ATP hydrolysis to ADP

In the cell, the AG of ATP hydrolysis is substantially larger, because the concentrations of ATP, ADP and Pj are much lower than in standard conditions and there is an excess of ATP over ADP (see p. 18). The pH value and Mg concentration also affect the value of AG. The physiological energy yield of ATP hydrolysis to ADP and anorganic phosphate (Pj) is probably around -50 kJ mol . 

Next, in steps 7 and 8, N-l of the purine ring is contributed by aspartate. Aspartate forms an amide with the 4-carboxyl group, and the succinocarboxamide so formed is then cleaved with release of fumarate. Energy for carboxamide formation is provided by ATP hydrolysis to ADP and phosphate. These reactions resemble the conversion of cit-rulline to arginine in the urea cycle (chapter 22) and the conversion of IMP to AMP (see fig. 23.11). 

The spheres removed from SMPs do not support ATP synthesis but do hydrolyze ATP to ADP and phosphate. Thus, ATP synthesis is carried out by Fq/Fi—ATPase (ATP synthase). The subscript o in Fq indicates that it contains the site at which a potent antibiotic inhibitor, oligomycin, binds and inhibits oxidative phosphorylation. Oligomycin does not bind Fi-ATPase and does not inhibit ATP hydrolysis to ADP and phosphate. 

Fructose is present outside a cell at 1 /iM concentration. An active transport system in the plasma membrane transports fructose into this cell, using the free energy of ATP hydrolysis to drive fructose uptake. Assume that one fructose is transported per ATP hydrolyzed, that ATP is hydrolyzed on the intracellular surface of the membrane, and that the concentrations of ATP, ADP, and Pi are 3 mM, 1 mM, and 0.5 mM, respectively. T = 298 K. What is the highest intracellular concentration of fructose that this transport system can generate Hint Kefer to Chapter 3 to recall the effects of concentration on free energy of ATP hydrolysis.)... 

However, release of ADP and P from myosin is much slower. Actin activates myosin ATPase activity by stimulating the release of P and then ADP. Product release is followed by the binding of a new ATP to the actomyosin complex, which causes actomyosin to dissociate into free actin and myosin. The cycle of ATP hydrolysis then repeats, as shown in Figure 17.23a. The crucial point of this model is that ATP hydrolysis and the association and dissociation of actin and myosin are coupled. It is this coupling that enables ATP hydrolysis to power muscle contraction. 

The charging of the tRNA molecule with the aminoacyl moiety requires the hydrolysis of an ATP to an AMP, equivalent to the hydrolysis of two ATPs to two ADPs and phosphates. The entry of the aminoacyl-tRNA into the A site results in the hydrolysis of one GTP to GDP. Translocation of the newly formed pep-tidyl-tRNA in the A site into the P site by EF2 similarly results in hydrolysis of GTP to GDP and phosphate. Thus, the energy requirements for the formation of one peptide bond include the equivalent of the hydrolysis of two ATP molecules to ADP and of two GTP molecules to GDP, or the hydrolysis of four high-energy phosphate bonds. A eukaryotic ribosome can incorporate as many as six amino acids per second prokaryotic ribosomes incorporate as many as 18 per second. Thus, the process of peptide synthesis occurs with great speed and accuracy until a termination codon is reached. 

Some processes do involve direct hydrolysis of ATP (or GTP), however. For example, noncovalent binding of ATP (or of GTP), followed by its hydrolysis to ADP (or GDP) and Pi can provide the energy to cycle some proteins between two conformations, producing me-... 

Fast transport in cells requires vehicles (the organelles or vesicles), motors and fuel. The motors can be divided into two families, the cytoplasmic dyneins and the kinesins (Fig. 5-35). Each uses ATP as a fuel, the energy being provided by its hydrolysis to ADP and P . The motors can sense the polarity of the microtubules dyneins drive toward the minus end, while most kinesins drive toward the plus end. Movement is generated by cyclic hydrolysis of ATP, conformational changes, and the reversible attachment to microtubules. 

This sequence has been termed the central dogma of bioenergetics by analogy to the universal sequence of biological information transfer, DNA RNA proteins. Hydrolysis of ATP back to ADP and Pi is the common energy source for virtually all energy-requiring processes in the cell, such as mechanical work, active transport, and biosynthesis of macromolecules. 

Many reactions in a cell would not take place spontaneously if left alone. These reactions can use the spontaneity of ATP hydrolysis to take place by coupling with the ATP —> ADP reaction. ATP hydrolysis thus allows these other nonspontaneous reactions to take place. 

It can easily be overlooked just how important ATP s3mthesis is, for it is the hydrolysis of ATP back to ADP that generates the energy we need to live. For example, an adult male in a sedentary occupation bums about 3000 kcal of energy per day. This requires the daily hydrolysis of about 200 kg of ATP, which presents a problem as the body only has, on average, 50 g of the stuff So ATP has to be hydrolysed down to ADP and resynthesised back to ATP many thousands of times in the course of the day. Cytochrome c is just one of the links in the chain of electron carrier proteins that ultimately produce ATP. 

In all eukaryotic cells, the eflEcient use of carbohydrate, fat or protein as an energy source depends on the complete oxidation of constituent carbon atoms to carbon dioxide. The energy available from these oxidations is conserved through the coupled synthesis of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and phosphate (Pi) and used to drive the kinetic, biosynthetic and homeostatic processes of the cell through numerous concerted reactions involving the hydrolysis of ATP back to ADP. 

During ATP hydrolysis, rat-liver mitoehondria transloeate nearly 2 protons outwards per ATP hydrolysed to ADP-t-Pj (ref. 208). The ATP-driven proton translocation is sensitive to oligomycin. These and other observations indicate that the mitochondrial ATPase eouples the hydrolysis of ATP to the translocation of 2 protons across the membrane in which it resides (see ref. 43). 

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