Hydrolysis of ADP

The phosphate esters (81) and (82) are also subject to catalysis by metal ions275,276 and possible reactive complexes are illustrated. Metal complexes of adenosinediphosphoric acid and adenosinemonophosphoric acids have been studied277 and the effect of divalent metal ions on the hydrolysis of ADP and ATP has been investigated.278,279... 

The high reactivity of the 2 1 complex can be attributed to the availability on the second cobalt of a coordinated hydroxide which can attack a phosphorus center (preferably Pg). The other cobalt, in the six-membered ring, can assist the process by functioning as an electron withdrawing center. The acceleration for the hydrolysis of ADP is marked ( 105) and comparable to the effect we have previously seen for pyrophosphate and ATP. 

Preparations are listed in Table 47. Main interest lies in determining how metal ions catalyze the hydrolysis of ADP and ATP. ATP plays a crucial role in the energy metabolism of all living cells and divalent metal ions (Mg +, Mn " and Ca ) play an important role in these phosphoryl transfer processes. Divalent metal ions such as Mg +, Ca, Zn, Cu and Mn " provide only modest in vitro catalysis "" and stronger, more specific coordination to phosphate units appears to be required by the enzyme. Co " (and Cr" ) complexes of ADP and ATP have been shown to mimic many of the biological functions of the Mg" enzyme, and since the cobalt(III)-phosphatc coordination remains intact, the specificity of alternative coordination sites, and the stereochemical requirements at phosphorus, have been elucidated in some cases. Often the Co "-enzyme species is biologically active and several enzymic functions of ATP have been examined in this manner. 

ANIMATED FIGURE 15.7 Decrease in electrostatic repulsion on hydrolysis of ATP. Hydrolysis of ATP to ADP (and/or hydrolysis of ADP to AMP) leads to relief of electrostatic repulsion. Sign in atwww.thomsonedu.com/login to see an animated version of this figure. 

Chapter 3 introduces ATP as a nucleoside triphosphate building block involved in RNA synthesis. The hydrolysis of the terminal phosphate anhydride bond when ATP is converted into adenosine diphosphate (ADP) releases energy. Further hydrolysis of ADP is possible, giving rise to adenosine monophosphate (AMP), with the release of a slightly smaller amount of energy. 

ATP is a NUCLEOTIDE consisting of adenine and ribose with three phosphate groups attached. Hydrolysis of the terminal phosphate bond releases energy (30.6 kj moH) and is coupled to an energy-requiring process. Further hydrolysis of ADP to AMP sometimes occurs, releasing more energy. 

It was proposed over 20 years ago that arsenate functions as an alternate substrate for the membrane-bound, proton-translocating ATP-synthetase. This enzyme was thought to catalyze the formation of ADP-arsenate which would hydrolyze rapidly (Avron, Jagendorf 1959). Only recently this has been confirmed in mitochondria (Moore, Gresser 1982) and R. rubrum chromato-phores (Slooten, Nuyten 1983). Aside from the coupled enz)mie assay used to trap ADP-arsenate (Moore, Gresser 1982), we demonstrate here two other methods for the measurement of the synthesis and hydrolysis of ADP-arsenate, and apply them to a determination of some kinetic constants for arsenylation and phosphorylation of ADP and GDP. 

TABLE II (LEFT). Effect of salts on the pseudo-first order rate constant for non-enz)7mic hydrolysis of ADP-arsenate. The basal mixture contained 0.4 mM arsenate and 25 uM ADP. 

Non-enzymic hydrolysis of ADP-arsenate is accelerated by a number of ions, specifically arsenate and Mg (Table II). These data were calculated from log plots of the dark decay of the rapid phase in the luciferase measurements. Similar results were obtained from the ratio VAs/(ADP-As)sg (not shown, see eq. 1). Phosphate has an effect comparable to that of arsenate (not shown) other ions are ineffective. We propose that these effects are due to formation of transient intermediates such as pyro-arsenate or arsenato-pyro-phosphate. These compounds are stable as solids, but not in solution (Dolique 1958). 

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