The joining of two molecules is energetically unfavorable in an aqueous medium when the substrates correspond to hydrolysis products. In biochemistry, such ligations are driven by the free energy released by the hydrolysis of MgATP or an energetically equivalent molecule. The ATP-dependent synthetases and ligases catalyze reactions in which water is extracted from two molecules that become joined. The amount of free energy available depends on the site at which the ATP molecule is cleaved. The most common cleavage modes and the free energy change under standard conditions, which are pH = 7.0, 25°C, and 1 mM free Mg2+, are given in (Alberty, 1994; Arabshahi and Frey, 1995). Hydrolysis of the α,β-phosphoanhydride linkage to form AMP and PPi releases 3.2 kcal mol–1 more free energy than hydrolysis of the β,γ-linkage. In the actions of ATP-dependent ligases and synthetases, the free energy released in the hydrolysis of MgATP is used to overcome the energetic barrier to the elimination of water. The general principle is exemplified by the free energy barrier for the formation of ethyl acetate from acetate and ethanol under standard conditions, which is ΔG' = +4 kcal mol−1 (Jencks and Regenstein, 1970) The free energy change in the hydrolysis of MgATP to MgADP and Pi is ΔG' = –7.7 kcal mol−1 under the same conditions (Alberty, 1994). If these two reactions can be made to be interdependent, or coupled, the overall process would be the reaction of acetate, ethanol, and ATP to produce ethyl acetate, MgADP, and Pi, and the overall standard free energy change would be ΔG' = –3.7 kcal mol−1, making it a spontaneous or energetically downhill process. In the action of an ATP-dependent synthetase or ligase, the enzyme links the hydrolysis of MgATP with the ligation of the molecules by catalyzing the phosphorylation or adenylylation of one substrate and then the displacement of phosphate or AMP by the other substrate. Two types of glutamine synthetases are found in bacteria and eukaryotes. The bacterial glutamine synthetases, designated GS I (EC 6.3.1.2), are the most thoroughly studied. All species of GS I are dodecameric, 600- to 640-kDa enzymes assembled as two layers of hexameric rings associated face to face (Eisenberg et al., 2000; Stadtman and Ginsburg, 1974). Eukaryotic synthetases, designated GS II, are less understood, but essential aspects of their reaction mechanisms appear to be similar to that of GS I (Eisenberg et al., 2000; Meister, 1974a). Both GS I and GS II can be found in bacteria, although GS I is predominant. Eukaryotes contain only GS II. In this chapter, we discuss the reaction mechanism of GS I and GS II and the structure of GS I.
The equilibrium constants of the glutamate dehydrogenase systems
