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What are Peripheral Proteins?
Peripheral proteins, also known as peripheral membrane proteins, are a class of physiologically active molecules made primarily of amino acids that interact with the lipid bilayer of cell membranes on the surface.
Peripheral proteins, unlike integral membrane proteins, do not permeate the cell membrane’s hydrophobic region. Peripheral proteins, on the other hand, contain particular amino acid sequences that allow them to bind to the phosphate heads of lipid molecules or integral proteins.
Peripheral proteins can operate on the cell membrane’s surface because they can connect to it but not be locked to it. A variety of mechanisms can be used to activate or deactivate peripheral proteins.
Many peripheral proteins are also involved in a variety of complicated metabolic processes. They may have a role in transporting chemicals within or outside of a cell, activating other proteins and enzymes, or interacting with other cells.
Structure of Peripheral Proteins
Several peripheral proteins are indicated in the picture below. A peripheral protein does not have a defined structure, but it does have many essential characteristics that distinguish it from other proteins.
To begin with, all peripheral proteins are bound to the cell membrane. These proteins’ amino acid sequences are unusual in that they attract them to the membrane and cause them to cluster on the membrane’s surface. This permits them to be in the right place at the right time to carry out their mission.
Orange peripheral proteins may be observed linked to either the phosphoglyceride lipid molecules that make up the lipid bilayer or integral proteins in the picture. A protein that lacks these amino acid regions will not bind to the membrane.
It would not be a peripheral protein and would be dispersed uniformly throughout the cytoplasm. Second, peripheral proteins lack a hydrophobic amino acid region.
This, along with the polarity of other amino acid groups, maintains peripheral proteins on the cell membrane’s surface. This is owing to phosphoglycerides’ amphipathic character. The blue “head” area is hence polar and hydrophilic.
The hydrophobic yellow “tails” that make up the membrane’s centre are hydrophobic. Peripheral proteins generally have a number of hydrophilic amino acids exposed on their surface to prevent being pulled into the membrane.
Hydrophobic amino acids are exposed in the centre of integral proteins, while hydrophilic amino acids are exposed in the portions that are exposed to water. They are effectively trapped within the membrane as a result of this.
Functions of Peripheral Proteins
i. Support
One of the most important functions of peripheral proteins is to control and maintain the intracellular cytoskeleton as well as extracellular matrix components. Organelles, filaments, and tubules combine to produce both of these structures.
These tiny structures can give stiffness or tension, but they must be attached to something. This site of attachment to the cell membrane might be provided by peripheral proteins.
The cytoskeleton and extracellular matrix are used by cells in a variety of ways. They’re most commonly employed to manage the cell’s form and size. The cytoskeleton also has the ability to move metabolic products around, and it may be terminated or activated by a variety of peripheral proteins.
A packet of proteins freshly packed in the Golgi apparatus, for example, might use the cytoskeleton to travel across the cytoplasm. Specific peripheral proteins identify the package and begin the process of expelling it when it reaches the cell membrane to be ejected.
ii. Communication
The extracellular matrix is a large network for collecting information in numerous cells, in addition to providing structural support. To activate peripheral proteins, bacteria, for example, employ a sequence of events that starts in the filaments of their extracellular matrix.
The message is subsequently passed on to integral proteins, and it is transported throughout the cell. It is then transferred to another peripheral protein, which triggers a reaction.
A tiny creature or cell may learn a lot about its local surroundings in this way. Cells growing together to create a multicellular creature react in this way, and they cease growing at the proper time.
Peripheral proteins, like many other proteins and chemical signals, cause chain reactions that might cause the DNA or other organelles to respond. Based on its surroundings and the signals it gets, a cell can grow more, react to a hazard, or even create its own poisons.
Furthermore, depending on variables like pH and temperature, numerous peripheral proteins can adhere and detach from the membrane. This enables a cell to adapt its strategies to diverse surroundings and manage activities like cell signalling and hormone receipt.
iii. Enzymes
On the surface of cell membranes, several peripheral proteins exist to perform a specialised activity on a specific substrate. It might be broken down or combined with another molecule.
Because the chemicals they produce are needed within or near to the cell membrane, peripheral proteins with basic enzymatic activities are frequently peripheral proteins. Peripheral proteins, for example, are enzymes that regulate the synthesis and destruction of the cell membrane.
iv. Molecule Transfer
The transfer of tiny molecules or electrons is also a function of many peripheral proteins. Because of their affinity for the cell membrane, these proteins allow reactions to take place in a small space and with great precision.
Peripheral proteins make up a large portion of the electron transport chain. These proteins can transmit electrons to other proteins and chemicals by transferring electrons from the integral proteins to which they are linked.
This effectively stores the energy released by the breakdown of glycolysis products into conveniently accessible molecules, such as ATP. Other hydrophobic compounds can attach to peripheral proteins and be transferred over or through the membrane in a variety of ways.
Nucleotide Citations
- Peripheral membrane proteins of sarcoplasmic and endoplasmic reticulum. Comparison of carboxyl-terminal amino acid sequences. Biochem Cell Biol . 1989 Oct;67(10):696-702.
- Peripheral Membrane Proteins: Promising Therapeutic Targets across Domains of Life. Membranes (Basel) . 2021 May 8;11(5):346.
- The interactions of peripheral membrane proteins with biological membranes. Chem Phys Lipids . 2015 Nov;192:51-59.
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