Chemical classification of hormones
To understand pharmacokinetics and the pathways of effect realization, all hormonal compounds are divided into three groups:
- Amino acid derivatives — thyroid hormone compounds.
- Peptide and protein hormones — secreted by the hypothalamus, pituitary gland, pancreas, and parathyroid glands.
- Steroid hormones — produced by the adrenal cortex and gonads.
Localization of receptors in the cell
The location of the target receptor directly depends on the chemical nature of the hormone itself:
- Outer cell membrane — signals from peptide structures that are unable to independently overcome the membrane barrier arrive here.
- Intracellular localization (in the cytoplasm) — characteristic of lipophilic steroid hormones that penetrate inside the cell.
Mechanism of action of protein and peptide hormones
These compounds act without penetrating inside the cell, using the system of secondary messengers (messengers).
The reaction cascade includes the following stages:
- Stimulation of the membrane receptor.
- Activation of G proteins.
- Increase in the enzymatic activity of adenylate cyclase.
- Accumulation of cyclic adenosine monophosphate (cAMP).
- Activation of cAMP-dependent protein kinases.
The final result of such a cascade is a change in channel permeability, ion transport, increased secretion, substance synthesis, and cell proliferation. A special case is observed with triiodothyronine: in high doses, it increases cAMP levels by inhibiting the enzyme phosphodiesterase.
Genomic mechanism of steroid hormones
Steroids exert their effect directly on the genetic apparatus of the cell. The process proceeds in stages:
- Penetration: high lipophilicity allows molecules to easily pass through histohematic barriers and membranes.
- Binding: a steroid-receptor complex with high affinity for DNA is formed in the cytoplasm.
- Translocation and nucleus: the complex penetrates the cell nucleus and binds to DNA.
The result of interaction with the genome is the regulation of transcription — the initiation (induction) or suppression of the synthesis of specific protein molecules.
Briefly: answers to topical questions
Which secondary messengers, besides cAMP, participate in signal transduction from peptide hormones?
The following secondary messengers participate in signal transduction from peptide hormones:
- cGMP (cyclic guanosine monophosphate) — cyclic guanosine monophosphate;
- IP3 (inositol trisphosphate) — inositol trisphosphate;
- DAG (diacylglycerol) — diacylglycerol;
- Ca²⁺ (calcium ions) — calcium ions.
Which hormones are classified as amino acid derivatives, besides thyroid hormones?
The following hormones are classified as amino acid derivatives:
- Catecholamines (catecholamines) — dopamine, norepinephrine, epinephrine;
- Serotonin (serotonin) and melatonin (melatonin) as derivatives of tryptophan;
- Histamine (histamine) as a derivative of histidine.
Where are receptors for thyroid hormones located?
Receptors for thyroid hormones are located inside cells, being cytoplasmic or nuclear receptors (soluble proteins in the cytosol or nucleus).
Which types of G proteins participate in the regulation of adenylate cyclase activity?
The following types of G proteins participate in the regulation of adenylate cyclase activity and other target proteins:
- Stimulatory (stimulatory) — accelerate processes and activate target proteins;
- Inhibitory (inhibitory) — slow down the action of target proteins.
Which specific DNA regions does the steroid-receptor complex bind to in the nucleus?
The provided sources do not contain specific names of specific DNA regions (nucleotide sequences) to which this complex binds; only their regulatory function is mentioned.
Peptides knock from the outside (membrane, cAMP), while steroids enter directly to visit DNA through the cytoplasm.
Class and exam questions
Why do peptide hormones not cross the cell membrane?
This is due to their chemical nature: such compounds cannot independently pass through the lipid layer of the outer cell membrane, which is why their receptors are located on the surface.
What role do G proteins play in the action of protein hormones?
After stimulation of the membrane receptor by a peptide hormone, G proteins are activated, which trigger an increase in adenylate cyclase activity and the accumulation of intracellular cAMP.
How do steroid hormones affect protein synthesis?
Steroids penetrate into the cytoplasm, bind to the receptor, are transported into the nucleus, and interact with DNA, regulating the transcription process — inhibiting or activating the formation of necessary proteins.
What else to study on this topic
- Pharmacokinetics of various hormone groups
- Features of the action of triiodothyronine in high doses
- The role of phosphodiesterase in cAMP regulation
- Transcription processes in the genomic mechanism
- Functions of G proteins in signal transduction
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