Mechanism of cell signaling
The cell surface, known as the glycocalyx, acts as a unique sensory panel. It contains a specific set of receptors responsible for reactions with various agents that are unable to independently dissolve in lipids and cross the membrane.
The signal transduction process can be divided into three consecutive stages:
- Interaction of the membrane receptor with a ligand molecule on the outer surface of the cell.
- Transmission of the received signal across the thickness of the plasma membrane.
- Activation of secondary signaling molecules inside the cell.
Numerous biologically active substances act as ligands that bind to receptors. These include: estrogen, growth hormone, insulin, insulin-like growth factor (IGF), as well as follicle-stimulating (FSH), luteinizing, adrenocorticotropic (ACTH), and thyroid-stimulating (TSH) hormones.
The result of such activation is the initiation of secondary transmission. This leads to the activation of specific enzymes, the activation of DNA-binding proteins, and the possible translocation of the substrate directly into the cell nucleus. The end result is often the initiation of transcription for a strictly specific set of genes.
Adenylate cyclase system (using insulin as an example)
A classic example of the receptor function is the operation of the adenylate cyclase system. Let us examine it using the insulin receptor as an example. Structurally, it is a large glycoprotein that completely spans the plasmalemma (it is transmembrane). It consists of an external receptor portion and an internal catalytic domain.
Mechanism of action:
- The hormone specifically binds to the external receptor portion.
- This interaction instantly activates the intracellular catalytic portion—the enzyme adenylate cyclase.
- Adenylate cyclase begins actively catalyzing the synthesis of cyclic adenosine monophosphate (cAMP) from ATP molecules.
The resulting cAMP plays a crucial role as a secondary messenger. It regulates the rate of intracellular processes, suppresses or activates metabolic enzymes, and significantly amplifies the cell's primary response to the hormonal signal.
In the case of insulin, this cascade of reactions leads to increased permeability of the cell plasmalemma to glucose, acceleration of the conversion of glucose into reserve glycogen, and a marked enhancement of lipid synthesis.
Pathologies of the receptor apparatus
Disruption of the normal structure or function of receptors inevitably leads to the development of severe diseases, as cells lose their ability to adequately respond to the body's regulatory signals.
- Type 2 diabetes mellitus. Develops due to pronounced impairment of insulin receptors located in muscle and fat cells. Tissues cease to absorb glucose.
- Morris syndrome (testicular feminization). The cause of this disease is genetic mutations in the genes encoding sex hormone receptors. The consequence of this insensitivity is a profound alteration in sex differentiation in the developing organism.
- Curare poisoning. Ingestion of the toxin causes severe blockade of receptors intended for binding the neurotransmitter acetylcholine. A direct consequence of this is a complete disruption of nerve impulse conduction, leading to respiratory arrest and death.
Briefly: answers to topical questions
What are the types of membrane receptors according to their signal transduction mechanism?
Membrane receptors are divided into three main groups depending on how the signal is transmitted to intracellular effectors. These include:
- Ionotropic receptors — contain a subunit that binds the signaling molecule and forms an ion channel.
- Catalytic receptors — are directly coupled to enzymes and exhibit intrinsic enzymatic activity upon ligand binding.
- Metabotropic receptors — transmit signals to intracellular enzymes via membrane G-proteins, which activate the generation of secondary messengers.
What secondary messengers (other than cAMP) function in the cell?
Inside the cell, several other molecules involved in signal transduction perform the function of secondary messengers besides cAMP. These include:
- cGMP — cyclic guanosine-3',5'-monophosphate;
- IP₃ — inositol 1,4,5-trisphosphate;
- DAG — diacylglycerol;
- Ca²⁺ — calcium ions.
What is the role of G-proteins in the functioning of the adenylate cyclase system?
G-proteins act as a link between the membrane receptor and the enzyme adenylate cyclase. After the hormone binds to the receptor, the G-protein is activated. The activated G-protein, in turn, activates the enzyme adenylate cyclase. This catalyzes the formation of cyclic AMP (cAMP) from ATP, which acts as a secondary messenger transmitting the signal to cytosolic proteins and enzymes to alter their functional activity.
Which chemical structures and molecules form the glycocalyx of the cell membrane?
The glycocalyx is a supramembrane complex on the outer surface of the membrane formed by carbohydrate components.
In sources, it is described as long branching carbohydrate chains, as well as oligosaccharides linked to proteins or lipids.
Its formation involves:
- Glycoproteins — carbohydrate chains linked to membrane proteins.
- Glycolipids — carbohydrate chains linked to membrane lipids.
Regarding membrane carbohydrates, it is noted that glycoproteins and glycolipids make up to 10% of the membrane composition. In one of the sources, the composition of the supramembrane layer is also described as membrane glycoproteins and various enzymes.
What is the true mechanism of action of the insulin receptor (involving tyrosine kinase), as opposed to the simplified adenylate cyclase model?
The mechanism of action of the insulin receptor is based on its intrinsic tyrosine kinase activity.
The receptor has two extracellular α-subunits that bind insulin, and two transmembrane β-subunits possessing tyrosine kinase activity.
Sequence of activation:
- Insulin binds to the α-subunits of the receptor.
- The conformation of adjacent β-subunits changes, bringing them closer together.
- Transphosphorylation or autophosphorylation occurs: one β-subunit phosphorylates the other at tyrosine residues.
- Phosphorylated tyrosine residues become "docking sites" for insulin receptor proteins.
- Insulin receptor substrate IRS-1 is phosphorylated.
- IRS-1 binds to PI3 kinase.
- PIP₃ is formed, leading to the activation of protein kinase B (Akt/PKB).
Consequences of this cascade include the translocation of GLUT-4 from the cytoplasm to the plasma membrane, activation of glycogen synthase, hydrolysis of cAMP by phosphodiesterase, and dephosphorylation of key metabolic enzymes by protein phosphatases.
The rule of the three "P"s helps to remember the stages of signal transduction: Presentation (ligand binding) → Passage (transmission across the membrane) → Proxy (activation of secondary molecules).
Class and exam questions
What is a secondary messenger?
A secondary messenger (e.g., cAMP) is synthesized inside the cell in response to an external signal. It regulates the rate of internal processes, manages the activity of metabolic enzymes, and significantly amplifies the cell's response.
How is the insulin receptor structured?
This is a transmembrane glycoprotein. It consists of an external receptor portion that directly binds the hormone and an intracellular catalytic portion that activates the adenylate cyclase enzyme.
Why is curare poisoning fatal?
Curare poison specifically blocks acetylcholine membrane receptors. This interrupts the conduction of nerve impulses to the muscles, causing respiratory arrest due to muscle paralysis.
What else to study on this topic
- Classification of receptors by activation mechanisms
- Features of signal transduction by peptide hormones
- Process of substrate translocation into the nucleus and initiation of transcription
- Genetics of receptor mutations in Morris syndrome
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