Hydrolysis of AMP

The concepts of destabilization of reactants and stabilization of products described for pyrophosphate also apply for ATP and other phosphoric anhydrides (Figure 3.11). ATP and ADP are destabilized relative to the hydrolysis products by electrostatic repulsion, competing resonance, and entropy. AMP, on the other hand, is a phosphate ester (not an anhydride) possessing only a single phosphoryl group and is not markedly different from the product inorganic phosphate in terms of electrostatic repulsion and resonance stabilization. Thus, the AG° for hydrolysis of AMP is much smaller than the corresponding values for ATP and ADP. 

There are two anhydride linkages in ATP, but nucleophilic attack in the enzyme-controlled reaction usually occurs on the terminal P=0 (hydrolysis of ATP to ADP), and only occasionally do we encounter attack on the central P=0 (hydrolysis of ATP to adenosine monophosphate, AMP). Both reactions yield the same amount of energy, AG—34 kJmoD This is not surprising, since in each case the same type of bond is being hydrolysed. The further hydrolysis of AMP to adenosine breaks an ester linkage and would liberate only a fraction of the energy, AG — 9 kJmol and this reaction is not biochemically important. 

This enzyme [EC 3.S.4.6], also known as AMP amino-hydrolase and adenylic acid deaminase, catalyzes the hydrolysis of AMP to yield IMP and ammonia. 

Tsai and Chang showed that 5 -nucleotidase-catalyzed hydrolysis of AMPS to adenosine and phosphorothioate proceeds with overall inversion of configuration [28], They synthesized the (Rp) and (Sp) isomers of AMPS, l80 used them as substrates for 5 -nucleotidase in H(70, and determined the configurations of the two samples of [ l60, nO, 180]phosphorothioate by the procedure of Webb and Trentham outlined in Fig. 18 [48]. The configurations of chiral phosphorothioate samples obtained from the (Rp) and (Sp) isomers of AMPS, l80 were (/ p) and (Sp), respectively, the configurations corresponding to inversion. The stereochemistry was thereby shown to be that of Equation 13 ... 

Amino acids ate not the only source of ammonium ions produced in the body. Much of the ammonia produced, especially in the brain, arises from the hydrolysis of purines. Adenylate deaminase catalyzes the hydrolysis of AMP, yielding IMP and ammonium ions Cooper and Plum, 1987). IMP is inosine monophosphate (inosinic acid), GMP may also be hydrolyzed in this manner, yielding xanthosine and ammonium ions. Further details on purine metabolism occur at the end of this chapter and under Folate in Chapter 9. 

Mentch, F., Parkin, D.W. and Schramm, V.L. (1987) Transition-state structures for V-glycoside hydrolysis of AMP by acid and by AMP nucleosidase in the presence and absence of allosteric activator. Biochemistry, 26, 921-930. 

Write an equation for the complete hydrolysis of AMP to its component parts. 

Excessive hydrolysis of polyacrylamide in situ can promote undesirable polymer precipitation in the reservoir. The rate of this hydrolysis decreases with increasing level of anionic comonomers such as AMPS (130). 

FIGURE 15.20 The adenylyl cyclase reaction yields 3, 5 -cyclic AMP and pyrophosphate. The reaction is driven forward by subsequent hydrolysis of pyrophosphate by the enzyme inorganic pyrophosphatase. 

FIGURE 24.7 The acyl-CoA synthetase reaction activates fatty acids for /3-oxidation. The reaction is driven by hydrolysis of ATP to AMP and pyrophosphate and by the subsequent hydrolysis of pyrophosphate. 

There is some evidence to suggest that these drugs may owe their activity to inhibition of the enzyme that is responsible for hydrolysis of 3, 5 -cyclic AMP (itself a guanine derivative) and thus prolong the action of cyclic AMP. 

The tethering of PKA through AKAPs by itself is not sufficient to compartmentalize and control a cAMP/ PKA-dependent pathway. Cyclic AMP readily diffuses throughout the cell. Therefore, discrete cAMP/PKA signalling compartments are only conceivable if this diffusion is limited. Phosphodiesterases (PDE) establish gradients of cAMP by local hydrolysis of the... 

Cyclic nucleotide phosphodiesterases (PDEs) are a class of enzymes that catalyze the hydrolysis of 3, 5 -cyclic guanosine monophosphate (cGMP) or 3, 5 -cyclic adenosine monophosphate (cAMP) to 5 -guanosine monophosphate (GMP) or 5 -adenosine monophosphate (AMP), respectively. 

Figure29-1. Partial reactions in the attachment of ubiquitin (UB) to proteins. (1) The terminal COOH of ubiquitin forms a thioester bond with an -SH of E, in a reaction driven by conversion of ATP to AMP and PP. Subsequent hydrolysis of PP by pyrophosphatase ensures that reaction 1 will proceed readily. (2) A thioester exchange reaction transfers activated ubiquitin to Ej. (3) E3 catalyzes transfer of ubiquitin to e-amino groups of lysyl residues of target proteins.

Condensation of CO2, ammonia, and ATP to form carbamoyl phosphate is catalyzed by mitochondrial carbamoyl phosphate synthase I (reaction 1, Figure 29-9). A cytosolic form of this enzyme, carbamoyl phosphate synthase II, uses glutamine rather than ammonia as the nitrogen donor and functions in pyrimidine biosynthesis (see Chapter 34). Carbamoyl phosphate synthase I, the rate-hmiting enzyme of the urea cycle, is active only in the presence of its allosteric activator JV-acetylglutamate, which enhances the affinity of the synthase for ATP. Formation of carbamoyl phosphate requires 2 mol of ATP, one of which serves as a phosphate donor. Conversion of the second ATP to AMP and pyrophosphate, coupled to the hydrolysis of pyrophosphate to orthophosphate, provides the driving... 

The charging of the tRNA molecule with the aminoacyl moiety requires the hydrolysis of an ATP to an AMP, equivalent to the hydrolysis of two ATPs to two ADPs and phosphates. The entry of the aminoacyl-tRNA into the A site results in the hydrolysis of one GTP to GDP. Translocation of the newly formed pep-tidyl-tRNA in the A site into the P site by EF2 similarly results in hydrolysis of GTP to GDP and phosphate. Thus, the energy requirements for the formation of one peptide bond include the equivalent of the hydrolysis of two ATP molecules to ADP and of two GTP molecules to GDP, or the hydrolysis of four high-energy phosphate bonds. A eukaryotic ribosome can incorporate as many as six amino acids per second prokaryotic ribosomes incorporate as many as 18 per second. Thus, the process of peptide synthesis occurs with great speed and accuracy until a termination codon is reached. 

Attention has been drawn to the potential of phosphoric acid anhydrides of nucleoside 5 -carboxylic acids (14) as specific reagents for investigating the binding sites of enzymes. For example, (14 B = adenosine) inactivates adenylosuccinate lyase from E. coli almost completely, but has little effect on rabbit muscle AMP deaminase. The rate of hydrolysis of (14) is considerably faster than that of acetyl phosphate, suggesting intramolecular assistance by the 3 -hydroxyl group or the 3-nitrogen atom. 

Hydrolysis of amide groups to carboxylate is a major cause of instability in acrylamide-based polymers, especially at alkaline pH and high temperatures. The performance of oil-recovery polymers may be adversely affected by excessive hydrolysis, which can promote precipitation from sea water solution. This work has studied the effects of the sodium salts of acrylic acid and AMPS, 2-acrylamido-2-methylpropanesulfonic acid, as comonomers, on the rate of hydrolysis of polyacrylamides in alkaline solution at high temperatures. Copolymers were prepared containing from 0-53 mole % of the anionic comonomers, and hydrolyzed in aqueous solution at pH 8.5 at 90°C, 108°C and 120°C. The extent of hydrolysis was measured by a conductometric method, analyzing for the total carboxylate content. 

The extent of hydrolysis of the copolymers was determined by conductometric titration. The increase in carboxylate content was determined by difference, before and after hydrolysis. (The AMPS content of the polymers, where measured, was determined by colloid titration with poly [diallyl dimethyl ammonium chloride].)... 

The rate of hydrolysis of acrylamide is assumed to be equal to the rate of formation of carboxylate groups in the early stages of reaction, for both sodium acrylate and AMPS copolymers. 

The rate of hydrolysis of acrylamide in copolymers with sodium acrylate or AMPS, 2-acrylamido-2-methylpropanesulfonic acid, decreased as the proportion of the anionic comonomers was increased. This effect was much more marked with AMPS than with sodium acrylate, and occurred at 90°, 108°, and 120°C. Typical results at 108°C [Figs. 1 and 2] show the increase in carboxylate content of acrylamide copolymers containing sodium acrylate and AMPS respectively. 

The calculated pseudo-unimolecular rate constants (k) for the hydrolysis reaction [Fig. 3], clearly show the inhibiting effect of AMPS, relative to sodium acrylate at all three temperatures. 

Figure 2. Hydrolysis of amide in AMPS (S) copolymers at 108 ° C, pH 8.5, 0.025 M amide.

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