RECEPTOR:
A receptor is typically a protein molecule that serves as a binding site for signal molecules. It is frequently located on the cell membrane’s surface. When the signal molecules attach to the receptor, it triggers significant events. The receptor may undergo a change in its shape which initiates a signal transduction pathway inside the cell where one molecule activates another molecule. This process continues until we get a response from the cell.
Receptors are categorized into two main types: cell surface receptors (CSRs) and intracellular receptors.
1. CELL SURFACE RECEPTORS (CSRs):
Cell surface receptors, as the name suggests, are receptors present on the surface of a cell, specifically bound to the cell’s plasma membrane. When a receptor is situated there, the signaling molecule or ligand is not required to enter the cell. Indeed, certain characteristics of the ligand might hinder its ability to traverse the cell membrane; for instance, if the ligand is hydrophilic, it makes penetration challenging.
Basic Structure Of Cell Surface Receptors (CSRs):
A cell surface receptor is composed of three fundamental elements. The initial segment, known as the Extracellular Ligand-Binding Domain, is located outside the cell. This is the place where the signaling molecules(ligands) can bind. The second segment, termed the Hydrophobic Transmembrane Domain, is embedded into the plasma membrane; and lastly, the third part, the Intracellular Domain, resides within the cell. It triggers a specific response when a signaling molecule binds to the extracellular domain. This response often initiates downstream pathways within the cell.
Types Of Cell Surface Receptors (CSRs):
Cell surface receptors are generally divided into three primary categories: G protein-coupled receptors (GPCRs), enzyme-linked receptors (ELRs), and ion channel receptors (ICRs). These three receptors hold significant importance in cell signaling, playing a big role in complex communication because they trigger multiple downstream pathways within the cell.
2. INTRACELLULAR RECEPTORS:
Within a cell, there are special receptors called intracellular receptors that are always present inside the cell. These receptors are designed to respond to certain types of molecules that don’t mix well with water, known as hydrophobic ligands, which can cross the cell’s outer membrane. When one of these ligands attaches to the intracellular receptor, it causes the receptor to change its shape, revealing a site on the protein that can bind to DNA. The ligand-receptor complex then enters the nucleus, where it attaches to specific regions of the DNA on the chromosomes and initiates transcription. What’s interesting is that these internal receptors can directly influence the activity of genes, all without needing to involve other receptors or messengers in the process.
Categories Of Intracellular Receptors:
They are categorically segregated into two classes.
- The first category includes receptors that are located in the cytoplasm and have the ability to translocate to the nucleus.
- The second category consists of receptors that permanently reside within the nucleus.
