2 -Acetyl 5’-adenylate

Figure 21.9 Formation of the thioester acetyl CoA by nucleophilic acyl substitution reaction of coenzyme A (CoA with acetyl adenylate.

Problem 21.22 Write the mechanism of the reaction shown in Figure 21.9 between coenzyme A and acetyl adenylate to give acetyl CoA. 

The frequency of bacterial aminoglycoside resistance encountered in clinical practice has remained nearly constant over the past 2 decades. Of the three recognized mechanisms of resistance that occur in aerobic gram-negative bacteria, plasmid-mediated expression of enzymes that acetylate, adenylate, or phosphorylate the aminoglycosides is the most important. Ring one is the primary target of these enzymes. 

Acknowledgements The authors are grateful to the European COST organization through the D27 action Prebiotic Chemistry and Early Evolution, to John Sutherland (University of Manchester, UK) and Franck Selsis (Centre de Recherche Astronomique de Lyon, France) for helpful discussions, to Prof. David Clark (University of Southern Illinois, USA) for making us aware of the free energy content of acetyl adenylate reported in [156], to the CNRS and to the French Ministry of Education and Research for financial support. 

Yeast acetyl-CoA synthetase is known to catalyze the conversion of ATP, acetate, and coenzyme A to acetyl-CoA, AMP and pyrophosphate by a mechanism involving the intermediate formation of acetyl-adenylate according to Equations 17 and 18 ... 

Step 1 Nucleophilic addition of the -SR group of CoA (after deprotonation) to acetyl adenylate to form a tetrahedral intermediate. 

Resistance to this type of antibiotic is usually mediated by acetylation, adenylation and phosphorylation mostly in rings A and B causing thus lowered or abolished antibiotic affinity to the RNA [87]. 

Acetyl-CoA synthetase from mammalian tissues and yeast catalyzes the reaction of acetate with ATP and CoA to form acetyl-CoA by a chemical mechanism similar to that of the aminoacyl-tRNA synthetases. The catalytic pathway is similar to that of reactions (29a) and (29b), substituting acetate for the amino acid and CoA for tRNA (93). The activation of acetate via the intermediate acetyl adenylate also occurs with inversion of configuration at P of ATP (94). Thus, as for aminoacyl-tRNA synthetases, acetyl-CoA synthetase appears to catalyze the activation of acetate by a single-displacement mechanism. 

SOLUTION Acetate reacts with ATP to form acetyl adenylate, which then reacts with CoASH to form acetyl-CoA (Section 17.20). Because malonyl-CoA is prepared from acetyl-CoA, the thioester carbonyl carbon of malonyl-CoA will also be labeled. Examining each step of the mechanism for the biosynthesis of isopentenyl pyrophosphate from acetyl-CoA and malonyl-CoA allows you to determine the locations of the radioactively labeled carbons in isopentenyl pyrophosphate. Similarly, the locations of the radioactively labeled carbons in geranyl pyrophosphate can be determined from the mechanism for its biosynthesis from isopentenyl pyrophosphate. And the locations of the radioactively labeled carbons in farnesyl pyrophosphate can be determined from the mechanism for its biosynthesis from geranyl pyrophosphate. Knowing that squalene is obtained from a tail-to-tail linkage of two farnesyl pyrophosphates tells you which carbons in squalene will be labeled. 

Acetyl adenylate Phosphoeijol pyruvate Creatine phosphate Acetyl phosphate Adenosine triphosphate (ATP) Glucose-i-phosphate Fructose-6-phosphate Fructose-1 -phosphate... 

The second group of compounds studied by Jencks et al. were acyl phosphates (Table 19.2). The acetyl phosphate dianion, which is the predominant ionic species at neutral pH, has a carbonyl band at 1713 cm Acylated phosphate monoesters, such as acetyl adenylate and acetyl phenyl phosphate absorb at higher frequencies of 1737 to 1747 cm and the structurally similar acetyl phosphate monoanion absorbs... 

Acetyl adenylate 1737 1372 2922m, 1620. 1577w, 1480m. 1427m, 1337w, 1299w... 

The structurally similar amino acid-RNA derivatives also react rapidly with hydroxylamine and are energy-rich compounds, since they are in reversible equilibrium with ATP (Hoagland et al, 1958 Mager and Lipmann, 1958 Holley and Goldstein, 1959). Within the acyl phosphate series, acetyl adenylate (1737 cm ) has a free energy of hydrolysis at pH 7 which is similar to that of acetylimidazole and is... 

The direct route of acyl coenzyme A synthesis from a free carboxylic acid is catalysed by a group of nucleoside triphosphate-requiring en mes, collectively known as thiokinases. The general mechanism, as exemplified for acetate activation by acetyl thiokinase, proceeds as follows. The carboxylic acid is first activated by acetyl adenylate formation with the displacement of pyrophosphate from ATP. While the initial reaction is fully reversible, subsequent action of pyrophosphatase drives the reaction... 

There is still discussion which enzyme, ACS or pPDHC, provides the major acetyl-CoA pool for de novo fatty acid biosynthesis, and this seems to be dependent on plant species, development of plants and plastids and on the plastidic levels of pyruvate and acetate [4,6]. In order to obtain new information on the contribution of both enzymes to de novo fatty acid biosynthesis we looked for enzyme activities in developing chloroplasts and for specific inhibitors of the ACS and the pPDHC. The tests were performed in isolated chloroplasts and etioplasts which are capable of incorporating exogenously applied [ C]-acetate as well as [2- C]-pyruvate Into the total fatty acid fraction [6,7]. A specific, potent Inhibitor of the plant ACS was found to be ethyl-adenylate (ethyl-AMP) (Figure 1) which mimics the tightly enzyme-bound ACS-intermedlate acetyl-adenylate [7]. acetylmethylphosphinate (Figure 1) is a known inhibitor of mitochondrial PDHC [8] which also blocks the plastidic pPDHC as is shown here. We also demonstrate that etioplasts prefer pyruvate and chloroplasts acetate for fatty acid biosynthesis. 

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