Introduction
Caffeine has become a staple of student life and academia alike thanks to its ability to promote alertness. This is achieved through higher blood pressure, increased heart rate and greater oxygen delivery to muscles, which are all physiological consequences of caffeine. But have you ever wondered what the molecular effects of caffeine are? While it may seem as though this molecule is an excitatory one, it is actually an inhibitor of an enzyme found in the PKA signalling pathway.
Continue reading to find out what this signalling pathway consists of and what the molecular effects of caffeine are in our cells.
PKA signalling pathway
Since this is a complicated pathway to address, a brief explanation of the most important proteins and molecules which intervene appears below.
β-adrenergic receptor
As a G-protein coupled receptor or GCPR, the β-adrenergic receptor plays a key role in the first step of activation of the pathway. This protein binds to epinephrine, a hormone, and activates the G-protein it is coupled with. Another interesting characteristic is that the β-adrenergic receptor is a transmembrane protein made up of seven alpha helices.
Cyclic AMP (cAMP)
This odd molecule is a nucleotide of adenosine monophosphate (AMP) which has been cycled at the phosphate group. The adenine portion is shown to the right in the image above. Caffeine is a very similar molecule to adenine which is key information to understand its function.
Protein Kinase A (PKA)
Also known as cAMP-dependent kinases, this is a family of proteins which phosphorylate (add a phosphate group to) other proteins. This is an essential process which regulates many enzymes by activating or inactivating them because of the bulkiness and negative charge of the phosphate group.
In reality, this is the protein responsible for the physiological effects that were mentioned in the introduction, such as increased blood pressure. Caffeine is only one of the many molecules that interact in PKA’s regulation.
Pathway mechanism
Now we can take a closer look to the pathway. The first step is the binding of epinephrine (adrenaline) to the beta-adrenergic transmembrane receptor. Next, a trimeric G-protein, which was coupled to this receptor, is activated by binding GTP and releasing GDP. The activation causes a change in conformation whereby the β and γ subunits dissociate and the α subunit goes on to activate an adenylyl cyclase.
Adenylyl cyclase is a transmembrane enzyme which converts ATP into cAMP. Four of these cyclic nucleotides activate PKA by causing another conformation change. Finally, this kinase is free to phosphorylate other proteins so that they may function. An example is the stimulation of gluconeogenesis in skeletal muscle.
What does caffeine trigger?
If you were paying close attention, you will remember that I mentioned caffeine and adenine were similar molecules. In the image below, you can see that they both contain a five membered ring and a six membered ring, with two nitrogens in each.
Caffeine can then act as a competitive inhibitor with cAMP of another enzyme called phosphodiesterase (PDE). When there is too much cAMP, PDE breaks one of its bonds, creating AMP. This molecule no longer binds to PKA and, therefore, the effects that this enzyme had triggered are reverted.
Thus, caffeine is an enzyme inhibitor, not an activator. Since cAMP is not degraded in the presence of caffeine, the effects of high blood pressure and elevated heart rate persist.
Amplification effects
Key to this signalling process is the effect of amplification. Consider only one molecule of epinephrine were to bind the β-adrenergic receptor. In this case, there would still be G-protein activation and as soon as that trimer dissociates, another G-protein can bind to the GPCR and undergo the same reaction.
Furthermore, each activated G-protein can promote various cycles of adenylyl cyclase, meaning that there is even more amplification. Only four molecules of cAMP are required for each PKA to work, which translates to a very high number of active proteins after the binding of a single epinephrine hormone with its receptor!
Altogether, this is called a reaction cascade and it is not unique to this pathway. Blood clotting also works in a similar way with each clotting factor amplifying the signal to produce a faster response as time goes by.
Conclusion
In summary, caffeine acts as an inhibitor of PDE, which is in charge of breaking a bond in cAMP, converting it to AMP. If this reaction does not take place, there will be an increased production of cAMP due to the amplification you have seen. cAMP activates PKA which is in charge of the activation of many proteins that can alter our behaviour and make us feel more alert.
The next time you drink a cup of coffee or tea, or even a soft drink, think about the number of complex reactions that are taking place in your cells just because of a single molecule: caffeine.
Extra: Nobel Prizes for signalling pathways
If you found this explanation of signalling pathways interesting, I not only recommend you study Biochemistry, but also want to stress the fact that you are not alone.
At least five Nobel Prizes were awarded for small discoveries which have helped to create our understanding of this pathway. Although it may not have seemed like much at the time, without these significant advances, we could not be able to explain the effects of caffeine on our cells at a molecular level.
- Nobel Prize in Physiology or Medicine 1971: Earl Sutherland “for his discoveries concerning the mechanisms of the action of hormones”. In particular, he showed how cAMP was a secondary messenger in many pathways.
- Nobel Prize in Physiology or Medicine 1994: Alfred Gilman and Martin Rodbell “for their discovery of G-proteins and the role of these proteins in signal transduction in cells”. Another critical discovery that has been central to understanding multiple processes in the cell.
- Nobel Prize in Chemistry 2012: Brian Kobilka and Robert Lefkowitz “for studies of G-protein coupled receptors”. Like the β-adrenergic receptor we have seen throughout this article.
Bibliography
Lilly Pubillones
Trinity College Hartford
Biochemistry
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