A-Hydroxylation reaction

Comparison of the predicted amino acid sequence of the protein encoded by pTOM 13 with the sequence of flavanone 3-hydroxylase showed considerable similarity (A. Prescott and C. Martin, personal communication, cited in Hamilton etal., 1990). This is particularly interesting, since it has been suggested that the conversion of ACC to ethylene might involve a hydroxylation reaction (Yang, 1985). Armed with this information and using conditions developed for extraction and purification of flavanone 3-hydroxylase, Ververidis John (1991) showed that it is possible to solubilise the ethylene forming enzyme from plants and retain full... 

PAH, TH and TPH are highly homologous enzymes. These enzymes catalyze a hydroxylation reaction of aromatic amino acids that requires reduced pterin cofactor 43, molecular oxygen, and iron (Scheme 28). Iron is present at the active sites of the enzymes. Ferrous iron (Fe(II)) is essential for the catalysis, although, the iron was found to be in the ferric form (Fe(III)) when the enzymes were purified from tissues or cells. The ferric iron at the active site of the enzymes was found to be reduced to the ferrous form by BH4 [125]. Thus, BH4 serves a bi-functional role for aromatic amino acid hydroxylases one is the reduction of iron at the active sites from the ferric form to the ferrous form and the other is an electron donor for the hydroxylation reaction. 

In a more comprehensive study, Madani et al. (165) quantified CYP2D6 protein in 20 human jejunum and 31 human livers. They found that the median microsomal-specific CYP2D6 content was less than 8% of the hepatic microsomal content (0.85 vs. 12.8 pmol/mg) and that there was extensive interindividual variability in protein content for both tissues. These investigators also characterized the catalytic activity of the same jejunal microsomes toward the recognized CYP2D6 substrate metoprolol and found that a-hydroxylation reaction rate was significantly correlated with CYP2D6 protein content (r = 0.75). 

Aerobic cometabolism also may play a role in the oxidation of 1,1,1-TCA, where a hydroxylation reaction is the initial step [27] ... 

Tyrosine is not an essential amino acid in animals because it is synthesized from phenylalanine in a hydroxylation reaction. The enzyme involved, phenylala-nine-4-monoxygenase, requires the coenzyme tetrahydrobiopterin (Section 14.3), a folic acid-like molecule derived from GTP. Because this reaction also is a first step in phenylalanine catabolism, it is discussed further in Chapter 15. 

Lithium enolates of esters containing an a-tertiary carbon atom also undergo this a-hydroxylation reaction. One example of successful oxygenation of an ester with an a-methylene group was reported by Wasserman and Lipshutz. ... 

Phenylalanine is converted to tyrosine by a hydroxylation reaction. Tyrosine, pro-dnced from phenylalanine or obtained from the diet, is oxidized, nltimately forming acetoacetate and fnmarate. The oxidative steps required to reach this point are, surprisingly, not energy-generating. The conversion of fnmarate to malate, followed by the action of malic enzyme, allows the carbons to be used for gluconeogenesis. The conversion of phenylalanine to tyrosine and the production of acetoacetate are considered further in section IV of this chapter. 

Neurons that secrete norepinephrine synthesize it from dopamine in a hydroxylation reaction catalyzed by dopamine (3-hydroxylase (DBH). This enzyme is present only within the storage vesicles of these cells. Like tyrosine hydroxylase, it is a mixed-function oxidase that requires an electron donor. Ascorbic acid (vitamin C) serves as the electron donor and is oxidized in the reaction. Copper (Cu ) is a bound cofactor required for the electron transfer. 

MnOij is added. This reaction is typical of alkenes. Certain oxidizing agents (such as RMnOi ) convert alkenes into glycols. This is a hydroxylation reaction. 

Kirsch and coworkers have further investigated the reactions of IBX with carbonyl compounds and found that, depending on a functional group at the a-position of a carbonyl compound, the reaction may lead either to oxidative dehydrogenation or to a-oxygenation [1128, 1186, 1187], In particular, 3-keto esters and some other suitably substituted carbonyl compounds can be selectively a-hydroxylated by treatment with IBX in aqueous DMSO at 50 °C a representative example of the a-hydroxylation reaction of 3-keto ester (869) is shown in Scheme 3.348 [1128]. 

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