Hydroxy synthetase

In E. coli GTP cyclohydrolase catalyzes the conversion of GTP (33) into 7,8-dihydroneoptetin triphosphate (34) via a three-step sequence. Hydrolysis of the triphosphate group of (34) is achieved by a nonspecific pyrophosphatase to afford dihydroneopterin (35) (65). The free alcohol (36) is obtained by the removal of residual phosphate by an unknown phosphomonoesterase. The dihydroneoptetin undergoes a retro-aldol reaction with the elimination of a hydroxy acetaldehyde moiety. Addition of a pyrophosphate group affords hydroxymethyl-7,8-dihydroptetin pyrophosphate (37). Dihydropteroate synthase catalyzes the condensation of hydroxymethyl-7,8-dihydropteroate pyrophosphate with PABA to furnish 7,8-dihydropteroate (38). Finally, L-glutamic acid is condensed with 7,8-dihydropteroate in the presence of dihydrofolate synthetase. 

Figure26-2. Biosynthesis of squalene, ubiquinone, dolichol, and other polyisoprene derivatives. (HMG, 3-hydroxy-3-methylglutaryl x, cytokinin.) A farnesyl residue is present in heme a of cytochrome oxidase. The carbon marked with asterisk becomes C or C,2 in squalene. Squalene synthetase is a microsomal enzyme all other enzymes indicated are soluble cytosolic proteins, and some are found in peroxisomes.

The reaction is catalyzed by the fifth synthetase enzyme-hydroxy acyl hydratase, to produce crotonyl. 

N-Hydroxy arylamines are also converted to N-acetoxy arylamines (V), but apparently by an acetyl coenzyme A-dependent enzymatic O-esterification (7, 8). Similarly, N-sulfonyloxy arylamines (VI) are thought to arise by a PAPS-dependent enzymatic O-sulfonylation of N-hydroxy arylamines (9,10) while 0-seryl or 0-prolyl esters (VII) are formed by their corresponding aminoacyl tRNA synthetases in a ATP-dependent reaction (11,12). 

The formation of 0-seryl or 0-prolyl esters (Figure 1) of certain N-hydroxy arylamines has been inferred from the observations that highly reactive intermediates can be generated in vitro by incubation with ATP, serine or proline, and the corresponding aminoacyl tRNA synthetases (11,12,119). For example, activation of N-hydroxy-4-aminoquinoline-l-oxide (119,120), N-hydroxy-4-aminoazobenzene (11) and N-hydroxy-Trp-P-2 (121) to nucleic acid-bound products was demonstrated using seryl-tRNA synthetase from yeast or rat ascites hepatoma cells. More recently, hepatic cytosolic prolyl-, but not seryl-, tRNA synthetase was shown to activate N-hydroxy-Trp-P-2 (12) however, no activation was detectable for the N-hydroxy metabolites of AF, 3,2 -dimethyl-4-aminobiphenyl, or N -acetylbenzidine (122). 

Although aminoacyl-tRNA synthetases are necessary for protein synthesis in all tissues, their importance in chemical carcinogenesis is difficult to assess. Mutation induction by this pathway has been studied extensively (123), yet metabolic activation in a carcinogen-target tissue has not been demonstrated. The only exception is hepatic prolyl-tRNA synthetase activation of N-hydroxy-Trp-P-2 however, hepatic O-acetylation of this substrate also occurs to an appreciable extent (12). Further investigations involving the use of specific enzyme inhibitors would be helpful in addressing this problem. 

Recently, bacterial NRPS modules with the organization of A-KR-PCP have been discovered in the valino-mycin and cereulide synthetases. The A domains of these modules selectively activate a-keto acids. After the resulting adenylate is transferred to the PCP domain, the a-ketoacyl- -PCP intermediate is reduced to a PCP-bound, a-hydroxythioester by the KR domain. These domains use NAD(P)H as a cofactor and are inserted into A domains between two conserved core motifs analogous to MT domains. Their substrate specificity differs from that of polyketide synthase KR domains, which reduce /3-ketoacyl substrates. Similar fungal NRPSs, such as beauvericin synthetase, utilize A domains that selectively activate a-hydroxy acids. These molecules are thought to be obtained using an in trans KR domain, which directly reduces the necessary, soluble a-keto acid. 

Thymine derivatives - 5-[7V-(2-Amino-4-hydroxy-6-methyl-5-pyrimidinyl-propyl)-p-carboxyanilinomethyl] uracil (XXXIII) was synthesized for study as a possible intermediate in the enzymatic synthesis of thymidylate. It is active as an enzyme inhibitor against thymidylate synthetase isolated from E. coli [298]. Certain thymine derivatives containing a 2-thioimidazole moiety (XXXIV, R = alkyl) inhibit growth of Ehrlich ascites carcinoma (fluid form) in mice [299]. 

AMINOACYL-tRNA SYNTHETASES 2-AMINO-4-HYDROXY-6-HYDROXY-... 

Generally amino acid conjugation is a detoxication reaction. However, amino acid conjugation with hydroxylamino groups (N-hydroxy) can lead to the formation of reactive nitrenium ions, as already discussed with sulfate conjugation and acetylation. For example, the conjugation of serine with N-hydroxy-4-aminoquinoline-l-oxide (Fig. 4.40 for structure) leads to such a reactive nitrenium ion. This requires the enzyme serine-tRNA synthetase. 

Elaborate cascades initiate the clotting of blood (Chapter 12) and the action of the protective complement system (Chapter 31). Cascades considered later in the book are involved in controlling transcription (Fig. 11-13) and in the regulation of mammalian pyruvate dehydrogenase (Eq. 17-9), 3-hydroxy-3-methyl-glutaryl-CoA reductase and eicosanoids (Chapter 21), and glutamine synthetase (Chapter 24). 

Each synthetase module contains three active site domains The A domain catalyzes activation of the amino acid (or hydroxyacid) by formation of an aminoacyl- or hydroxyacyl-adenylate, just as occurs with aminoacyl-tRNA synthetases. However, in three-dimensional structure the A domains do not resemble either of the classes of aminoacyl-tRNA synthetases but are similar to luciferyl adenylate (Eq. 23-46) and acyl-CoA synthetases.11 The T-domain or peptidyl carrier protein domain resembles the acyl carrier domains of fatty acid and polyketide synthetases in containing bound phos-phopantetheine (Fig. 14-1). Its -SH group, like the CCA-terminal ribosyl -OH group of a tRNA, displaces AMP, transferring the activated amino acid or hydroxy acid to the thiol sulfur of phosphopan-tetheine. The C-domain catalyzes condensation (peptidyl transfer). The first or initiation module lacks a C-domain, and the final termination module contains an extra termination domain. The process parallels that outlined in Fig. 21-11.1... 

The conversion of androstenedione to estrone is catalyzed by aromatase. Inhibition of aromatase (human estrogen synthetase) by several naturally occurring flavonoids, including quercetin, chrysin, and apigenin, has been described. The synthetic flavone 7,8-benzoflavone was most active. Aromatization of androstenedione was affected by several flavonoids, of which 7-hydroxy-flavone and 7,4-dihydroxyflavone were the most potent. 

The anti-inflammatory activity of curcumin and its derivatives is associated with the hydroxyl and phenol groups in the molecule, which are also essential for the inhibition of prostaglandins, PG synthetase and leucotriene synthesis (LT) (Kiuchi et al., 1982, 1992 Iwakami, et al, 1986). Claeson et al. (1993, 1996) suggested that the antiinflammatory action and the antiparasitic activity were associated with the (3-dicarbo-nylic system with conjugated double bonds (dienes) (Araujo et al., 1998, 1999). The better skin penetration and lipophylicity is attributed to the presence of a diene ketone system. Calebin-A, a novel curcuminoid isolated from turmeric, protects neuronal cells from (3-amyloid insult. The hydroxy group at para-position of this compound is most critical for the expression of biological activity (Kim et al., 2001). 

Fatty acid synthetase has all the activities that would be necessary to reverse the /8-oxidation pathway. Thus, a cis double bond is generated from /3-hydroxy fatty acyl-CoA residues by a dehydration process. 

Urinary organic acid analysis is useful for differentiating isolated carboxylase deficiencies from the biotin-responsive multiple carboxylase deficiencies. P-Hydroxyisovalerate is the most common urinary metabolite observed in isolated P-methylcrotonyl-CoA carboxylase deficiency, biotinidase deficiency, biotin holo-carboxylase synthetase deficiency, and acquired biotin deficiency. In addition to P-hydroxy-isovalerate, elevated concentrations of urinary lactate, methylcitrate, and P-hydroxypropionate are indicative of multiple carboxylase deficiency. 

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