Aspartic acid and glutamic acid are the two naturally occurring a-amino acids that have

Chapter 25, Problem 25.4

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Aspartic acid and glutamic acid are the two naturally occurring a-amino acids that have carboxylate groups in their side chains. Asparagine and glutamine are the corresponding a-amino acids in which the side chains are primary amides. (All of these amino acids have the S configuration.) aspartic acid NH3 CO2 C O O + H C CH2 asparagine NH3 CO2 C NH2 O + H C CH2 glutamine C NH2 NH O 3 CO2 + H C CH2CH2 glutamic acid C O NH O 3 CO2 + H C CH2CH2 Asparagine synthetase catalyzes the biosynthesis of asparagine from glutamine according to the equation shown in Fig. P25.40. Note that the amide NH2 group in asparagine comes from the amide NH2 group of glutamine. (a) Suggest a sequence of steps for this reaction that involves the adenylation of aspartic acid. (b) Glutamine and asparagine could in principle be equilibrated without ATP. Why is ATP utilized in this way? (c) Calculate the DG for the biosynthesis of asparagine using the following DG values for hydrolysis (in kJ mol1 ): ATP to AMP 1 pyrophosphate, 245.6; asparagine to aspartic acid, 215.1; glutamine to glutamic acid, 214.2. (The corresponding values in kcal mol1 are 210.9, 23.61, and 23.39.) (d) It is sometimes said that asparagine biosynthesis is coupled to ATP hydrolysis (to AMP). What is meant by this statement? Is ATP actually hydrolyzed? (e) What other reaction (catalyzed by a different enzyme) ensures that asparagine biosynthesis goes to completion?