Introduction:
Membrane proteins are proteins that are embedded in the lipid bilayer of a cell membrane. They are an important class of proteins that play many critical roles in the cell, including acting as receptors, channels, pumps, and enzymes.
Location:
- Membrane proteins are located in the lipid bilayer of the cell membrane. The lipid bilayer is a thin, flexible barrier that surrounds cells and encloses the cell’s internal environment. It is made up of two layers of phospholipid molecules, which are arranged with their hydrophobic (water-fearing) tails facing inward and their hydrophilic (water-loving) heads facing outward.
- Membrane proteins can be found either embedded within the lipid bilayer or on the surface of the membrane. Integral proteins are fully embedded in the lipid bilayer and are not easily removed. They often span the entire membrane and have hydrophobic regions that interact with the lipids in the membrane. Peripheral proteins, on the other hand, are not fully embedded in the lipid bilayer and can be more easily removed. They often bind to integral proteins or the surface of the membrane.
- The location of a membrane protein within the lipid bilayer or on the surface of the membrane can determine its function. For example, integral proteins that span the entire membrane can act as channels or pumps, allowing molecules to pass through the membrane. Peripheral proteins on the surface of the membrane may act as receptors, binding to specific molecules and transmitting signals across the membrane.
Features:
- Membrane proteins are amphipathic, meaning that they have both hydrophobic (water-fearing) and hydrophilic (water-loving) regions. This allows them to interact with the hydrophobic region of the lipid bilayer and anchor themselves in the membrane.
- Membrane proteins can be integral or peripheral. Integral membrane proteins are completely embedded in the lipid bilayer and cannot be easily removed without disrupting the integrity of the membrane. Peripheral membrane proteins are not embedded in the lipid bilayer and can be more easily removed.
- Membrane proteins can have multiple subunits, which can be arranged symmetrically or asymmetrically.
- Some membrane proteins are anchored to the cytoskeleton, which helps to maintain the shape and integrity of the cell.
- Membrane proteins can be highly specific and selective in their interactions with other molecules, allowing them to perform specific functions within the cell.
- Membrane proteins can be regulated by various factors, including changes in temperature, pH, and the concentration of other molecules.
Types:
Membrane proteins are classified into various types:
Transport proteins: These proteins help transport substances across the membrane, such as ions, sugars, and amino acids. Examples include ion channels and pumps.
Enzyme-linked receptors: These proteins contain an enzyme that is activated when a specific molecule binds to the receptor. This can trigger a signalling pathway within the cell.
G protein-coupled receptors: These proteins are activated when a specific molecule binds to the receptor, causing a change in the shape of the protein. This can trigger a signalling pathway within the cell.
Integral membrane proteins: These proteins are embedded in the membrane and are important for the structure and function of the membrane. Examples include ion channels and pumps.
Peripheral membrane proteins: These proteins are not embedded in the membrane, but are instead attached to the outer surface of the membrane. They may be involved in signalling pathways or in the attachment of the cell to its surroundings.
Anchoring proteins: These proteins anchor other proteins to the membrane. They may be integral membrane proteins or peripheral membrane proteins.
Structures:
There are several structural types of membrane proteins.
Alpha helices: These are long, helical structures that span the membrane. They are composed of alpha helices, which are secondary structures formed by the folding of a protein chain into a spiral shape. Alpha helices are typically hydrophobic, so they interact with the hydrophobic region of the lipid bilayer.
Fig: Different types and structures of membrane proteins
Beta sheets: These are flat, sheet-like structures that span the membrane. They are composed of beta sheets, which are secondary structures formed by the folding of a protein chain into a sheet-like structure. Beta sheets can be either hydrophobic or hydrophilic, depending on the side chains of the amino acids that make up the sheet.
Alpha-beta barrels: These are barrel-shaped structures that span the membrane. They are composed of both alpha helices and beta sheets, which are arranged in a cylindrical structure. Alpha-beta barrels are typically hydrophilic on the inside and hydrophobic on the outside.
Pore-forming proteins: These are proteins that form channels or pores in the membrane through which small molecules can pass. They are typically composed of multiple subunits that assemble to form a pore.
G protein-coupled receptors: These are receptors that bind to specific ligands, such as hormones or neurotransmitters, and transmit a signal across the membrane by activating a G protein on the intracellular side.
Fig: G protein-coupled receptors
Ion channels: These are proteins that form channels in the membrane through which ions, such as sodium or potassium, can pass. Ion channels are important for maintaining the concentration gradients of ions across the membrane, which is necessary for many cellular processes.
Fig: Ion channels receptor
The Fluid Mosaic Model and membrane proteins:
The fluid mosaic model is a widely accepted model of the structure of biological membranes. It proposes that the membrane is composed of a mosaic of proteins embedded in a fluid lipid bilayer. The lipid bilayer is composed of two layers of phospholipid molecules, which have a hydrophobic tail and a hydrophilic head. The hydrophobic tails face each other and form the interior of the bilayer, while the hydrophilic heads face outward and interact with the watery environment inside and outside the cell.
Membrane proteins play important roles in the fluid mosaic model. They can be integral membrane proteins, which are completely embedded in the lipid bilayer and cannot be easily removed, or peripheral membrane proteins, which are not embedded in the lipid bilayer and can be more easily removed. Integral membrane proteins can be found in the lipid bilayer in a variety of configurations, including alpha helices, beta sheets, and alpha-beta barrels. They are typically anchored in the lipid bilayer by hydrophobic interactions between their hydrophobic residues and the hydrophobic tails of the phospholipid molecules.
Fig: Fluid Mosaic Model
Peripheral membrane proteins are not embedded in the lipid bilayer and can be more easily removed. They are typically associated with the membrane through non-covalent interactions, such as hydrogen bonding or ionic interactions. They can be found on either the intracellular or extracellular surface of the membrane and can perform a variety of functions, including signalling, transport, and enzyme activity.
The fluid mosaic model suggests that the membrane is a dynamic structure, with proteins and lipids constantly moving within the bilayer. This movement allows the membrane to respond to changes in the environment and perform its various functions.
Membrane proteins and Transporter:
Membrane proteins can act as transporters to move molecules across the membrane. Transporters are proteins that bind to a specific molecule and facilitate its movement across the membrane by using energy from ATP or from the concentration gradient of the molecule. Transporters are important for maintaining the concentration gradients of various molecules, which is necessary for many cellular processes. They can be regulated by various factors, including changes in the concentration of the molecule being transported and the availability of ATP.
The several types of transporters:
- Uniporters: These transport a single molecule across the membrane.
- Symporters: These transport two or more molecules in the same direction across the membrane.
- Antiporters: These transport two or more molecules in opposite directions across the membrane.
- Facilitated diffusion: This is the passive transport of molecules down their concentration gradient. It does not require energy and is mediated by specific transporters.
- Active transport: This is the transport of molecules against their concentration gradient, which requires energy. It is mediated by specific transporters that use energy from ATP to move the molecules across the membrane.
Fig: Different Types of Membrane Transport
Functions:
- Transport: Membrane proteins can act as channels, pumps, or transporters to move molecules across the membrane. This can include the movement of ions, such as sodium and potassium, and the movement of small molecules, such as sugars and amino acids.
- Signal transduction: Membrane proteins can act as receptors that bind to specific ligands, such as hormones or neurotransmitters, and transmit a signal across the membrane. This can activate intracellular signalling pathways that lead to changes in gene expression or other cellular processes.
- Enzyme activity: Some membrane proteins are enzymes that catalyze chemical reactions within the cell. These enzymes can be found on the intracellular or extracellular surface of the membrane.
Fig: Functions of membrane proteins
- Cell-cell communication: Membrane proteins can act as receptors or ligands that mediate communication between cells. This can include the communication between immune cells, such as T cells and B cells, and the communication between neurons.
- Structural support: Some membrane proteins are anchored to the cytoskeleton, which helps to maintain the shape and integrity of the cell.
- Defence: Membrane proteins can act as part of the immune system, helping to defend the body against pathogens. This can include proteins that act as receptors for pathogen-associated molecular patterns (PAMPs) or proteins that form part of the antimicrobial defence system.
References:
- Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K. and Walter, P., 2003. Molecular biology of the cell. Scandinavian Journal of Rheumatology, 32(2), pp.125-125.
- Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K. and Walter, P., 2002. An overview of the cell cycle. Molecular Biology of the Cell. 4th edition.
- Alberts, B., Bray, D., Hopkin, K., Johnson, A.D., Lewis, J., Raff, M., Roberts, K. and Walter, P., 2015. Essential cell biology. Garland Science.