Adenosine triphosphate magnesium complex

Figure 10-4. Adenosine triphosphate (ATP) shown as the magnesium complex. ADP forms a similar complex with Mg A...

For example, Bachelard used [Mgtotai]/[ATPtotai ] = 1 in his rate studies, and he obtained a slightly sigmoidal plot of initial velocity versus substrate ATP concentration. This culminated in the erroneous proposal that brain hexokinase was allosterically activated by magnesium ions and by magnesium ion-adenosine triphosphate complex. Purich and Fromm demonstrated that failure to achieve adequate experimental control over the free magnesium ion concentration can wreak havoc on the examination of enzyme kinetic behavior. Indeed, these investigators were able to account fully for the effects obtained in the previous hexokinase study. ... 

Reactions involving the formation and hydrolysis of phosphate and polyphosphate esters are of vital importance in biological systems in which it is found that magnesium ions are almost invariably implicated. The formation and decomposition of adenosine triphosphate are the fundamental reactions involved in energy storage in living systems. In this context, it is perhaps relevant to note that the hydrolysis of ATP is enhanced, albeit in a very modest manner, by some cobalt(m) complexes. 

Cayley and Hague [70a] have employed the temperature-jump relaxation method to measure the rate of formation and dissociation of the 1 1 complex of magnesium(II) and 5-nitrosalicyclic acid (NSA) and also of the ternary complexes formed between this ligand and Mg(II) nitrilotriacetate (NTA), adenosine triphosphate (ATP) and polytriphosphate (TP) complexes, the last in an effort to gather information about the effect of bound ligands on the rate of substitution of a metal ion by a second type of group. It is expected that the presence of a bound ligand should... 

Metal cofactors do not always bind to the enzyme but rather bind to the primary substrate. The resulting substrate-metal complex binds to the enzyme and facilitates its activity. Creatine kinase catalyses the transfer of phosphoryl groups from adenosine triphosphate (ATP), which is broken down to adenosine diphosphate (ADP). The reaction requires the presence of magnesium ions. These, however, do not bind to the enzyme but bind to ATP, forming an ATP Mg complex. It is this complex that binds to the enzyme and allows transfer of the phosphoryl group ... 

Chromium(III) forms stable complexes with adenosine-S -triphosphate.840,841,842 These are kinetically inert analogues of magnesium ATP complexes and may be used to study enzyme systems. The complexes prepared are chiral and may be distinguished in terms of chirality at the metal centre (198,199).843 The related complex of chromium(lll) with adenosine-5 -(l-thiodiphosphate) has been prepared the diastereoisomers were separated.844 The stereospecific synthesis of chromium(III) complexes of thiophosphates has been reported845 by the method outlined in equation (47), enabling the configuration of the thiophosphoryl centre to be determined. The availability of optically pure substrates will enable the stereospecificity of various enzyme systems to be investigated.845... 

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