Signal Transduction Pathway - The cAMP Pathway

cAMP stands for cyclic-adenosine monophosphate and we all know that this is the secondary messenger. The concept as to why cAMP is the secondary messenger was first discovered by Earl Sutherland in 1958, when he discovered that the action of hormone, epinephrine, was mediated by an increase in the concentrations of cAMP. Why it is the secondary messenger? Because the first messenger is the hormone itself (here, epinephrine is the primary messenger) and the cAMP mediates the activity of primary messenger.

How is cAMP formed and then degraded? So, cAMP is formed by the action of an enzyme, adenylyl cyclase which acts on ATP (adenosine triphosphate) as can be seen in the above diagram. This cAMP is further degraded to AMP by cAMP phosphodiesterase.

cAMP has an important function that it mediates the breakdown of glycogen to glucose in muscle cells for energy. But how? This effect and some more effects of cAMP in turn is mediated by action of another enzyme, known as protein kinase A (abbreviated as PKA), also called as cAMP dependent protein kinase. Now, next question is how is PKA regulated? To understand this, first we will have to understand the structure of PKA. The enzyme PKA has four subunits. Two of the subunits are regulatory while the other two are catalytic (figure on the right side). Now, when cAMP binds to regulatory subunits, there is conformational change which leads to the dissociation of the catalytic subunits. These free catalytic subunits are enzymatically active and they phosphorylate the serine residues on the target molecules (proteins).


Now, in effect of glycogen metabolism, how does PKA regulates glycogen metabolism?
The PKA has two enzymes upon which it acts (as can be seen in the adjacent diagram). The first one is another protein kinase called, phosphorylase kinase. This kinase is phosphorylated on serine residue by PKA; which then activates glycogen phosphorylase. The glycogen phosphorylase is an enzyme in glycogenolysis or glycogen breakdown pathway and thus is responsible for breakdown of glycogen to glucose-1-phosphate. Another enzyme which PKA phosphorylates is glycogen synthase. The glycogen synthase is the enzyme which is responsible for glycogen synthesis. The point to note here is that the phosphorylation of glycogen synthase inhibits its enzymatic activity. Hence, from here, we can infer that, the increase in cAMP levels results in the activation of PKA and this stimulates glycogen breakdown and at the same time, also inhibits glycogen synthesis.

As already mentioned, there are many other effects of cAMP. Lets have a look at another example. When there is an increase in cAMP levels, there is activation of transcription of various target genes in many animal cells. These target genes are known to contain the specific regulatory sequence called as the cAMP response element, generally abbreviated as CRE (diagramatic representation on the right side). Again, as described above, when cAMP binds to regulatory subunit of PKA, the catalytic subunit is released which carries the signal from cytoplasm to nucleus. Within the nucleus this activated PKA phosphorylates a transcription factor called CREB (CRE binding protein) at serine residue. This in turn recruits various co-activators and transcription of cAMP inducible genes takes place. This regulation of gene expression plays an important role in various processes like proliferation, differentiation, survival etc.

It is important to note that protein phosphorylation needs to be balanced which is done by the activity of protein phosphatases. Some protein phosphatases are transmembrane receptors while others are cytosolic. These phosphatases remove the phosphate group from tyrosine or serine or threonine residues in the substrate proteins thereby terminating the responses which is initiated by the activity of protein kinases. Lets make it more clear by taking the example of protein kinase A. The serine residues of the target proteins (phosphorylase kinase, CREB) which are phosphorylated by PKA are usually dephosphorylated by phosphatase called as protein phosphatase 1. Thus, the levels of phosphorylation of the target proteins which are phosphorylated by PKA is counter-balanced by the activities of protein phosphatases (diagram of regulation of phosphorylation by PKA and protein phosphatase 1 shown below).

Just note that although most of the effects of cAMP are mediated by PKA. However, cAMP can also directly regulate ion channels with no requirement of protein phosphorylation.