Phosphoryl Group Transfers and ATP:- ATP Donates Phosphoryl, Pyrophosphoryl, and Adenylyl Groups
The reactions of ATP are generally SN2 nucleophilic dis placements (p. II.8), in which the nucleophile may be, for example, the oxygen of an alcohol or carboxylate, or a nitrogen of creatine or of the side chain of arginine or histidine. Each of the three phosphates of ATP is susceptible to nucleophilic attack (Fig. 13–10), and each position of attack yields a different type of product. Nucleophilic attack by an alcohol on the phosphate (Fig. 13–10a) displaces ADP and produces a new phosphate ester. Studies with 18O-labeled reactants have shown that the bridge oxygen in the new com pound is derived from the alcohol, not from ATP; the group transferred from ATP is a phosphoryl (-PO3-2), not a phosphate (-OPO3-2). Phosphoryl group transfer from ATP to glutamate (Fig. 13–8) or to glucose involves attack at the position of the ATP molecule. Attack at the phosphate of ATP displaces AMP and transfers a pyrophosphoryl (not pyrophosphate) group to the attacking nucleophile (Fig. 13–10b). For example, the formation of 5-phosphoribosyl-1-pyrophosphate (p. XXX), a key intermediate in nucleotide synthesis, results from attack of an -OH of the ribose on the β phosphate.
Nucleophilic attack at the position of ATP displaces PPi and transfers adenylate (5-AMP) as an adenylyl group (Fig. 13–10c); the reaction is an adenylylation (a-den-i-li-la--shun, probably the most ungainly word in the biochemical language). Notice that hydrolysis of the –phosphoanhydride bond releases considerably more energy (~46 kJ/mol) than hydrolysis of the β- γ bond (~31 kJ/mol) (Table 13–6). Furthermore, the PPi formed as a byproduct of the adenylylation is hydrolyzed to two Pi by the ubiquitous enzyme inorganic pyro phosphatase, releasing 19 kJ/mol and thereby providing a further energy “push” for the adenylylation reaction. In effect, both phosphoanhydride bonds of ATP are split in the overall reaction. Adenylylation reactions are therefore thermodynamically very favorable. When the energy of ATP is used to drive a particularly unfavor able metabolic reaction, adenylylation is often the mechanism of energy coupling. Fatty acid activation is a good example of this energy-coupling strategy.
The first step in the activation of a fatty acid— either for energy-yielding oxidation or for use in the syn thesis of more complex lipids—is the formation of its thiol ester (see Fig. 17–5). The direct condensation of a fatty acid with coenzyme A is endergonic, but the formation of fatty acyl–CoA is made exergonic by stepwise removal of two phosphoryl groups from ATP. First, adenylate (AMP) is transferred from ATP to the carboxyl group of the fatty acid, forming a mixed anhydride (fatty acyl adenylate) and liberating PPi. The thiol group of coenzyme A then displaces the adenylate group and forms a thioester with the fatty acid. The sum of these two reactions is energetically equivalent to the exergonic hydrolysis of ATP to AMP and PPi (ΔG0=- 45.6 kJ/mol) and the endergonic formation of fatty acyl–CoA (ΔG0=-31.4 kJ/mol). The formation of fatty acyl–CoA is made energetically favorable by hydrolysis of the PPi by inorganic pyrophosphatase. Thus, in the activation of a fatty acid, both phosphoanhydride bonds of ATP are broken. The resulting ΔG0 is the sum of the ΔG0 values for the breakage of these bonds, or -45.6 kJ/mol+ (-19.2) kJ/mol:
ATP+2H2O → AMP+2Pi ΔG0=- 64.8 kJ/mol
The activation of amino acids before their polymerization into proteins (see Fig. 27–14) is accomplished by an analogous set of reactions in which a transfer RNA molecule takes the place of coenzyme A. An interesting use of the cleavage of ATP to AMP and PPi occurs in the firefly, which uses ATP as an energy source to produce light flashes (Box 13–2).
FIGURE 13–10 Nucleophilic displacement reac tions of ATP.Any of the three P atoms (α, β, or γ) may serve as the electrophilic target for nucleophilic attack—in this case, by the labeled nucleophile R-18O: The nucleophile may be an alcohol (ROH), a carboxyl group (RCOO-), or a phosphoanhydride (a nucleoside mono- or diphosphate, for example). (a)When the oxygen of the nucleophile attacks the position, the bridge oxygen of the product is labeled, indicating that the group transferred from ATP is a phosphoryl (OPO32-), not a phosphate (OOPO3 2-). (b)Attack on the position displaces AMP and leads to the transfer of a pyrophosphoryl (not pyrophosphate) group to the nucleophile. (c)Attack on the position displaces PPi and transfers the adenylyl group to the nucleophile.
