Carrier Protein Definition:
Carrier proteins are proteins that carry substances from one side of a biological membrane to the other. Many carrier proteins are found in a cell’s membrane, though they may also be found in the membranes of internal organelles such as the mitochondria, chloroplasts, nucleolus, and others.
Carrier proteins and channel proteins are the two types of membrane transport proteins. While channel proteins are exactly what they sound like proteins that open channels in the cell membrane, allowing molecules to flow in and out along their concentration gradient carrier proteins are only open to one side of the membrane in question at a time.
While a sodium-potassium channel may simply open and allow ions to flow from one side to the other, for example, the carrier protein known as the sodium-potassium pump binds to ions on one side of the membrane, then changes shape to carry them through to the other side without opening a channel.
This makes carrier proteins useful for active transport, where a substance needs to be carried against its concentration gradient in a direction it would not normally flow. However, carrier proteins can also be used for facilitated diffusion, a form of passive transport.
Carrier proteins typically have a “binding site” which will only bind to the substance they’re supposed to carry. The sodium-potassium pump, for example, has binding sites that will only bind to those ions.
Once the carrier protein has bound to a sufficient quantity of its target substance, the protein changes shape to “carry” the substance from one side of the membrane to the other. A textbook example of this process is the action of the sodium-potassium pump, illustrated below:
Some carrier proteins require no energy sources but the diffusion gradient that their substrate “wants” to pass down, making them a form of passive transport. Others may require energy in the form of ATP or may perform “secondary active transport,” where the transport of one substance against its diffusion gradient is powered by a different diffusion gradient that is created by ATP-using carrier proteins.
We will discuss examples of all passive, active, and secondary active transport using carrier proteins below in the “examples” section.
Function of Carrier Protein
Carrier proteins are some of the most common proteins in the world, and some of the most important to sustaining life. A cell’s ability to perform the functions of life depends on its ability to maintain a difference between the intracellular and extracellular environment. That’s where carrier proteins come in.
Within our own bodies, the action of all of our nerve cells is powered by the sodium-potassium gradient that is created by the sodium-potassium pump. This carrier protein binds to ions of sodium on one side of the membrane, and ions of potassium on the other side. Then the carrier protein binds with ATP and uses the energy of ATP to pump these ions across the cell membrane in opposite directions.
It is ultimately this sodium-potassium gradient that allows our nerve cells to fire, which is what allows us to move, think, perceive the world around us, and even keep our hearts beating.
Carrier proteins that transport protons across the mitochondrial membrane to create a concentration gradient there are also responsible for the creation of most of the ATP made by eukaryotic cells. The mitochondria use the enzyme ATP synthase to turn the energy of that concentration gradient into the energy of ATP.
Some of the common purposes served by carrier proteins include:
- Creating ion gradients which allow nerve cells to function
- Creating ion gradients which allow the mitochondria to function
- Creating ion gradients which allow chloroplasts to function in photosynthesis
- Transporting large molecules such as sugars and fats in and out of cells
- Many other tasks not listed here
Types of Carrier Proteins
Active transport carrier proteins require energy to move substances against their concentration gradient. That energy may come in the form of ATP that is used by the carrier protein directly, or may use energy from another source.
Many active transport carrier proteins, such as the sodium-potassium pump, use the energy stored in ATP to change their shape and move substances across their transportation gradient.
Pumps which practice “secondary active transport,” are sometimes referred to as “coupled carriers.” These pumps use the “downhill” transport of one substance to drive the “uphill” transport of another.
“Coupled carriers” like the sodium-glucose cotransport protein do end up costing the cell energy because the cell must use ATP to maintain the sodium concentration gradient that this carrier uses as its energy source. But the carrier protein does not use ATP directly.
Other carrier proteins, such as some that are found in bacteria and in organelles such as mitochondria and chloroplasts, might use energy sources directly from the environment without requiring ATP.
Carrier proteins can also carry substances in a “downhill” direction – that is, carry them down their concentration gradient, in the direction that the substance “wants” to go.
One example is the valinomycin potassium carrier, which binds to potassium ions and changes shape to release them on the other side of the membrane.
Examples of Carrier Proteins
The sodium-potassium pump uses ATP to transport both sodium and potassium ions against their transportation gradient.
The protein binds to sodium ions inside the cell, while simultaneously binding to potassium ions inside the cell. Once it has bound to a sufficient number of ions on both sides, it binds to a molecule of ATP. By releasing the energy stored in ATP, it changes shape to move both sets of ions to the opposite side of the membrane.
The sodium-potassium pump is crucial for the nerve function of animals, and is estimated to use about 20-25% of all the ATP in the human body!
This is because nerve cells fire using electrochemical signals – which are created by moving charged particles, i.e. sodium and potassium ions, from one side of the nerve cell membrane to the other very quickly. These potentials can only be created if there is an extreme difference in concentration between sodium and potassium ions inside the cells vs. outside them.
One reason why diseases like anorexia and cholera can be so dangerous is that extreme dehydration or malnutrition can disrupt the amount of sodium and potassium available to our cells, disrupting this gradient. In extreme cases, these ionic imbalances can cause the nerve cells that power our heart muscles to fail.
This is also why diseases that affect the kidneys, which control how we export or retain ions in our urine, can be dangerous. A rare side effect of diabetes, for example, is hypokalemia – not enough potassium in the blood, which can disrupt the function of the nerve cells driving the heart muscle.
The glucose-sodium cotransport protein is a good example of a protein that uses “secondary active transport, by “indirectly” using ATP.
In the example above, we discussed how the cell uses ATP to maintain the sodium and potassium gradients between the inside and outside of the cell. Generally, cells try to keep a higher concentration of sodium outside, and a higher concentration of potassium inside.
So to power the glucose-sodium pump, the cell allows a couple of sodium ions inside along with the glucose. The carrier protein binds to both the glucose molecule – which doesn’t “want” to move inside the cell – and the two sodium ions, which do want to move down their concentration gradient into the cell.
The energy of the sodium ions “wanting” to get into the cell overrides the glucose’s resistance, and all three particles are moved into the cell together.
This means more work for the sodium-potassium pump in the cell membrane, which will have to use ATP to pump the sodium back out in order to preserve this vital gradient. But the glucose-sodium cotransport protein does not use ATP itself – it only takes advantage of the energy of ATP indirectly.
This type of secondary active transport is called “symport,” from the Greek words “sym” for “together” and “port” for “transport.” Symport transports two substances together in the same direction in order to assure that they both get transported.
Valinomycin: A Passive Transport Carrier
Valinomycin is a protein that binds to potassium and carries it across the cell membrane down its concentration gradient, in the direction that the potassium “wants” to move.
It is found in the cell membranes of strep bacteria, who use it when they “want” to move potassium out of their cells. Its high degree of selectivity for potassium only gives it an advantage over other means to accomplish this transport, which might be more likely to move other ions such as sodium.
If you think “valinomycin” sounds like the name of an antibiotic, you’re right! Valinomycin is also used as an antibiotic to fight bacteria like strep because artificially introducing it to bacteria can destroy their electrochemical gradient.
For the same reason, valinomycin can also be a powerful neurotoxin: if it gets into nerve cells, it can dangerously disrupt their sodium-potassium gradient too!