ATP cleavage

The simplest mechanism to explain the much faster rate of dissociation of actomyosin-S-1 by ATP than that of ATP cleavage is that actin activates the myosin ATPase by accelerating the rate at which ADP and Pj are released. That is when ATP is added to actomyosin-S-1, ATP rapidly binds and dissociates actomyosin, myosin ATPase then hydrolyzes ATP to form myosin-ADP.Pj, this state then reattaches to actin and phosphate is released much faster from actomyosin. ADP.Pj than it is from myosin.ADP.Pj, as shown in the scheme below ... 

Some 20 different amino acid tRNA ligases in the cytoplasm each bind one type of tRNA (see p. 82) with the corresponding amino acid. This reaction, known as amino acid activation, is endergonic and is therefore coupled to ATP cleavage in two steps. 

The myosin head hydrolyzes the bound ATP to ADP and Pj, but initially withholds the two reaction products. ATP cleavage leads to allosteric tension in the myosin head. 

The peculiar metal ion specificity of the ATP cleavage reaction may perhaps be explained by reference to some studies on the metal complexes of Schiff bases, which have provided clues to many aspects of biological metal catalysis. It was shown that metal ions will split the carbon-nitrogen double bond in thiophenalde-hyde-ethylenediamine (18, 21) as a consequence of the electronic-drift-to-metal... 

Returning to the ATP cleavage reaction, it may be supposed that some metal ions, probably the weakest complexers, form a monocyclic chelate by attaching to the terminal pyrophosphate only (C), whereas others, probably the strongest complexers, form a bicyclic chelate by binding the a-phosphate as well(D). By... 

Fig. 6. ATP cleavage, ATP—ADP exchange and phosphoprotein formation during calcium uptake. The assay medium contained 7 mM MgClj,...

From the description of the coupling between ion movement and ATP cleavage or synthesis it infers that the transport protein must be able to interact with at least five reactants. In the following, these interactions will be analyzed. To facilitate the analysis, the reaction sequence will be dissected in its elementary steps whenever the analysis does not require the complete transport system. Partial reactions can most favourably be studied with modified preparations or the isolated transport protein. These preparations do not only provide simplicity but enable one to gain inside into the role of the various constituents of the system. 

D is the common intermediate and can serve as a carrier of chemical energy between the two reactions. Many coupled reactions use ATP to generate a common intermediate. These reactions may involve ATP cleavage—that is, the transfer of a phosphate group from ATP to another molecule. Other reactions lead to ATP synthesis by transfer of phosphate from an energy-rich intermediate to ADP, forming ATP. 

The majority of enzymes that are apt to be mentioned in any discussion of metabolism catalyze nucleophilic displacement reactions (Type 1, Table 10-1). These include most of the reactions by which the energy of ATP cleavage is harnessed and by which polymers are assembled from monomers. They include reactions by which pieces, large or small, are transferred onto or off of polymers as well as the reactions by which polymers are cleaved into pieces.1 3,1... 

C. Multiple Displacement Reactions and the Coupling of ATP Cleavage to Endergonic Processes... 

The first step in coupling ATP cleavage to any process is transfer of part of the ATP molecule to a nucleophile Y, usually by displacement on one of the three phosphorus atoms. The nucleophilic attack may be (a) on the terminal phosphorus (Py) with displacement of ADP or (b) on the internal phosphorus (Pa) with displacement of inorganic pyrophosphate. In the first case, Y-P03H is formed in the latter, Y-adenylyl (sometimes shortened to Y-adenyl) is formed. More rarely, displacement occurs (c) on the central phosphorus (Pp) with transfer of a pyrophospho group to the... 

Group activation usually takes place at the expense of ATP cleavage. 

Fatty acid chains are taken apart two carbon atoms at a time by (3 oxidation. Biosynthesis of fatty acids reverses this process by using the two-carbon acetyl unit of acetyl-CoA as a starting material. The coupling of ATP cleavage to this process by a carboxylation-decarboxylation sequence, the role of acyl carrier protein (Section H,4), and the use of NADPH as a reductant (Section I) have been discussed and are summarized in Fig. 17-12, which gives the complete sequence of... 

The idea of conformational coupling of ATP synthesis and electron transport is especially attractive when we recall that ATP is used in muscle to carry out mechanical work. Here we have the hydrolysis of ATP coupled to motion in the protein components of the muscle. It seems reasonable that ATP should be formed as a result of motion induced in the protein components of the ATPase. Support for this analogy has come from close structural similarities of the F, ATPase P subunits and of the active site of ATP cleavage in the muscle protein myosin (Chapter 19). 

Only one electron is transferred to the MoFe-protein in each catalytic cycle of the Fe-protein. Thus, the cycle must be repeated eight times to accomplish the reduction of N2 + 2 H+. Where in the MoFe-protein does a transferred electron go EPR spectroscopic and other experiments with incomplete and catalyti-cally inactive molybdenum coenzyme40 have provided a clear answer. The electron is transferred first to one of the two P-clusters, both of which are close to the Fe4S4 cluster of the Fe-protein. The transfer causes an observable change both in the spectroscopic properties and in the three-dimensional structure of the P-cluster.23/40a Since protons are needed at the active site for the reduction reactions (the FeMo-coenzyme), it is probable that hydrolysis of ATP in the Fe-protein is accompanied by transport of protons across the interface with the MoFe-protein. Tire electron transfer from the P-cluster on to the FeMo-co center would be assisted by a protic force resulting from ATP cleavage. 

Thiocysteine can also arise in a similar manner through action of cystathionine (3 lyase on cystine. Thiocysteine is eliminated with production of pyruvate and ammonia from the rest of the cystine molecule 467 One of the nifS-like proteins of E. coli is thought to transfer a selenium atom from selenocysteine (pp. 823-827) into selenophosphate 466a f The latter can be formed by transfer of a phospho group from ATP to selenide HSe-. The other products of ATP cleavage are AMP and P . Reduction of Se° to HSe- is presumably necessary. 

Type I and type II topoisomerases relax negatively supercoiled DNA in steps of one and steps of two, respectively. Type II topoisomerases can also add additional negative supercoils (as indicated by the double arrow). The latter reaction requires energy input, which is encoded by ATP cleavage. 

Coupling ATP Cleavage to an Unfavorable Reaction To explore the consequences of coupling ATP hydrolysis under physiological conditions to a thermodynamically unfavorable biochemical reaction, consider the hypothetical transformation X — Y, for which AG ° = 20 kJ/mol. 

Ligation reactions form bonds by using free energy from ATP cleavage. Reaction 3 illustrates the ATP-dependent formation of a carbon-carbon bond, necessary to combine smaller molecules to form larger ones. Oxaloacetate is formed from pyruvate and CO2. 

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