Pump transports coupling ratio

The sarcoplasmic transport system can be fueled in addition to ATP by a great number of phosphate compounds which differ considerably in their chemical nature. Not only the natural nucleoside triphosphates126 but also para-nitrophenylphosphate,27 acetyl phosphate128 or carbarmyl phosphate129 can drive calcium transport. While there are considerable differences between the rates with which calcium transport proceeds with the different substrates, they are all used with the same coupling ratio of two and the pump can establish similar maximal concentration ratios (Fig. 7)... 

This simple experiment was important in that it clearly established the key notion that cellular extrusion of sodium ions by the sodium pump was coupled to metabolism. Because in this and subsequent experiments of the same sort the electrochemical gradient for sodium was known precisely, and since the fluxes of sodium (and later potassium) both into and out of the cell could be measured independently, this study also laid the groundwork for a theoretical definition of active transport, a theory worked out independently by Ussing in the flux ratio equation for transepithelial active transport of ions (see below). 

Pumps are proteins that can transport ions against electrochemical potential gradients using adenosine-5-triphosphate (ATP) as an energy source. Sodium-potassium pumps maintain intracellular sodium and potassium concentrations in animal cells and also control salt and water absorption by the epithelial cells in the intestine and kidney. The sodium-potassium pump transports three sodium ions out of the cell and two potassium ions into the cell at the cost of one molecule of ATP. The 3 2 coupling ratio results in net loss of sodium ions into the cell down an electrochemical gradient and maintains cell volume. Currently, considerable research is attempting to elucidate the structures of the various isoforms and subunits of sodium potassium pumps. 

In real cells, multiple transmembrane pumps and channels maintain and regulate the transmembrane potential. Furthermore, those processes are at best only in a quasi-steady state, not truly at equilibrium. Thus, electrophoresis of an ionic solute across a membrane may be a passive equilibrative diffusion process in itself, but is effectively an active and concentra-tive process when the cell is considered as a whole. Other factors that influence transport across membranes include pH gradients, differences in binding, and coupled reactions that convert the transported substrate into another chemical form. In each case, transport is governed by the concentration of free and permeable substrate available in each compartment. The effect of pH on transport will depend on whether the permeant species is the protonated form (e.g., acids) or the unprotonated form (e.g., bases), on the pfQ of the compound, and on the pH in each compartment. The effects can be predicted with reference to the Henderson-Hasselbach equation (Equation 14.2), which states that the ratio of acid and base forms changes by a factor of 10 for each unit change in either pH or pfCt ... 

I-III as pictured in Fig. 18-5. One site of pumping is known to be in the cytochrome c oxidase complex. When reconstituted into phospholipid, the purified complex does pump protons in response to electron transport, H+/e ratios of 1 being observed. 437,i47,i9i As mentioned in Section B,3 a large amoimt of experimental effort has been devoted to identifying proton transport pathways in cytochrome c oxidase and also in the cytochrome bcj (complex II). Proton pumping appears to be coupled to chemical changes occurring between intermediates P and F of Fig. 18-11, between F and and possibly between O and r3 7138... 

Cantley al. (50) found that vanadate was transported to the red blood cell where it inhibited the sodium pump by binding to (Na, K)-ATPase from the cytoplasmic side (the site of ATP hydrolysis). They suggested that the vanadium in mammalian tissue acts as a regulatory mechanism for the sodium pump that maintains a high intracellular id" to Na" ratio by coupling with ATP hydrolysis. 

Membrane ion pumps consist of assemblies of large macromolecules that span the thickness of the phospholipid cell membrane, as illustrated in Fig. 17.2. The pumps for sodium and potassium ion are coupled and appear to be a single structure that acts somewhat like a turnstile. Their physical construction transports sodium and potassium in opposite directions in a 3 2 atomic ratio, respectively. 

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