ATP magnesium complex

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... 

The function of magnesium in enzyme activity may either be to form a complex with the substrate, as in the magnesium-ATP complex formed in creatine kinase and phosphofhictokinase, or to bind to the enzyme and either produce an allosteric activation or play a direct role in catalysis. If an enzyme is known to utilize a nucleotide as one of its substrates, it can be assumed that magnesium is also required for catalysis. The magnesium ion possibly acts as an electrostatic shield. The enzyme pyravate kinase, described earlier, and shown in Figure 1, requires both magnesium and potassium ions for maximal activity. 

Nonetheless, there remained some uncertainty regarding the actual intracellular concentration of uncomplexed magnesium ion, which controls the fraction of total ATP complexed with Mg ... 

Magnesium is distinguished by the fact that it is required by most ATP-using enzymes. Here, Mg occurs as a complex with ATP, as shown in Figure 10,49. In other words, the true substrate for most ATF-requiring enzymes is not ATP, but the Mg-ATP complex. A deficiency in magnesium is uncommon. When it does occur, the physiological funchon that is most sensitive is neuromuscular activity. In molecular terms, the enzymes involved in neuromuscular activity that appear to be sensitive to Mg deficiency are those involved in the transport of sodium, potassium, and calcium, these enzymes are Na.K-ATPase and the calcium pump (Ca-ATPase). 

Most or all ATP-requiring enzymes use ATP in the form of the Mg-ATP complex. ATP chelates the magnesium ion, which is a divalent metal ion. The glycolytic pathway features a number of Mg-requiring enzymes. One of these enzymes is phosphofructokinase. The Mg requirement for this enzyme is illustrated by the data in Figure 10-50- Ihe study involved phosphofructokinase purified from rabbit muscle. Each point in Figure 10-50 represents the catalytic activity of the enzyme that was expressed during incubation in separate test tubes. All of the test tubes... 

Magnesium ions bind to inorganic phosphate [P,] and to citrate. The levels of these anions, respectively, are about 3 3 and 1,2 mM in the cytosol and 17.0 and 5-2 mM in the mitochondria. The association constants for Mg-P and Mg-citrate formation are small, compared with that for Mg-ATP formation (see Table 10.16), Pj and citrate might be ex peeled to only slightly impair the formation of the Mg-ATP complex in the cell,... 

This formula indicates that the concentration of the MP complex is dependent on those of M and P. The formula can be used to estimate the concentration of MP, with a knowledge of the total Mg and total ATP. The cytosolic level of total Mg is about 10 mM and that of ATP is about 2 mM. The value of K is taken from Table 10.16. The concentration [M] of free magnesium is 10 - [MP]. The concentration [P] of free ATP is 2.0 - [MP]. The value for the concentration [MP] of the Mg-ATP complex can easily be foimd from the following formula using a computer ... 

ALA-dehydratase, and isocitrate dehydrogenase and decreases Na+, K -ATPase activity, Mg +-ATPase activity, and choline uptake into synaptosomes. In vitro, aluminum displaces magnesium from Mg +-ATP complexes, and it could thus antagonize virtually any phosphatetransferring reaction that uses Mg +-nucleotide triphosphate complexes. 

The transformation of the carboxylic acid substrate into acyl-CoA requires one ATP molecule and leads to the release of AMP and pyrophosphate. The reaction mechanism involves the formation of an acyl-AMP intermediate. Extensive and detailed kinetic studies provide details on the ordered addition of the substrate and ATP to the enzyme and the subsequent release of products (Fig. 31.34). The reaction proceeds via a Bi-Uni-Bi-Ping-Pong mechanism whereby ATP complexed to magnesium binds first, thus allowing the fixation of carboxylic acid to the enzyme. Acyl-AMP is thereafter formed and pyrophosphate released. Then... 

The insertion of D-alanine into teichoic acids is thought to occur after the polymer backbone has been constructed. An enzyme which is putatively involved in its activation , prior to addition, has been isolated from a strain of Lactobacillus easel It requires ATP and magnesium ions and appears to form an enzyme-bound amino-acyl-ATP complex. However, the addition of D-alanine in vivo occurs in a highly ordered environment and attempts to imitate it in vitro have not achieved rates of synthesis comparable with physiological levels. Very possibly the true acceptor in vivo is a lipoteichoic acid. 

Possibly the reason for this apparent contradiction is that the Mn +-ATP complex contains a water molecule which is simultaneously co-ordinated to the metal ion and hydrogen-bonded to N-7 of the adenine ring (see Figure 1 a similar structure has been proposed for the ATP complexes of Co + and Ni +), Thus there is an interaction between the ring and the metal ion but it is not a direct one. Further Raman work with the calcium and magnesium complexes of ATP has indicated that ATP-M-ATP interactions are more important with the larger metal than with the smaller. The relaxation spectra have been reported for the reaction of nickel(n) with 9-methylpurine, adenine, and hypoxanthine. It appears that the rate... 

All organisms seem to have an absolute need for magnesium. In plants, the magnesium complex chlorophyll is the prime agent in photosynthesis. In animals, magnesium functions as an enzyme activator the enzyme which catalyses the ATP hydrolysis mentioned above is an important example. 

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

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