Carrier Proteins

Carrier Proteins Definition

Organic compounds are essentially chemical compounds that contain carbon bonds. Carbon is essential for life, as it quickly forms bonds with other molecules and components, allowing life to occur readily. Proteins are another type of organic compound, like Carbohydrates, but their main functions include acting as Antibodies to protect our Immune System, Enzymes to speed up chemical reactions, etc.

Now, let's look at the definition of carrier proteins.

The cell membrane's Selective Permeability is why carrier proteins are necessary. Carrier proteins allow polar molecules and ions that cannot easily pass through the cell membrane to enter and exit the cell.

Because of the cell membrane's structure, polar molecules and ions cannot easily enter the cell. The cell membrane is made of phospholipids arranged into two layers making it a phospholipid bilayer.

Phospholipids are a type of lipid. Lipids are organic compounds containing Fatty Acids and are insoluble in water. A phospholipid molecule consists of a hydrophilic or water-loving head, shown in white in Figure 1, and two hydrophobic tails, shown in yellow.

The hydrophobic tails and hydrophilic head make the phospholipids an amphipathic molecule. An amphipathic molecule is a molecule that has both hydrophobic and hydrophilic parts.

Polar and ion molecules have a more challenging time passing through because polar and ionic molecules are water-loving or hydrophilic, and the way the cellular membrane is structured has the hydrophilic heads facing the outside and the hydrophobic tails facing the inside.

This means that small non-polar or hydrophobic molecules don't need carrier proteins to help them go in and out of the cell.

Other ways phospholipids can organize themselves beside the phospholipid bilayer are liposomes and micelles. Liposomes are spherical sacs made of phospholipids, usually formed to carry nutrients or substances into the cell. Liposomes can artificially be used to deliver drugs into our bodies, as illustrated in Figure 2.

Micelles are a bunch of molecules forming a colloidal mixture, as illustrated in Figure 1. Colloidal particles are particles in which one substance is suspended in another due to its inability to dissolve.

Figure 1: Different structures of phospholipids shown. Wikimedia, LadyofHats.

Figure 2: Liposome used for drug delivery shown. Wikimedia, Kosigrim.

Carrier proteins function

Carrier proteins function by changing shape. This change in form allows molecules and substances to pass through the cell membrane. Carrier proteins attach or bind themselves to specific molecules or ions and transport them across the membrane in and out of Cells.

Carrier proteins participate in both active and passive modes of transport.

• In passive transport, substances diffuse from high to low concentrations. Passive transport occurs because of the concentration gradient created by the difference in concentrations in two areas.

Keep in mind that there are other types of proteins besides transport proteins. Still, here we are focusing on carrier proteins that fall under transport, as their job is to facilitate the diffusion of molecules.

Membrane proteins can be found either integrated or in the periphery of the phospholipid bilayer. Membrane proteins have many functions, but some of them are carrier proteins that allow transport to occur in and out of the cell. Carrier proteins are considered membrane transport proteins.

As for the active mode of transport, we'll elaborate on that in the next section.

Carrier Proteins Active Transport

Carrier proteins also participate in active transport.

Both active and passive means of transport involve carrier proteins changing shape as they move molecules from one side of the cell to the other. The difference is that active transport requires chemical energy in the form of ATP. ATP, or adenosine phosphate, is a molecule that provides cells with a usable form of energy.

One of the most famous examples of active transport that uses carrier proteins is the sodium-potassium pump.

The general steps to the sodium-potassium pump are as follows and shown in Figure 3:

1. Three sodium ions bind to a carrier protein.

2. ATP is hydrolyzed into ADP, releasing one phosphate group. This one phosphate group attaches to the pump and is used to supply the energy for the change in the shape of the carrier protein.

3. The pump or carrier protein undergoes conformational or change in shape and allows the sodium \((Na^+)\) ions to cross the membrane and go out of the cell.

4. This conformational change allows two potassium \((K^+)\) to bind to the carrier protein.

5. The phosphate group is released from the pump, allowing the carrier protein to return to its original shape.

6. This change to the original shape allows the two potassium \((K^+)\) to travel across the membrane and into the cell.

Figure 3: The sodium-potassium pump illustrated. Wikimedia, LadyofHats.

Carrier Proteins vs. Channel Proteins

Channel proteins are another type of transport protein. They act similar to pores on the skin, except in the cell membrane. They act like channels, hence the name, and can let small ions through. Channel proteins are also membrane proteins that are permanently positioned in the membrane, making them integral membrane proteins.

Unlike carrier proteins, channel proteins stay open to the outside and inside the cell, as shown in Figure 4.

The transport rate of channel proteins occurs much faster than the rate of transport for carrier proteins. This is because carrier proteins don't remain open and have to undergo conformational changes.

Channel proteins also deal with passive transport, while carrier proteins deal with both passive and active transport. Channel proteins are highly selective and often only accept one type of molecule. Other channel proteins besides aquaporin include chloride, calcium, potassium, and sodium ions.

Overall, transport proteins deal with either 1) larger hydrophobic molecules or 2) small to large ions or hydrophilic molecules. Non-facilitated diffusion, or simple diffusion, only occurs for small enough hydrophobic molecules.

An example of a simple, but vital, diffusion that frequently occurs in our bodies is oxygen diffusing or moving into cells and tissues. If diffusion of oxygen didn't happen quickly and passively, we'd most likely get oxygen deprivation which could lead to seizures, comas, or other life-threatening effects.

Figure 4: Protein channel (left) compared to carrier proteins (right). Wikimedia, LadyofHats.

Carrier Protein Example

Carrier proteins can be categorized based on the molecule that they transport in and out of the cell. Facilitated diffusion for carrier proteins usually involves sugars or amino acids.

Carrier proteins also perform transport actively. We can categorize active transports by the energy source used: chemical or ATP, photon, or electrochemically driven. Electrochemical potentials can drive the diffusion of substances through the difference in concentration inside and outside the cell and the charges of the molecules involved.

For example, if we refer back to the sodium-potassium pump, the two molecules involved are potassium and sodium ions. The difference between the concentrations of both ions inside and outside the cell creates a membrane potential that drives nerve impulses. On the other hand, a photon refers to particles of light, so we can also call this type of transport light-driven, which can be found in bacteria.

Bacteria are single-celled organisms that do not have structures that are membrane-bound.

The most common examples of carrier proteins are:

• ATP-driven transport can use carrier proteins. This type of active transport couples ATP or chemical energy to drive the transport of molecules in and out of cells.

  • For example, the sodium-potassium pump discussed earlier is ATP-driven, as ATP is used to facilitate the transport of sodium and potassium ions. Sodium-potassium pumps are essential as they drive nerve impulses and maintain homeostasis in our bodies. Homeostasis is the process by which our bodies maintain stability.

  • The sodium-potassium pump is also an antiporter. An antiporter is a transporter that moves the molecules involved in opposite directions, such as sodium ions out and potassium ions into the cell.

Other types of transporters besides antiporters include uniporters and symporters. Uniporters are transporters that only move one kind of molecule. In turn, symporters transport two types of molecules, but unlike antiporters, they do it in the same direction.

• Sodium-glucose pump uses the electrochemical gradient of the sodium ion making it secondary active transport, unlike the sodium-potassium pump, which directly uses ATP, making it a primary active transport.

  • Cells generally keep a higher sodium concentration inside and a higher potassium concentration outside the cell. The sodium-glucose pump works by a carrier protein binding to glucose and two sodium ions simultaneously. This is because glucose and sodium both don't want to go against their gradient, resulting in glucose not wanting to go into the cell and sodium wanting to go into the cell.

  • Energy gradient caused by sodium wanting to go into the cell drives the glucose along with it. If the cells wish to keep sodium at a lower concentration inside the cell relative to the outside, the cell ends up having to use the sodium-potassium pump to drive out sodium ions.

  • All in all, the sodium-glucose pump doesn't use ATP directly, making it secondary active transport. It is also a symport because glucose and sodium go into the cell or in the same direction, unlike the sodium-potassium pump.

Figure 5: Types of transporter illustrated. Wikimedia, Lupask.