Active transport occurs when molecules or ions are moved against their concentration gradients across cell membranes (Figure 14.1). Energy is required to move these solutes into and out of cells. The energy used is derived from adenosine triphosphate or ATP. The membrane proteins that bind to and transport the molecules or ions against their gradients possess the enzymatic activity to hydrolyze or break down ATP to harness its energy. Known as ATPases, these transporters are ATP-powered pumps that directly hydrolyze ATP to ADP and inorganic phosphate (Pi) in primary active transport.
FIGURE 14.1.Active transport moves substrates against their concentration gradients.
A consequence of primary active transport is the establishment of ion gradients. Certain ions such as sodium are pumped out of cells while others, such as potassium, are pumped into cells by primary active transport. Sodium is therefore found at much higher concentration outside of cells than inside of cells and potassium is at higher concentration inside cells than outside as a consequence of primary active transport. The tendency of these ions will be to move down their concentration gradients.Such strong concentration gradients, like those for sodium, can be used to power the transport of other solutes against their own concentration gradient. These other molecules can make use of the concentration gradient and travel along with it, even against the direction of their own concentration gradient. Secondary active transport is the process by which ion gradients generated by ATP-powered pumps are used to power transport of other molecules and ions against their own concentration gradients.
PRIMARY ACTIVE TRANSPORT
Four classes of transport proteins function as ATP-powered pumps to transport ions and molecules against their concentration gradients (Figure 14.2). All have ATP-binding sites on the cytosolic side of the membrane. ATP hydrolysis is coupled to the transport of the substrates of the ATP-powered pump. ATP is only broken down to ADP + Pi when ions or molecules are transported. The classes differ in the types of ions/molecules they transport and in the mechanisms they use to catalyze ATP-powered transport.
FIGURE 14.2.Four classes of primary active transporters.
The P class of pumps is named for the phosphorylation of one of the subunits of the transport protein that occurs during the transport process. The transported substrates are moved through the phosphorylated subunit of the transport protein. A member of this class is the sodium-potassium ATPase that is present on the plasma membranes of all animal cells (Figure 14.3). It functions to maintain extracellular sodium concentrations and intracellular potassium concentrations. Three sodium ions are pumped out of the cell and two potassium ions are pumped in for each ATP hydrolyzed by the sodium-potassium ATPase.
FIGURE 14.3.The sodium-potassium ATPase pump.
Other members of this class are calcium ATPases that pump calcium out of the cytosol into either the extracellular environment or the intracellular storage locations that are also members of the P class of primary active transporters. Even very small increases in the concentration of free calcium ions within the cytosol can cause cellular responses to occur. Maintaining the concentration of free calcium ions in the cytosol at a low level is an important function of these calcium ATPases.
Inhibition of the sodium-potassium ATPase to decrease heart rate
Cardiac glycosides are inhibitors of the sodium-potassium ATPase and include ouabain and digoxin. These agents prevent cells from maintaining their normal sodium-potassium balance. When heart cells (myocytes) are exposed to a cardiac glycoside, there is an increased intracellular concentration of sodium since sodium is not pumped out by the inhibited sodium-potassium ATPase. The sodium ion gradient is much less than normal. Thus, transport via a sodium-calcium exchanger is altered since it depends on the sodium gradient in order to transport calcium out of cells.The intracellular concentration of calcium then rises since less calcium is transported out of the cell. As a result, there is an increased cardiac action potential (the electrical signal generated by nerves that results from changes in the permeability of nerve cells to certain ions), an increase in the force of contraction, and a decrease in heart rate. Drugs such as digoxin are now most commonly used to treat atrial fibrillation, an abnormal heart rhythm involving the upper chambers (atria) of the heart.
F-class and V-class pumps
Both F-class and V-class pumps transport protons (H+). V-class pumps maintain the low pH of lysosomes by pumping protons into the lysosomes, against their electrochemical gradient, in an ATP-dependent process. Bacterial F-class pumps transport protons. In mitochondria, the F-class pump known as ATP synthase works in reverse. There, movement of electrons between protein complexes allows protons to be pumped from the mitochondrial matrix to the intermembrane space, creating an electrical gradient across the inner mitochondrial membrane (with more positive charges on the outside of the membrane) and also a pH gradient (the outside of the membrane is at a lower pH than the inside). The proton gradient is used by ATP synthase to drive synthesis of ATP from ADP + Pi by allowing for passive flux of protons across the membrane back into the matrix along their concentration gradient (see also LIR Biochemistry, pp. 77–80).
The fourth class of primary active transport proteins is known as the ABC superfamily. This name is derived from ATP-binding cassettes characteristic of these proteins. All ABC proteins have two cytosolic ATP-binding domains and two transmembrane domains that form the passageway for the transported molecules (Figure 14.4). The ABC cassettes bind and hydrolyze ATP, causing conformational changes in the membrane-spanning domains, inducing the translocation of substrate from one side of the membrane to the other. Forty-eight human ABC transporters have been characterized.All are involved in transport of ions, drugs, or xenobiotic compounds (natural substances that are foreign to the human body). CFTR, the cystic fibrosis transmembrane regulator, is a chloride ion channel defective in cystic fibrosis which is a unique ABC transporter since it functions as a channel. CFTR uses ATP to regulate the flux of chloride ions. The molecular basis for CFTR’s activity as an ATPase continues to be explored (see also Chapter 13 for more discussion of CFTR).
FIGURE 14.4.ABC-class transporters have ATP-binding cassettes.
ABC transporters and multidrug resistance
Cells exposed to toxic compounds can develop resistance to them by decreasing their uptake, by increasing their detoxification, by altering target proteins of the drug, and/or by increasing excretion of the drug. In this way, cells can become resistant to several drugs in addition to the initial compound. Cells resistant to a drug no longer respond to its therapeutic effects. This phenomenon is known as multidrug resistance (MDR) and is a major limitation to cancer chemotherapy.Cancer cells often become resistant to the effects of several different drugs designed to kill them. The ABC transporter ABCB1 or P-glycoprotein is associated with MDR. Inhibitors of ABCB1 have been developed and clinical research to attempt to block the development of drug resistance has been conducted. The high concentrations of inhibitors necessary to inhibit ABCB1 are often toxic to the patient. Investigation of new approaches to circumvent MDR continues to be explored.
SECONDARY ACTIVE TRANSPORT
In addition to ATP-powered pumps, cells may use an active form of transport powered by the energy stored in electrochemical gradients. Ion concentration gradients of protons and sodium, generated by primary active transport, can power the movement of substrates against their gradients (Figure 14.5). Because the transport of one solute is coupled to and dependent upon the transport of another solute, this process is described as cotransport.Transport proteins that function in secondary transport do not possess ATPase activity. Instead, they depend indirectly on ATP hydrolysis since it is required for the primary active transport that establishes the ion gradients that power secondary active transport. Cotransport can allow substrates to cross the membrane in the same direction as each other or can allow for entry of one substrate and exit of another. While uniporters transport one type of molecule by facilitated transport, symporters and antiporters are cotransporters that function in secondary active transport.
FIGURE 14.5.Cotransport of substrates in secondary active transport.
Symporters are secondary active transporters that move substrates in the same direction across plasma membranes (Figure 14.6). Both substrates will enter or both substrates will exit a cell by symport. One substrate is transported in an energetically favorable direction along its concentration gradient established by primary active transport. The second substrate is actively transported, against its gradient, with the energy of the concentration gradient of its cotransported substrate powering the process.The sodium-glucose transporter is a well-described symporter that transports glucose against its concentration gradient into the intestinal epithelial cells using the strong concentration gradient of sodium to power the transport. Both glucose and sodium are taken into the cells (see also Chapter 15 for more discussion of glucose transport). Sodium-dependent amino acid transport proteins also function in symport.
FIGURE 14.6.Symport involves cotransport of substrates in the same direction across membranes.
Antiporters cotransport molecules in opposite directions across plasma membranes (Figure 14.7). The movement of one substrate into a cell is coupled to the movement of another substrate out of the cell. One substrate is moved along its gradient and the other moved against its gradient. The sodium-calcium antiporter in cardiac muscle cells is an example of an antiporter. This system maintains low calcium in the cytosol so that an increase in calcium can then trigger muscle contraction. Three sodium ions are moved into the cell, along the sodium gradient, while one calcium ion is moved out of the cell, against its gradient.The sodium-calcium antiporter thereby decreases the cytosolic concentration of calcium and reduces the strength of heart muscle contraction (see also information about inhibition of the sodium-potassium ATPase earlier in this chapter). Another antiporter is the sodium-proton exchanger that catalyzes the electroneutral exchange of sodium ions and protons and regulates salt concentration and pH. The bicarbonate/chloride antiporter in the stomach is another example.
FIGURE 14.7.Antiport involves cotransport of substrates in opposite directions across membranes.
- Active transport involves the movement of ions or molecules against their concentration gradient in an energy-dependent process.
- Primary active transport requires the direct hydrolysis of ATP by the primary active transport protein.
- Four classes of ATPases function in primary active transport. P-class transporters move ions against their gradients, F-class and V-class transporters pump protons against their concentration gradients, and ABC-class transporters move drugs, ions, and xenobiotics against their gradients.
- Ion gradients are generated and maintained by primary active transport.
- The ion gradients established by primary active transport can drive the transport of ions and small molecules against their concentration gradients in secondary active transport.
- Symporters are secondary active transporters that move substrates in the same direction (to the inside or to the outside) across membranes.
- Antiporters move one substrate into the cell and another substrate out of the cell by secondary active transport.