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Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002.

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Biochemistry. 5th edition.

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Section 13.4Secondary Transporters Use One Concentration Gradient to Power the Formation of Another

Many active-transport processes are not directly driven by the hydrolysis of ATP. Instead, the thermodynamically uphill flow of one species of ion or molecule is coupled to the downhill flow of a different species. Membrane proteins that pump ions or molecules uphill by this means are termed secondary transporters or cotransporters. These proteins can be classified as either antiporters or symporters. Antiporters couple the downhill flow of one species to the uphill flow of another in the opposite direction across the membrane; symporters use the flow of one species to drive the flow of a different species in the same direction across the membrane (Figure 13.10).

Figure 13.10. Secondary Transporters.

Figure 13.10

Secondary Transporters. These transporters employ the downhill flow of one gradient to power the formation of another gradient. In antiporters, the chemical species move in opposite directions. In symporters, the two species move in the same direction. (more...)

The sodium—calcium exchanger in the plasma membrane of an animal cell is an antiporter that uses the electrochemical gradient of Na+ to pump Ca2+ out of the cell. Three Na+ ions enter the cell for each Ca2+ ion that is extruded. The cost of transport by this exchanger is paid by the Na+-K+- ATPase pump, which generates the requisite sodium gradient. Because Ca2+ is a vital messenger inside the cell, its concentration must be tightly controlled. The exchanger has lower affinity for Ca2+ than does the Ca2+ATPase (Section 13.2.1), but its capacity to extrude Ca2+ is greater. The exchanger can lower the cytosolic Ca2+ level to several micromolar; submicromolar Ca2+ levels are attained by the subsequent action of the Ca2+ ATPase. The exchanger can extrude about 2000 Ca2+ ions per second, compared with only 30 ions per second for the Ca2+-ATPase pump.

Glucose is pumped into some animal cells by a symporter powered by the simultaneous entry of Na+. The entry of Na+ provides a free-energy input of 2.2 kcal mol-1 (9.2 kJ mol-1) under typical cellular conditions (external [Na+] = 143 mM, internal [Na+] = 14 mM, and membrane potential = -50 mV). This free-energy input is sufficient to generate a 66-fold concentration gradient of an uncharged molecule such as glucose.

Image tree.jpg Secondary transporters are ancient molecular machines, common today in bacteria and archaea as well as in eukaryotes. For example, approximately 160 (of approximately 4000) proteins encoded by the E. coli genome appear to be secondary transporters. Sequence comparison and hydropathy analysis suggest that members of the largest family have 12 transmembrane helices that appear to have arisen by duplication and fusion of a membrane protein with 6 transmembrane helices. Included in this family is the lactose permease of E. coli. This symporter uses the H+ gradient across the E. coli membrane generated by the oxidation of fuel molecules to drive the uptake of lactose and other sugars against a concentration gradient. The permease has a proton-binding site and a lactose-binding site (Figure 13.11). A proton and a lactose molecule bind to sites facing the outside of the cell. The permease, with both binding sites full, everts, releasing first the proton and then the lactose inside the bacterium. Another eversion places the empty sites on the outside. Thus, the energetically uphill transfer of one lactose molecule is coupled to the downhill transport of one proton. Further analysis of the three-dimensional structures is underway and should provide more information about their mechanisms of action as well as the evolutionary relationships within this large group of ancient proteins.

Figure 13.11. Action of Lactose Permease.

Figure 13.11

Action of Lactose Permease. Lactose permease pumps lactose into bacterial cells by drawing on the proton-motive force. The binding sites evert when a lactose molecule (L) and a proton (H+) are bound to external sites. After these species are released (more...)

These observations reveal how different energy currencies are interconverted. A single energy currency, ATP, is used by P-type ATPases to generate gradients of a small number of types of ions, particularly Na+ and H+, across membranes. These gradients then serve as an energy source for the large number of secondary transporters, which allow many different molecules to be taken up or transported out of cells (Figure 13.12).

Figure 13.12. Energy Transduction by Membrane Proteins.

Figure 13.12

Energy Transduction by Membrane Proteins. The Na+-K+ ATPase converts the free energy of phosphoryl transfer into the free energy of a Na+ ion gradient. The ion gradient can then be used to pump materials into the cell, through the action of a secondary (more...)

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2002, W. H. Freeman and Company.
Bookshelf ID: NBK22533


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