Sodium pump mechanism

Carbohydrates, or starches, which are complex polysaccharides, are hydrolyzed to oligosaccharides and disaccharides by the action of pancreatic amylase. Disaccharides, including lactase, sucrase, and maltase, are enzymatically split by enzymes contained in the microvilli of enterocytes. Glucose and other monosaccharides are absorbed by an active transport mechanism and this action is coupled to energy derived from a sodium pump mechanism. 

Scheiner-Bobis G (2002) The sodium pump. Its molecular properties and mechanics of ion transport. Eur J Biochem. 269 2424—2433... 

Sodium hexakis(formato)molybdate, 3, 1235 Sodium hypochlorite alkene epoxidation manganese catalysts, 6,378 Sodium ions biology, 6, 559 selective binding biology, 6, 551 Sodium molybdate, 3, 1230 Sodium peroxoborate, 3,101 Sodium/potassium ATPase, 6, 555 vanadate inhibition, 3, 567 Sodium pump, 6, 555 mechanism, 6, 556 Sodium pyroantimonate, 3, 265 Sodium salts... 

Kaplan, J.H. and DeWeer, P. (Eds.) (1991) The Sodium Pump Structure, Mechanism, and Regulation, Rockefeller University Press, New York. 

Therien, A. G. and Blostein, R. (2000). Mechanisms of sodium pump regulation, Am. J. Physiol., 279, C541-C566. 

ATP is used not only to power muscle contraction, but also to re-establish the resting state of the cell. At the end of the contraction cycle, calcium must be transported back into the sarcoplasmic reticulum, a process which is ATP driven by an active pump mechanism. Additionally, an active sodium-potassium ATPase pump is required to reset the membrane potential by extruding sodium from the sarcoplasm after each wave of depolarization. When cytoplasmic Ca2- falls, tropomyosin takes up its original position on the actin and prevents myosin binding and the muscle relaxes. Once back in the sarcoplasmic reticulum, calcium binds with a protein called calsequestrin, where it remains until the muscle is again stimulated by a neural impulse leading to calcium release into the cytosol and the cycle repeats. 

Amino add reabsorption in the renal tubules Amino acids are small, easily filtered molecules. Efficient reabsorption mechanisms are vital to conserve amino acids which are metabolically valuable resources. Transport of individual amino acids and small peptides is symport carrier mediated mechanisms in which sodium is co-transported. The process is indirectly ATP dependent because Na is returned to the lumen of the nephron by the sodium pump , Na+/K+ dependent ATPase. 

Figure 12.2 Mechanism of action of carbonic anhydrase inhibitors on the proximai convoiuted tubuie. Carbonic anhydrase is an enzyme that cataiyses the interconversion of C02and H20 to H2C03and is found in the iuminai epitheiium of the proximai, and to a iesser extent, the distai convoiuted tubuie. It is essentiai for the conservation of body base in the form of HCO-3. An antiporter (1) mechanism (the movement of substances across a barrier in opposite directions) exchanges fiitrate Na+for ceiiuiar H+. The H+combines with fiitrate HCO-3to form carbonic acid which is converted to C02and H20 in the presence of carbonic anhydrase (CA). The C02is reabsorbed by the ceii thereby conserving HCO-3. Acetazoiamide inhibits the activity of carbonic anhydrase and limits the conversion of HCO-3to absorbable C02. The concentration of HCO-3in the filtrate increases as does the urinary loss. P, the sodium pump ECF, extracellular fluid.

Figure 12.4 Mechanism of action of Na+/K+symport inhibitors (thiazides) on the distal convoluted tubule. As in the other parts of the nephron, Na+movement is powered by the energy-requiring sodium pump (P) in the basolateral membrane which exchanges intracellular Na+for K-i-in the extracellular fluid (ECF). The transport of Na-rand Cl- into the cell from the filtrate against the prevailing electrochemical gradient is facilitated by the symporter (S). The Na-Hons are then transported by the pump mechanism described above and the Cl- ions diffuse passively Into the ECF through ion channels in the basolateral membrane. Thiazide diuretics inhibit the symporter by disabling the Cl- binding site with the loss of Na-rand Cl- in the urine.

Biotin has been shown to be an essential component of a bacterial oxaloacetate decarboxylase that pumps two sodium ions out of a cell for each oxaloacetate molecule decarboxylated. Propose a chemical mechanism for the functioning of biotin and also any ideas that you may have for the operation of the sodium pump. 

A full consideration of the mechanism of the sodium pump requires an account of the role of the lipid, the binding sites for Na+, K+, Mg2+ and ATP, the mechanism of hydrolysis of ATP and the way in which this is coupled to the transport of the cation. In addition it should be noted that the enzyme also functions as a K+-dependent phosphatase, a reaction usually studied with p-nitrophenyl phosphate as substrate. Studies with inhibitors have been informative, notably with ouabain and with vanadate. Ouabain binds at one site per pump and so has been of value in quantitatively defining the enzyme in various preparations. 

Later studies from the same group established, using microinjection, that ATP within the cell was required for this process, and also showed—with axons from which the cytoplasm had been extruded mechanically and replaced by an artificial intracellular ionic medium—that the sodium pump was a property of the axon membrane since such cells were still able to extrude this cation when supplied with an appropriate source of ATP. In their original paper, Hodgkin and Keynes (1955b) found that in addition to needing metabolism, sodium pumping was dependent on... 

Alan L. Hodgkin (1914—1998) and Andrew F. Huxley (1917- ), both English physiologists, first work out the mechanism of nerve-impulse transmission, showing that a sodium pump system works to carry impulses. 

Maunsbach, A.B., Skriver, E., Hebert, H. (1991). Two-dimensional crystals and three-dimensional structure of Na,K-ATPase analyzed by electron microscopy. In The Sodium Pump Structure, Mechanism, and Regulation (Kaplan, J.H. De Weer, P., eds.), pp. 159-172, The Rockefeller University Press, New York. 

Bamberg, E. Schoner, W. (eds.) (1994). The Sodium Pump. Structure, Mechanism, Hormonal Control and its Role in Disease. Steinkopff, Darmstadt. 

The relationship of intracellular sodium to intracellular calcium is such that a very small increase in sodium in terms of percentage increase leads to a disproportionately large increase in calcium. Therefore, a direct effect on the sodium/potassium-ATPase to inhibit sodium pump activity is the primary mechanism of the positive inotropic effect of the cardiac glycosides, while secondary elevation of intracellular calcium provides the ionic punch to increase contractility. A diagram of the relationship between sodium/potassium-ATPase and calcium is shown in Figure 12.6. 

At this point, however, we cannot ignore the fact that the evolution of protein synthesis started before the origin of the first cells, in systems which could not have cell walls, cytoskeleton filaments or sodium pumps, for the very good reason that all these structures require well-developed proteins. How could precellular systems have high potassium concentrations, and low sodium levels, without any of the molecular mechanisms that cells employ to this end The most plausible answer is that those concentrations did not have to be produced in prebiotic systems because they already existed in the environment of the primitive seas. The ribotype world, in short, was also a potassium world. 

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