For example, the binding of a single molecule (such as glycogen or epinephrine) at the cell surface can activate many effector G proteins and an adenylyl cyclase each of which can produce a large number of cAMP messengers in a short period of time. Thus, the production of a second messenger provides a mechanism to greatly amplify the signal generated from the original message. There are many steps in the reaction cascade, amplification of the signal via cAMP molecules which activate protein kinase K which involve in phosphorylation of Ser, Thr and tyrosine of target protein. PKA is tertameric protein which is made up of two catalytic and two regulatory subunits. Binding of cAMP to the regulatory subunits induces a conformational change that leads to dissociation of the catalytic subunits, which elicit formation of enzymatically active form of protein kinase A, are now able to phosphorylate Ser and Thr residues on their target proteins. In signal amplification, each PKA catalytic subunit phosphorylates a large number of phosphorylase kinase molecules, which in turn phosphorylate an even larger number of glycogen phosphorylase molecules, which in turn can catalyze the formation of a much larger number of glucose phosphates. Thus, what begins as a hardly noticeable stimulus at the cell surface is rapidly transformed into a major mobilization of glucose within the cell.

Figure 8: Signal amplification Pathways for Glycogen degradation

The different effect of some hormone in different tissues in shown in the following table:

Table 1: Example of hormone induced response mediated by cAMP

In the upcoming chapter we will study about each component of signal transduction in detail.

 

Interesting Facts:

1. Most signaling molecules are found in such low concentration that their effect in cytoplasm would be minimal unless the signal is amplified.
2. Most enzymes linked and G-protein linked receptor use a chain of other protein messenger to amplify the signal as it being relayed.