Hydroxylation enzymes

Pyridine-4-carboxylate is hydroxylated by Mycobacterium sp. strain INAl to 2,6-dihydroxypyridine-4-carboxylate. Two different hydroxylation enzymes were involved and were apparently Mo-dependent (Kretzer and Andreesen 1991). The formation of 2-ketoglutarate can, however, be rationalized equally as (3-oxidation to hexahydropyridine-2,3,6-trione-4-carboxy-CoA ester followed by hydrolysis. 

Natnre does not actually make a methylenedioxy group using formaldehyde. Instead, it modifies an existing ortAo-hydroxy-methoxy arrangement. Enzymic hydroxylation of the methoxy methyl converts this substitnent into what is identical to a hemiacetal of formaldehyde, and then acetal formation follows in a process analogous to a chemical synthesis. The hydroxylating enzyme involved is a cytochrome P-450 mono-oxygenase (see Box 11.4). 

McLean AEM, McLean EK. 1966. The effect of diet and 1,1,1-trichloro-2,2-bis-(p-chlorophenyl)ethane (DDT) on microsomal hydroxylating enzymes and on sensitivity of rats to carbon tetrachloride poisoning. Biochem J 100 564- 571. 

Adipate monoesters -enzymatic hydroxylation [ENZYMES IN ORGANIC SYNTHESIS] (Vol 9)... 

J. Schmitt, and R. D. Schmid, A continuous spectrophotometric assay for P450 BM-3, a fatty acid hydroxylating enzyme, and its mutant F87A, Anal Biochem. 1999, 269, 359-366. 

Fig. 6.6. Conformations of the HIF-la sequence containing the regulatory asparagine 803 that is hydroxylated under normoxic conditions, (a) Extended conformation in the X-ray crystal structure of the complex with the hydroxylating enzyme FIH [30]. (b) helical conformation in the NMR structure of the complex with the TAZ1 domain of CBP [21]...

Thus the active site of the hydroxylating enzyme ipears insensitive to the relative position of the amide group. This has also been observed for bicyclic ainides, for example both exo and endo isomers of the amide (34) are hydroxylated to the exo-alcohol (35) with the same regio- and stereo-selectivity (equation 10). Further, the hydroxylation can also be insensitive to the position of the carbonyl group. For example, bi-, tri- and tetra-cyclic amides, and the equivalent lactams, of which the amide (36a) and lactam (36b) are representative cases respectively, are both hydroxylated at the same position (Scheme 8a). 7 ... 

The sesquiteipene cedrol (46) can be hydroxylated regio- and stereo-selectively with Beauveria sulfu-rescens (equation 13). This transformation serves to illustrate the general principle that substrates with an electron rich substituent, to serve as an anchor at, or close to, the active site of the hydroxylating enzyme system, generally are transformed with improved selectivity over those with no such anchor. For example, in the above system the unsaturated substrate cedrene (47) gives low yields of a mixture of products. ... 

Stereoselectivity will be dictated in most cases by the binding of the substrate to the hydroxylating enzyme. One exception to this occurs in the 6p-hydroxylation of 3-keto A -steroids. In this case the stereochemistry of substitution at C-6 of the product is determined largely by conventional stereoelec-tronic processes, as the mechanism is believed to involve axial addition of oxidant to a conjugate of the substrate and the hydroxylating enzyme (equation 24). ... 

Strains of Pseudomonas putida are very versatile in metabolizing aromatic compounds, particularly to the corresponding 1,2-dihydro-l,2-diols. The hydroxylating enzyme of the P. putida mutant is not strongly substrate specific and alkyl, aryl and halogen functionalities are usually readily tolerated380. Thus, 4-bromobenzoic acid (1, R = Br) is converted to a. v-4-bro-mo-5,6-dihydroxy-l, 3-cyclohexadiene-l-carboxylic acid (2, R = Br) in 80% yield with 98% cc (determined by chiral NMR shift experiments on the 4-nitrobenzyl ester) 375. The absolute stereochemistry, (5R,6R), was determined by a single crystal X-ray analysis. 

Oral contraceptives increase the AUC and plasma concentrations of metoprolol, oxprenolol, and propranolol, but statistical significance is reached only with metoprolol. The changes are consistent with inhibition of hydroxylating enzymes, but are unlikely to be of clinical relevance (322). 

Co-administration of acetazolamide with primidone results in decreased gastrointestinal absorption of primidone and subsequent diminished plasma concentrations. Primidone administered in association with phenytoin produces a modest elevation of the phenobarbital/primidone ratio because phenytoin competes with the hepatic hydroxylating enzymes associated with phenobarbitaFs metabolism. Coadministration of valproic acid, for the same reasons out-fined for phenobarbital, causes a modest increase in both primidone and phenobarbital serum concentrations. 

Wagstaff, D.J. and Short, C.R. (1971). Induction of hepatic microsomal hydroxylaling enzymes by technical piperonyl butoxide and some of its analogs. Toxicol Appl Pharmacol 19, 54—61,... 

Safrole is potent inhibitor of liver microsomal hydroxylating enzymes, and thus could increase plasma levels of certain drugs. In addition, eugenol, a safrole metabolite, is a moderate enzyme inhibitor (Jaffe et al., 1968). It appears that safrole acts as a substrate for cytochrome P-450 isoenzymes, and upon metabolism covalent bonds form between the reactive metabolites and the enzyme, resulting in enzyme inhibition (Ionnaides et al., 1985). 

The role of vitamin C in this case is therefore to regenerate the biologically active form of iron by passing an electron to the oxidized form. The hydroxylating enzyme acts as a kind of merry-go-round that uses iron to attach oxygen to amino acids. By providing iron with electrons, vitamin C provides the motive force that keeps the merry-go-round turning. 

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