Following synthesis, the amino acid neurotransmitters are transported in vesicles where they are stored until released by exocytosis. Amines All amines except thyroid hormones are derived from amino acids in the cytosol by enzyme-catalyzed reactions. All the catecholamines are derived from tyrosine. First, dopamine is derived from tyrosine; norepinephrine is derived from dopamine; and epinephrine is finally derived from norepinephrine.
The amines are stored in cytosolic vesicles until their release is triggered. Peptides Peptides are synthesized as described earlier. Peptides that serve as hormones undergo further processing as follows Fig. A prepropeptide is synthesized and released into the rough ER. Proteolytic enzymes in the RER cleave off some amino acids to yield propeptides.
In the smooth ER propeptides are packaged into transport vesicles. Golgi complexes package the propeptide into secretory vesicles.
Either in the GA or secretory vesicles more amino acids are cleaved to yield the final peptide. Steroids Steroids are synthesized and released as needed by enzymes in smooth ER and mitochondria from cholesterol. All readily diffuse through cell membranes because of their lipophilic character. Steroids cannot be stored in membrane bound vesicles and are synthesized as needed and are released immediately.
Eicosanoids Eicosanoids are lipophilic and are synthesized from arachidonic acid and released on demand. The first step in eicosanoid synthesis involves the release of arachidonic acid from phospholipids in the cell membrane by phospholipase A 2. Eicosanoids are derived from arachidonic acid by one of two pathways. The cyclooxygenase pathway results in the synthesis of prostaglandins , prostacyclin s, and thromboxanes. Aspirin interferes with synthesis by this pathway.
The lipoxygenase leads to synthesis of leukotrienes. Prostacyclins and thromboxanes are important in blood clotting while prostaglandins and leukotrienes contribute to the inflammatory response. Transport of Messengers. Chemicals may reach their target cells by simple diffusion autocrines, paracrines, neurotransmitters and most cytokines or be transported in the blood some cytokines, hormones and neurohormones. Hydrophilic messengers that travel in blood may do so in dissolved form although catecholamines can be transported bound to carrier proteins.
Hydrophobic messengers such as steroids and thyroid hormones are transported bound to carrier proteins. Blood-borne messengers are degraded by the liver or are secreted by the kidneys. The persistence of a messenger in the blood is measured by their half-life.
Dissolved messengers have a shorter half-life than messengers bound to carrier proteins. Signal Transduction Mechanisms. Properties of Receptors Receptors bind to only one messenger or class of messengers.
This property called specificity. The strength of the binding between a messenger and its receptor is called affinity. A single messenger can bind to more than one receptor and have different affinities for each one. For example, both epinephrine and norepinephrine bind to adrenergic receptors. Different types of adrenergic receptors exist including a 1 and a 2 , and b 1 , b 2 and b 3.
The a receptors have a greater affinity for norepinephrine and b 2 receptors have a greater affinity for epinephrine. Cells vary in the receptors they have and a single target cell may have receptors for different chemical messengers. Receptor Binding and Target Cell Response Three factors influence the degree of the target cell's response to a messenger:. Messenger Concentration The greater the concentration of the messenger the greater the response.
Number of Receptors The more receptors the target cell has the greater the response. Cell can up-regulate or increase the number of receptors when messenger concentration is low or down-regulate or decrease receptors when messenger concentration is high. Affinity of Receptor The greater the affinity of the receptor for the messenger the greater the response. Ligands that bind to receptors to cause a biological response are called agonists.
When a ligand binds to a receptor and does not produce a response it is called an antagonist. Artificial receptor agonists and antagonists often have therapeutic and experimental usefulness. The receptor may be located in the cytosol or in the nucleus. The messenger binds to the receptor to form a hormone-receptor complex that binds to a certain region of DNA called the hormone response element.
This binding may then either activate or deactivate a gene resulting in either the synthesis or nonsynthesis of protein. Because activating or deactivating protein synthesis involves some time, the hormones have actions that are slow to develop and persist for some time. Signal Transduction Mechanisms by Membrane Bound Receptors Lipophobic messengers depend upon receptors in the plasma membrane facing the extracellular environment.
We will place these membrane-bound receptors into three categories:. Channel-Linked Receptors Ion-channels that open or close in response to the binding of chemical messengers are called ligand-gated channels. Ligand-gated channels are proteins that function as both receptor and ion channels and are fast. The binding of the messenger to the protein opens the channel and ions either enter or leave the cell. Sometimes, substances have both hydrophilic and hydrophobic portions.
The hydrophobic portion can be lipophilic or not. Examples for hydrophilic substances include compounds with hydroxyl groups such as alcohols. The solubility of a compound in a solvent depends on the chemical structure of the compound. Lipophilic substances have nonpolar structure, and hydrophilic compounds have polar structures.
Besides, the key difference between lipophilic and hydrophilic is that lipophilic substances tend to combine with or dissolve in lipids or fats and other lipophilic solvents whereas hydrophilic substances tend to combine with or dissolve in water and other hydrophilic solvents. Examples for lipophilic substances include fat-soluble vitamins, hormones, amino acids, hydrocarbon compounds, etc.
The terms lipophilic and hydrophilic are adjectives which describe the solubility of compounds. The key difference between lipophilic and hydrophilic is that lipophilic refers to the ability of a substance to dissolve in lipids or fats while hydrophilic refers to the ability of a substance to dissolve in water or other hydrophilic solvents.
Lipophilicity refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. These non-polar solvents are themselves lipophilic translated as "fat-loving" or "fat-liking"— the axiom that dissolves. Thus lipophilic substances tend to dissolve in other lipophilic substances, while hydrophilic substances tend to dissolve in water and other hydrophilic substances. Ipophilic substances interact within themselves and with other substances through the London dispersion force.
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