Amide transferases

The glutamo- and asparto-amide transferases are in another category. They transfer the amide (or hydroxylamine) radical without hydrolysis, i.e., the enzyme s specificity does not include water as a replacement agent. And with sufficient ATP at least one glutamotransferase can effect the synthesis of glutamine from glutamic acid and ammonia. 

Many NRPs such as cyclosporin, complestatin, actinomycin, and chondramide contain N-methyl amides. M-Methyl transferase (N-MT) domains utilize S-adenosylmethionine (SAM) as a cofactor to catalyze the transfer of the methyl group from SAM to the a-amine of an aminoacyl-S-PCP substrate. The presence of M-methylamides in NRPs is believed to protect the peptide from proteolysis. Interestingly, N-MT domains are incorporated into the A domains of C-A-MT-PCP modules, between two of the core motifs (A8 and A9). MT domains contain three sequence motifs important for catalysis. ° 0-Methyl transferase domains are also found in NRPSs and likewise use the SAM cofactor. For instance, cryptophycin and anabaenopeptilide synthetases contain 0-MT domains for the methylation of tyrosine side chains. These 0-MT domains lack one of the three core motifs described for N-MT domains. ... 

Phase II reactions (conjugate formation). Type II reactions couple their substrates (bilirubin, steroid hormones, drugs, and products of phase I reactions) via ester or amide bonds to highly polar negatively charged molecules. The enzymes involved are transferases, and their products are known as conjugates. 

Glutamine is one of the principal combined forms of ammonia that is transported throughout the body (Chapter 24). Glucosamine 6-phosphate synthase, which catalyzes the reaction of Eq. 20-5, is an amido-transferase of the N-terminal nucleophile hydrolase superfamily (Chapter 12).31 It hydrolyzes the amide... 

Amide and cyanoguanidine derivatives 265 and 266 containing a 5,ll-dihydro[l]benzothiepino[4,3-A pyridine ring system were evaluated in vitro and found to be good inhibitors of farnesyl-protein transferase <1998BML2521>. 

N,0-Acyltransferase. The /V-acyl transferase enzyme is believed to be involved in the carcinogenicity of arylamines. These compounds are first V-oxidized, and then, in species capable of their A-acetylation, acetylated to arylhydroxamic acids. The effect of N, O-transacetylation is shown in Figure 7.22. The A/-acyl group of the hydroxamic acid is first removed and is then transferred, either to an amine to yield a stable amide or to the oxygen of the hydroxylamine to yield a reactive N-acyloxyarylaminc. These compounds are highly reactive in the formation of adducts with both proteins and nucleic acids, and N, O -acy I Iransfcrasc, added to the medium in the Ames test, increases the mutagenicity of compounds such as A-hydroxy-2-acetylaminofluorine. 

Fig. 1. Targeted lipidomics of anandamide metabolism. Postulated pathways of anandamide metabolism. Abbreviations PC, phosphatidylcholine PE, phosphatidylethanolamine NAT, JV-acyl transferase LPA, lysophosphatidic acid PA, phosphatidic acid NAPE, jV-acyl-phosphatidylethanolamine Lyso-NAPE, l-lyso,2-acyl-OT-glycero-3-phosphoethanolamine-JV-acyl ABHD-4, a//3 hydrolase-4 GP-anandamide, glycerophospho-anandamide PAEA, phospho-anandamide PLA, phospholipase A NAPE-PLD, NAPE phospholipase D PLC, phospholipase C FAAH, fatty acid amide hydrolase P, phosphatase COX, cyclooxygenase LOX, lipoxygenase CYP450, cytochrome P450 PDE, phosphodiesterase.

In the first step of the peptidyl transferase reaction, a peptidyl tRNA molecule is bound in the P-site with its nascent peptide extending down the peptide exit tunnel (Fig. 4.1). An elongation factor binds to a factor binding site (FBS) and positions an aminoacyl-tRNA in the A-site. The a amino group of the aminoacyl-tRNA nucleophilically attacks the ester bond which connects the peptide to the tRNA bound in the P-site (Fig. 4.2). The ester bond is broken as an amide bond forms, and the peptide becomes one amino acid longer, and is now attached to the tRNA that in the A-site. Translocation of the products follows peptide bond formation, as the newly formed deacylated- tRNA of the P-site moves into the E-site, and as the newly elongated peptidyl-tRNA moves from the A-site into the P-site. 

The prototypical aminoacylated nucleoside analogue antibiotic is puromycin which inhibits the protein translation in all three domains of life. The chemical structure of puromycin is the same as that of tyrosylated adenosine, except for the presence of three added methyl groups and the replacement of an ester bond with an amide bond (Fig. 4.11). Puromycin mimics tyrosyl-tRNA so well that it binds to the A-site and gets incorporated into an elongating peptide. This leads to termination of translation because puromycin terminated peptides fall off the ribosome. Puromycin derivatives have been used crystallographically as peptidyl transferase substrates and have contributed to our understanding of the structure of the peptidyl transferase site (Fig. 4.5) [11, 16, 45],... 

Enzymes are classified into six categories depending on the kind of reaction they catalyze, as shown in Table 26.2. Oxidorediictases catalyze oxidations and reductions transferases catalyze the transfer of a group from one substrate to another hydrolases catalyze hydrolysis reactions of esters, amides, and related substrates lyases catalyze the elimination or addition of a small molecule such as H2O from or to a substrate isomerases catalyze isomerizations and ligases catalyze the bonding together of two molecules, often coupled with the hydrolysis... 

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