I recently learned the second messenger model, where adrenaline activates adenyl cyclase, which converts ATP into cAMP. Then cAMP acts as a second messenger which activates portein kinase enzymes. The protein kinase enzyme converts glycogen into glucose.

This is a very complicated pathway for me. Why doesn't the cell just use adrenaline to activate the protein kinase enzyme? Or even more simply, why doesn't adrenaline itself be an enzyme? Doesn't this make things much more easier?

Or in other words, why does natural selection choose to use the more complicated second messenger as opposed to one messenger only?

  • $\begingroup$ If you think that second messenger systems are complicated, then do not look into signal transduction cascades. More complexity allows a finer regulation as well as signal amplification. $\endgroup$
    – Chris
    Apr 20, 2023 at 20:51
  • 2
    $\begingroup$ The question seems to assume an underlying idea: that adrenaline has nothing to do other than cause the conversion of glycogen into glucose. It has many other functions; if you consider that, perhaps you'll understand why this multi-stage system evolved. $\endgroup$
    – mgkrebbs
    Apr 21, 2023 at 0:18
  • $\begingroup$ Does this answer your question? Apparent paradox in Glucagon action. It is obviously not the exact same question, but careful reading of my answer will help you understand how the integration of different biochemical responses between tissues is key to hormones and second messenger systems. $\endgroup$
    – David
    Apr 21, 2023 at 21:35

1 Answer 1


A simple explanation for why there are intermediate messengers between stimuli (in this case adrenaline) and the final effector (the kinase).

  1. Multiple messengers in a signalling pathway create different points at which signals can converge or diverge: Many stimuli may have the same end result, so a common secondary messenger is a point in the pathway for these signals to converge, as opposed to having multiple dedicated proteins to produce the same result for each stimulus. cAMP is a good example of such a secondary messenger where the signals from many stimuli can converge. Similarly, secondary messengers can also be points where signals diverge to have very different results, depending on other factors/proteins that may be active in the cell at the time. This way, a cell has common points to combine information from multiple stimuli (which may be be in agreement or conflicting) into an output. If there was a strict 1-1 relationship between stimuli and effectors with no cross-over points, it would be difficult for such cross-talk to happen.

  2. Signal amplification: Many times, cells receive only a few molecules of a stimulus, which may not be enough to trigger a response on its own. However, if there are intermediate steps in the signalling pathway where the signal can be amplified, then very feeble external signals can also be detected. This image illustrates this point:


Image source

  • $\begingroup$ There is already an explanation of amplification here which gives the detail needed for a naive student to understand. $\endgroup$
    – David
    Apr 21, 2023 at 21:42
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    $\begingroup$ The figure is also unintelligible on its own. What is D? I know what a GPCR is, but someone new to the subject would not. Please avoid using abbreviations without defining them. $\endgroup$
    – David
    Apr 21, 2023 at 21:48

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