Membrane proteins are proteins that are part of or interact with cell membranes, and they are responsible for carrying out the majority of the functions of these membranes. Membrane proteins account for approximately one-third of human proteins and are responsible for regulating processes that help biological cells survive.
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Membrane proteins have a range of different structures and are also situated in different areas of the membrane. They carry out a diverse range of functions, and the number of proteins and the types of proteins present on a particular membrane can vary.
Membrane protein structure
Cell membranes are made up of two phospholipid bilayers, which are called leaflets. These leaflets are present on all cells, forming a barrier that surrounds each cell. Membrane proteins are found on these phospholipid bilayers or they interact with these phospholipid bilayers.
There are non-polar membrane proteins that are hydrophobic (water repellent) and polar membrane proteins that are hydrophilic (able to mix with water), that are found inside the lipid bilayer. They are directly involved with the lipid bilayers that make a barrier around every cell.
Integral membrane proteins are a permanent fixture on the membrane.
Peripheral membrane proteins are not a permanent part of a membrane and can have hydrophobic, electrostatic, and other non-covalent interactions with the membrane or the integral proteins.
Integral proteins come in different types, such as monotopic, bitopic, polytopic, lipid-anchored proteins, or transmembrane proteins.
Monotopic integral proteins are only attached to one of the cell’s two leaflets.
Bitopic integral proteins are transmembrane proteins that can span lipid bilayers once. Polytopic proteins are also transmembrane proteins, which span lipid bilayers more than once.
A lipid-anchored protein has a covalent attachment to lipids that are embedded in the phospholipid bilayer.
Membrane protein functions
There is a diverse range of functions that membrane proteins carry out. These include:
- Junctions: connecting two cells together
All enzymes are a type of protein. As a result, a membrane protein that is embedded into the membrane can sometimes be an enzyme, which may have its active site facing substances outside of the lipid bilayer.
These types of enzymatic membrane proteins can work in teams to carry out the steps in a particular metabolic pathway, for instance breaking down lactose into carbohydrates and then monosaccharides.
Membrane proteins can allow hydrophilic molecules to pass through the cell membrane. Transport membrane proteins come in many forms, and some require energy to change shape and actively move molecules and other substances across the cell membrane. They do this by releasing ATP to use as an energy source.
- Anchorage: become points of attachment for the cytoskeleton and the extracellular matrix
Some membrane proteins can feature a binding site. These binding sites are characterized by specific shapes that match the shape of a chemical messenger. For example, these chemical messengers can be hormones.
When a hormone meets with the cell wall, it will connect with a receptor membrane protein that is embedded inside the cell wall. The hormone can change the receptor protein and cause a specific reaction, depending on the type of hormone or other substance, will take place within the cell.
Another important function of membrane proteins is in identification and recognition between cells. This particular function is useful in the immune system, as it helps the body to recognize foreign cells that may be causing infection, for instance. Glycoproteins are one type of membrane protein that can carry out cell recognition.
Adjacent cells may have membrane proteins that connect in a range of different junctions. Gap junctions and tight junctions.
This function helps cells to communicate with one another, and to transfer materials between one another.
Membrane proteins are important in the cytoskeleton, the system of filaments and fibers in the cytoplasm of a cell, and the extracellular matrix (ECM), which is the network of macromolecules found outside of cells, such as collagen, enzymes, and glycoproteins, to membrane proteins.
Attaching filaments or fibers in the cytoplasm found throughout the cell can help the cell to maintain its particular shape. It also keeps the location of membrane proteins stable.
Attaching membrane proteins to the extracellular matrix can help the ECM to mediate changes that occur in extracellular and intracellular environments.
Membrane Proteins in Disease
Several diseases are linked to mutations within membrane proteins. One example is a mutation called V509A, found in the thyrotropin receptor, thyrotropin being a hormone secreted by the pituitary gland that regulates the production of thyroid hormones.
This mutation increases the activity of the thyrotropin receptor and leads to congenital hyperthyroidism, a condition that can cause changes in mood, sleep problems, and stomach problems.
Other diseases that are linked to mutations in membrane proteins include hereditary deafness, Charcot-Marie-Tooth disease, which damages the peripheral nerves outside the central nervous system, and Dejerine-Sottas syndrome, which affects a person’s ability to move.
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Membrane proteins serve a range of important functions that helps cells to communicate, maintain their shape, carry out changes triggered by chemical messengers, and transport and share material.
Membrane proteins can also play a part in disease progression, as the immune system can use membrane proteins to identify potentially harmful foreign molecules within the body.