Wize University Biology Textbook > Cell Communication
Signal Propagation
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Signal Transduction: From Receptors to the Target
Once a signal is detected, it needs to somehow be communicated to the part of the cell that mediates a response to this signal. This is usually a multi-step process, and allows for amplification of the signal.
There are several different ways in which signal transduction can occur:
Phosphorylation Cascades
A phosphorylation cascade is when the signal is transmitted using a series of phosphorylation reactions.
- Phosphate groups are typically added to amino acid residues that have available -OH groups, such as tyrosine, threonine and serine.
- Kinases are the enzymes that add phosphates, while phosphatases are the enzymes that remove phosphate groups.
- Here's an example of how a phosphorylation cascade might occur:
- One enzyme becomes phosphorylated, which activates it.
- It then carries out its function, which is to phosphorylate the next enzyme in the cascade.
- This next enzyme is not activated and phosphorylates the next enzyme, and so on.
- Eventually, the target enzyme is phosphorylated, activated, and carries out the response.
Watch Out!
This is an example of a phosphorylation cascade in which phosphorylation activates enzymes. However, phosphorylation does not always mean activation!
Secondary Messengers
The signal may lead to the activation of an enzyme that produces a secondary messenger - a small molecule that regulates the activity of cellular processes.
- An example of a secondary messenger is cyclic AMP, also known as cAMP.
- This molecule regulated many enzymes in metabolic pathways, as well as the expression of many different genes.
Signal transduction:
Which of the following is incorrect about phosphorylation cascades?