ATP role

Potassium ions are essential to both plants and animals (see Mineral nutrients). Within cells, potassium serves the critical role as counterion for various carboxylates, phosphates, and sulfates, and stabilizes macromolecular stmctures. is the principal cation mediating the osmotic balance of the extra cellular fluids, and it is accumulated in cells in concert with the expulsion of (40). If a cell dies or has its metaboHsm blocked, the concentration gradient for intracellular potassium to extracellular potassium disappears as potassium ions slowly diffuse out across the cell membrane. This implies that metaboHc energy is expended in maintaining the gradient. The term sodium pump is used for the mechanism of active transport in rejection of sodium and accumulation of potassium by a cell. Energy for the sodium pump is provided by adenosine triphosphate (ATP) hydrolysis. This /Na separation has allowed the evolution of the reversible transmembrane electrical potentials essential for nerve and muscle action in animals. The /Na ratio for excitable membranes requires precise homeostatic control over internal electrolyte concentrations. Alkali metal ion levels in the circulating fluids are adjusted by the effect of thirst and salt-craving on intake and the influence of antidiuretic hormone and aldosterone on reabsorption in the kidney tubules.  [c.536]

Mobility is affected by the dielectric constant and viscosity of the suspending fluid, as indicated in Eq. (22-28). The ionic strength of the fluid has a strong effect on the thickness of the double layer and hence on As a rule, mobility varies inversely as the square root of ionic strength [Overbeek, Adv. Colloid Sci., 3, 97 (1950)b  [c.2007]

The role of ATP in muscular contraction has parallels to the role of GTP in G-protein activation  [c.296]

In the absence of nucleotides, the myosin nucleotide-binding cleft is open, the lever arm is "down," the actin-binding site is intact and this form binds strongly to actin (Figure 14.17a). This is the rigor state into which in the absence of nucleotides the muscle is locked as in rigor mortis. If ATP is added, the myosin head, bound to actin, will bind ATP, and then dissociate from the actin (Figure 14.17b). Binding of ATP to the nucleotide-binding domain in the cleft causes the P loop, which corresponds to the switch II region in G-proteins, described in Chapter 13, to change its conformation. The y-phosphate of ATP plays the same role in this conformational change as the y-phosphate of GTP in G-proteins. This change in the loop conformation is coupled to a major conformational change of parts of the head protein as a result the cleft closes and the region that binds actin releases the actin filament. The bound ATP is then hydrolyzed to ADP and phosphate (Figure 14.17c).  [c.296]

The role of ATP in muscular contraction has parallels to the role of GTP in G-protein activation 296  [c.417]

So far, as in Equation (3.33), the hydrolyses of ATP and other high-energy phosphates have been portrayed as simple processes. The situation in a real biological system is far more complex, owing to the operation of several ionic equilibria. First, ATP, ADP, and the other species in Table 3.3 can exist in several different ionization states that must be accounted for in any quantitative analysis. Second, phosphate compounds bind a variety of divalent and monovalent cations with substantial affinity, and the various metal complexes must also be considered in such analyses. Consideration of these special cases makes the quantitative analysis far more realistic. The importance of these multiple equilibria in group transfer reactions is illustrated for the hydrolysis of ATP, but the principles and methods presented are general and can be applied to any similar hydrolysis reaction.  [c.77]

Calcium, an ion acting as a cellular signal in virtually all cells (see Chapter 34), plays a special role in muscles. It is the signal that stimulates muscles to contract (Chapter 17). In the resting state, the levels of Ca near the muscle fibers are very low (approximately O.I ijlM), and nearly all of the calcium ion in muscles is sequestered inside a complex network of vesicles called the sarcoplasmic reticulum, or SR (see Figure 17.2). Nerve impulses induce the sarcoplasmic reticulum membrane to quickly release large amounts of Ca, with cytosolic levels rising to approximately 10 /jlM. At these levels, Ca stimulates contraction. Relaxation of the muscle requires that cytosolic Ca levels be reduced to their resting levels. This is accomplished by an ATP-driven Ca transport protein known as the Ca -ATPase. This enzyme is the most abundant protein in the SR membrane, accounting for 70 to 80% of the SR protein. Ca -ATPase bears many similarities to the Na, K -ATPase. It has an a-subunit of the same approximate size, it forms a covalent E-P intermediate during ATP hydrolysis, and its mechanism of ATP hydrolysis and ion transport is similar in many ways to that of the sodium pump.  [c.304]

Recall that, prior to synthesizing ATP in the phosphoglycerate kinase reaction, it was necessary to first make a substrate having a high-energy phosphate. Reaction 9 of glycolysis similarly makes a high-energy phosphate in preparation for ATP synthesis. Enolase catalyzes the formation of phosphoenolpyruvate from 2-phosphoglycerate (Figure 19.26). The reaction in essence involves a dehydration—the removal of a water molecule—to form the enol structure of PEP. The AG° for this reaction is relatively small at 1.8 kj/mol (Al<.q = 0.5) and, under cellular conditions, AG is very close to zero. In light of this condition, it may be difficult at first to understand how the enolase reaction transforms a substrate with a relatively low free energy of hydrolysis into a product (PEP) with a very high free energy of hydrolysis. This puzzle is clarified by real-  [c.628]

The role of non-equilibrium thennodynamics in molecular bioenergetics has experienced an experimental revolution during the last 35 years. Membrane energetics is now understood in tenns of chemiosmosis [40]. In chemiosmosis, a trans-membrane electrochemical potential energetically couples the oxidation-reduction energy generated during catabolism to the adenosine triphosphate (ATP) energy needed for chemosynthesis during anabolism. Numerous advances in experimental technology have opened up whole new areas of exploration [4T]. Quantitative analysis using non-equilibrium thennodynamics to account for the free energy and entropy changes works accurately in a variety of settings. There is a rich diversity of problems to be worked on m this area. Another biological application brings the subject back to its foundations. Rectified Brownian movement (involving a Brownian ratchet) is being invoked as the mechanism behind many macromolecular processes [42]. It may even explain the dynamics of actin and myosin interactions in muscle fibres [43]. In rectified Brownian movement, metabolic free energy generated during catabolism is used to bias boundary conditions for ordinary difhision, thereby producing a non-zero flux. In tliis way, themial fluctuations give the molecular mechanisms of cellular processes their vitality [44].  [c.713]

The primary and likely sole pathway of starch biosynthesis is the adenosine diphosphate (ADP) glucose pathway (57). In this pathway the first enzyme, ADPglucose pyrophosphorylase (ADPGPP), catalyzes the conversion of glucose-l-phosphate to ADPglucose. In plants, it has been proposed that sucrose synthase is involved in the production of the ADPglucose used in starch biosynthesis (58). This model is not considered to be accurate given a number of mutants characterized affecting both starch and sucrose biosynthesis, and this topic has recendy been reviewed (59). Another route for starch biosynthesis is through the action of starch phosphorylase. This enzyme is involved in the degradation of starch, forming glucose-l-phosphate from successive removal of glucose units from the polymer. The reaction is reversible in vitm, thus this enzyme potentially plays a role in the formation of starch. Through expression of antisense RNA, this enzyme has been eliminated in the amyloplast of potato tubers with no effect on starch content thus any role in biosynthesis is proposed to be very minor (60).  [c.254]

ADP Glucose Pyrophosphorylase. The rate-limiting reaction in both bacterial glycogen and plant starch biosynthesis is the first step, catalyzed by the enzyme ADPGPP. In bacteria the enzyme functions as a homotetramer subject to tight allosteric regulation by effector molecules that redect the energy state of the cell, and is the only enzyme in the pathway of glycogen biosynthesis subject to such regulation. The enzyme is activated by glycolytic intermediates and inhibited by adenosine monophosphate (AMP), ADP, and/or inorganic phosphate (Pi). Fmctose 1,6-bisphosphate is typically the primary activator and AMP the primary inhibitor (57,62,63). The role of the activator is to increase the affinity of the enzyme for its substrates, adenosine triphosphate (ATP) and glucose-l-phosphate, and increasing amounts of the activator reheves inhibition caused by AMP, ADP, or Pi The allosteric regulation of this enzyme has been shown to regulate the dux of carbon through this pathway and control the level of glycogen that is produced. Much of this work has been performed with mutants of E. coli and S. typhimurium affected in thek abiUty to accumulate glycogen.  [c.254]

T. S. Chow,/ Mater. Sci. 25, 957 (1990) Potymer, 32, 29 (1991) / Rheol 36, 1707 (1992y,Adv. Polym. Sci. 103, 149 (1992).  [c.207]

L ger, L., Raphael, E. and Hervet, H., Surface-anchored polymer chains their role in adhesion and friction. Adv. Polym. Sci., 138, 185-225 (1999).  [c.242]

It is clear that changes in the concentrations of these species can have large effects on AG. The concentrations of ATP, ADP, and Pj may, of course, vary rather independently in real biological environments, but if, for the sake of some model calculations, we assume that all three concentrations are equal, then the effect of concentration on AG is as shown in Figure 3.18. The free energy of hydrolysis of ATP, which is —35.7 kJ/mol at 1 M, becomes —49.4 kJ/mol at 5 mM (that is, the concentration for which pC = —2.3 in Figure 3.18). At 1 mMATP, ADP, and Pj, the free energy change becomes even more negative at —53.6 kj/mol. Clearly, the effects of concentration are much greater than the effects of protons or metal ions under physiological conditions.  [c.78]

Production of protons is a fundamental activity of cellular metabolism, and proton production plays a special role in the stomach. The highly acidic environment of the stomach is essential for the digestion of food in all animals. The pH of the stomach fluid is normally 0.8 to 1. The pH of the parietal cells of the gastric mucosa in mammals is approximately 7.4. This represents a pH gradient across the mucosal cell membrane of 6.6, the largest known transmembrane gradient in eukaryotic cells. This enormous gradient must be maintained constantly so that food can be digested in the stomach without damage to the cells and organs adjacent to the stomach. The gradient of H is maintained by an H", K -ATPase, which uses the energy of hydrolysis of ATP to pump H out of the mucosal cells and into the stomach interior in exchange for ions. This transport is electrically neutral, and the that is transported into the mucosal cell is subsequently pumped back out of the cell together with Cl in a second electroneutral process (Figure 10.16). Thus, the net transport effected by these two systems is the movement of HCl into the interior of the stomach. (Only a small amount of is needed because it is recycled.) The H, K -ATPase bears many similarities to the plasma membrane Na, K -ATPase and the SR Ca -ATPase described above. It has a similar molecular weight, forms an E-P intermediate, and many parts of its peptide sequence are homologous with the Na, K -ATPase and Ca -ATPase (Figure 10.13).  [c.307]

FIGURE 18.14 With NMR spectroscopy one can observe the metabolism of a living subject in real time. These NMR spectra show the changes in ATP, creadne-P (phosphocre-adne), and P levels in the forearm muscle of a human subjected to 19 minutes of exercise. Note that the three P atoms of ATP a, /3, and y) have different chemical shifts, reflecting their different chemical environments.  [c.582]

Aspects of the process still requiring clarification include details of the electron flow between redox centres the pathways for entry and exit of N2, NH3 and H2 (presumably structural rearrangements are needed) the role of Mg-ATP and the nature of the interaction between N2 and the FeMo cofactor which is central to the whole process. Persuasive arguments had been advanced for an intermediate involving 2 Mo atoms bridged by N2 yet in the determined structure the Mo atoms are too far apart to form a binuclear intermediate of this kind. On the other hand it has been plausibly suggested that a reduced form of the FeMo cofactor might be sufficiently open at its centre to allow the insertion of N2 so forming a bridged intermediate in which Fe-N interactions replace weak Fe-Fe bonds (Fig. 23.13c). The concomitant weakening of the N=N bond would facilitate subsequent reduction of the N2 bridge.  [c.1037]

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Introduction to protein structure (1999) -- [ c.296 ]