Structural Biochemistry/Membrane Proteins/Cotransporters

Cotransporters
Cotransporters are protein pumps used to export or import small molecules. It is sometimes equated with symporter, but the term "cotransporter" refers both to symporters and antiporters (though not uniporters). They utilize active transport, meaning that they require some sort of energy to carry out its process. Cotransporters are secondary active transporters, which means they use an electrochemical gradient as a means of energy. It works by binding to two molecules or ions at a time and using the gradient of one solute's concentration to force the other molecule or ion against its gradient.

Primary active transporters, on the other hand, use chemical energy like ATP. The electrochemical gradient for the cotransporter is due to the movement of Na + and H+ ions. This powers the movement of another substance that is pumped either in or out, against the concentration gradient. An example is the movement of glucose. In order for the transportation of glucose to be in the opposite direction of the concentration gradient, sodium ions are needed. Na+ ions are moved across the cell membrane by a transmembrane voltage gradient (movement from an area of positive charge to negative charge), and by a concentration gradient (from an area of high concentration to low concentration). Glucose is then coupled and moved against its concentration gradient. When the Na+ ion, an example of a cotransported ion, moves in the same direction as the coupled substance, it is call symporter; movement in the opposite direction of a substance is called antiporter.'

Transport of glucose in animal cells by symporters
A symporter is an integral membrane protein that is involved in movement of two or more different molecules or ions across a phospholipid membrane such as the plasma membrane in the same direction, and is therefore a type of cotransporter. Cotransporters are used for the transport of glucose in the cell. Glucose is needed in different organs, but there is already an abundance of glucose in these areas. This prevents glucose from entering by passive transport. Active transport is thus used, using a 2Na+/1Glucose symporter. The transporter moves one glucose in for every two sodium ion, and moves them out in the same ratio. What forces are used to drive the substances in and out of the cells? As mentioned before, the electrochemical gradient and the concentration gradient of the Na+ ion allows this process to happen.

This is done by the free energy change (ΔG) of transportation of Na+ ions. The concentration of Na+ ions is either higher or lower inside the cell than outside. The charge of the cell is also either inside negative or inside positive. The flow of Na+ ions is either to the inside or the outside, depending on which side has more Na+ ions and the charge of each side. Na+, like all particles, will go from an area of low concentration to high concentration. In addition, Na+ will move from an area of positive charge to negative charge, until there is a balance between the concentration of Na+ and a balance between the charges. This is the equilibrium potential, and it can be calculated by the Nernst equation. There is a free energy change of the cell for the transport of Na+ ions, which will affect the concentration of glucose inside and outside the membrane.

Transport of calcium ions from cardiac muscle cells by antiporter
An antiporter (also called exchanger or counter-transporter) is an integral membrane protein which is involved in secondary active transport of two or more different molecules or ions (i.e. solutes) across a phospholipid membrane such as the plasma membrane in opposite directions.

In secondary active transport, one species of solute moves along its electrochemical gradient, allowing a different species to move against its own electrochemical gradient. This movement is in contrast to primary active transport, in which all solutes are moved against their concentration gradients, fueled by ATP.

Antiporters play a key role in movement of calcium ions out of cardiac muscle cells. For every 3 Na+ ions out (or in) the membrane, 1 Ca2+ ions are pumped oppositely in (or out) the membrane. This is significant for different physiological functions, such as relaxation of cardiac muscles, maintenance of calcium ion concentration in different cellular organelles, control of neurosecretion, and other functions.