Fig. 10.4 Reaction catalyzed by the cytochrome P-450-dependent monooxygenase (5)-7V-methylcoclaurine 3 -hydroxylase (CYP80B1) along the (

Another important 2-OG dependent oxygenase in mammals is prolyl-4 hydroxylase, which catalyzes the hydroxylation of the proline residue in collagen (Scheme 5). This reaction is essential for the structure of the collagen triple helices (9,34 6). An overproduction of collagen is related to fibrotic diseases such as rheumatic arthritis. Thus collagen prolyl-4 hydroxylase is a target for therapeutics (34,36). 

This enzyme [EC 1.14.12.1], also known as anthranilate hydroxylase, decarboxylating, catalyzes the reaction of anthranilate with NAD(P)H, dioxygen, and two water molecules to produce catechol, carbon dioxide, NAD(P)+, and ammonia. The enzyme requires an iron ion as a cofactor. 

In the normal prolyl 4-hydroxylase reaction (Fig. 4a), one molecule of a-ketoglutarate and one of 02 bind to the enzyme. The a-ketoglutarate is oxidatively decarboxylated to form C02 and succinate. The remaining oxygen atom is then used to hydroxylate an appropriate Pro residue in procollagen. No ascorbate is needed in this reaction. However, prolyl 4-hydroxylase also catalyzes an oxidative decarboxylation of a-ketoglutarate that is not coupled to proline hydroxylation—and this is the reaction that requires ascorbate (Fig. 4b). During this reaction, the heme Fe2+ becomes oxidized, and the oxidized form of the enzyme is inactive—unable to hydroxylate proline. The ascorbate consumed in the reaction presumably functions to reduce the heme iron and restore enzyme activity. 

FIGURE 4 The reactions catalyzed by prolyl 4-hydroxylase, (a) The normal reaction, coupled to proline hydroxylation, which does not require ascorbate. The fate of the two oxygen atoms from 02 is shown in red. (b) The uncoupled reaction, in which a-ketoglutarate is oxidatively decarboxylated without hydroxylation of proline. Ascorbate is consumed stoichiometrically in this process as it is converted to dehydroascorbate. 

Phenylalanine and tyrosine Hydroxylation of phenylalanine leads to the formation of tyrosine (Figure 20.7). This reaction, catalyzed by phenylalanine hydroxylase, is the first reaction in the catabolism of phenylalanine. Thus, the metabolism of phenyl alanine and tyrosine merge, leading ultimately to the formation of fumarate and acetoacetate. Phenylalanine and tyrosine are, therefore, both glucogenic and ketogenic. 

In addition to these classical aromatic ring hydroxylations, many nitrogen heterocycles are substrates for molybdenum-containing enzymes, such as xanthine oxidase and aldehyde oxidase, which are present in the hepatic cytosolic fractions from various animal species. The molybdenum hydroxylases (B-75MI10902) catalyze the oxidation of electron-deficient carbons in aromatic nitrogen heterocycles. The reactions catalyzed by these enzymes are generally represented by equations (2) and (3). 

Of the three aromatic amino acid hydroxylases, the reaction catalyzed by L-phenylalanine hydroxylase has been subjected to mechanistic scrutiny most often (B-71MI11003, B-74MH1005, B-76MI11006). Of a number of isomeric dihydrobiopterins that are possible, it is the p-quinonoid dihydrobiopterin (20) that is the coenzyme-derived product in the reaction catalyzed by this enzyme (Scheme 7) (B-71MIH003). (20) is reduced back to (19) by an... 

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 origin of ricinoleic acid, an abundant constitu-tuent of castor beans, is also shown in Fig. 21-2. It is formed by an oleate hydroxylase that has an amino acid sequence similar to those of oleate desaturases.113 Both hydroxylation and desaturation are reactions catalyzed by diiron centers.114 Other fatty acid hydroxylases act on the alpha115 and the omega positions. The latter are members of the cytochrome P450 family.116 117... 

Scheme 2.1 Representation of the reaction catalyzed by aKG-dependent hydroxylases.

BH4 is converted to 4a-hydroxytetrahydrobiopterin (95) with incorporating one atom of dioxygen in the C(4a) position of pterin by the hydroxylation reaction catalyzed by aromatic amino acid hydroxylases (Scheme 32). The formation of 95 was observed in the reaction catalyzed by all three hydroxylases using UV spectroscopy [151]. Dehydration of 95 was carried out by the action... 

Figure 19-2. Aromatic amino acid hydroxylase reaction. Aromatic amino acids are hy-droxylated by a common mechanism catalyzed by a family of hydroxylases.The enzyme family consists of phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase. In addition to substrate, all three enzymes require molecular oxygen and the cofactor tetrahydrobiopterin.Tetrahydrobiopterin is consumed in this reaction and converted into pterin 4cx-carbinolamine. DOPA, dihydroxyphenylalanine.

Tyrosine is synthesized from phenylalanine in a reaction catalyzed by phenylalanine hydroxylase, which catalyzes two reactions. The reducing power in the reaction comes from NADPH, and the oxygen from molecular oxygen. 

Figure 10 Several reactions catalyzed by a-keto acid-dependent iron enzymes, (a) prolyl 4-hydroxylase, (b) deacetoxycephalosporin C synthase (DAOCS), (c) 4-hydroxyphenylpyruvate (HPP) dioxygenase and (d) clavaminate synthase 2 (CAS)...

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