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Unformatted text preview: MINIREVIEW The sodium pump Its molecular properties and mechanics of ion transport Georgios Scheiner-Bobis From the Institut fu r Biochemie und Endokrinologie, Fachbereich Veterina rmedizin, Justus-Liebig-Universita t Giessen, Germany The sodium pump (Na + /K +-ATPase; sodium- and potas- sium-activated adenosine 5 -triphosphatase; EC 126.96.36.199) has been under investigation for more than four decades. During this time, the knowledge about the structure and properties of the enzyme has increased to such an extent that specializedgroupshaveformedwithinthisfieldthatfocuson specific aspects of the active ion transport catalyzed by this enzyme. Taking this into account, this review, while some- what speculative, is an attempt to summarize the informa- tion regarding the enzymology of the sodium pump with the hope of providing tointerested readers from outside the field a concentrated overview and to readers from related fields a guide in their search for gathering specific information concerning the structure, function, and enzymology of this enzyme. Keywords : ATPase; P-type; ouabain; palytoxin; ion transport. T H E S O D I U M P U M P : A B R I E F R E T R O S P E C T I V E Today there is a vast amount of information concerning ion transport through biological membranes and primary structures, crystals, mutants, and chimeras of ion trans- porters. It is difficult to imagine that the impressive progress achieved thus far was originally generated by a few researchers who had the ability to observe simple phenom- ena connected with ion distribution, to question their origin, and to assemble experimental evidence in ways that did not allow any other conclusion but that there must a mechanism that enables ions to be actively transported against their electrochemical gradients. This mechanism, termed a sodium pump by Dean in 1941, originates from the observation that sodium ions within muscle fibers can exchange with radioactive sodium added to their environ- ment. Nevertheless, although a large amount of data and interpretation of it followed Deans proposal, it was not until 1954 that Gardos discovered that ion pumping in red blood cell ghosts was supported by ATP, which in turn became hydrolyzed. (Due to space limitations, some of the early, seminal work is not included in the reference list; instead, an up-to-date selection of papers from a variety of groups from which both the current progress in the field can be assessed and in which earlier, landmark discoveries are fully referenced is provided.) These observations, together with the finding that 18 sodium ions were transported for each molecule of oxygen consumed (4.5 Na + per electron or, in other words, 3 Na + per ATP) and the fact that ouabain had already been shown to inhibit sodium uxes on frog skin, contributed to the overall acceptance of Skous conclusion from 1957, which identified in crab nerve membrane preparations the sodium pump as an ATPase that was activated by Na...
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- Spring '08