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(13) Mode of Action of Hormones

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Mode of Action of Hormones
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Our knowledge here comes mainly from our knowledge of animals. But as you know, in
many aspects, the biological structure and function are highly conserved among living
organisms.
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In general, there are 3 sequential stages for any hormone to act:
Initial signal perception
Signal transduction pathway
Final response
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Signal Perception:
It is when a hormone recognizes its receptor and a reaction occurs. The receptors vary in
nature: proteins, glycoproteins… these receptors are found in the target cell of the
hormone.
A hormone will bind to its receptor forming a H-R complex  the conformation of the
protein receptor will change and it will become active, leading to the second stage.
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Note that there are many compounds that can bind to hormones. Example: an auxinbinding protein might or might not be a receptor. If it is a receptor, it must lead to the 2nd
and 3rd sequential stages by definition. These receptors are usually present on the plasma
membrane as well as on GA, ER…
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The idea of a receptor is essential for understanding the function of the hormone:
 Only the target cell will respond because the receptor is there  this is why not all
cells will respond to the hormone. So the presence or absence of the receptor will
determine whether the cell will respond to a hormone or not.
 The presence of a receptor explains why hormones have several effects. Because
simply, there are different receptors for these hormones that could be present on the
same cells or on different cells.
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The hormone is the first messenger.
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Signal Transduction:
The hormone acts as a first messenger. 1st messengers will lead to the production of
second messengers after binding to the receptors.
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The H-R complex activates a G protein which then activates an enzyme called adenylate
cyclase that will catalyze the reaction to form cAMP. cAMP is the 2nd messenger.
The second messenger will control a cascade of biochemical reactions that will lead to the
final response.
This is why hormones are needed in very low concentrations. 1 hormone molecule can
lead to the production of thousands of cAMP  amplification effect.
cAMP might activate a protein or NAD kinase. Once this is active, it will phosphorylate
certain proteins  final response.
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Some hormones might not require a 2nd messenger, especially steroid hormones that act
on the gene level. In this case, the hormone receptor complex is found in the nucleus and
will act directly on the gene level so the gene can be expressed and the protein can be
synthesized.
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The receptors are present in the membranes, in the nucleus, cytoplasm...
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What is the nature of 2nd messengers?
Calcium is an example of a 2nd messenger that will activate calmodulin which may
stimulate NAD kinase.
There are other 2nd messengers that are known in plants: IP3 (inisitol triphosphate) and
DAG (diacylglycerol). These are 2 more 2nd messengers that are present in plants.
There are enzymes called phospholipases that hydrolyze lipids. Through the action of
phospholipase C (breaks bond between glycerol and phosphate), the products are IP3 and
DAG. So if you have a hormone that activates phospholipase C, you have the production
of these 2 compounds that act as 2nd messengers.
IP3 acts by releasing another 2nd messenger which is Calcium.
In most of the cases, the hormones act at the gene level. You can prove that with new
techniques by isolating a certain gene, identifying it, and then cloning it. It was found that
these genes when they are cloned, if you apply hormones to them, you can stimulate the
production of hormones.
Examples:
IAA  nuclear: H+ ATPase synthesis
GA  α-amylase
Ethylene, cytokinins  all act on gene level
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