**5. Uniqueness of signals**

54 Biomedical Science, Engineering and Technology

However, there are many situations where a molecule needs to be present, or released, in a rapid manner. Cyclic AMP (cAMP) for example is needed in response to adrenaline (for an example of a signalling pathway in which cAMP is involved see Figure 2) while insulin is released in to the blood stream when required. One of the main underlying principles of signalling is that the system is able to convey the message when and where required, in a temporal manner appropriate to the required response. Therefore molecules need to be able

There are two main ways to make a signal. Either the molecule is produced when required, or it is made and stored, to be released when required (Figure 3). It a similar manner it could be argued that there are two main ways to listen to music. You either go to a concert and in the presence of the musical instruments you listen to the sounds being made, or you let the band record the music, store it until required and then play it. In this scenario the instruments are the enzymes, producing the message. At the concert instruments make the signal as needed, to the required amount, for the required time. This is just like an enzyme such as adenylyl cyclase which is turned on, generates cyclic AMP (cAMP) for a set period of time, and then turns off. Just like the person at the concert, the protein responding to cAMP can perceive its presence

Fig. 3. Signals can be generated using different scenarios. In (A) there is no signal produced until an enzyme is activated. At that point a compound X and be converted to compound Y, so generating a signal. That is Y can now be recognised and a response or effect produced due to its presence. In (B) the signalling molecule can be pre-made but sequestered into vesicles. On arrival of the appropriate stimulus the vesicles will translocate to the plasma membrane for example. The signalling molecule will then be released and be able to move to its site of recognition and action. In the case of hormones the site of perception may be a different organ or tissue, with the signalling compound being carried by the vascular system of the organism.

to partake in such signalling when called upon to do so.

and when it is all over revert back to a quiescent state – concert over.

Signals used by cells have to be specific and often are unique so that their presence does not get confused with another. If a signalling molecule needs to provoke a particular response it is vital that the cell's machinery recognises the presence of that specific molecule. If there was doubt then the cell may mount a response in the presence of the wrong molecule. A good example here is role of the molecules cAMP and cyclic GMP (cGMP). As can be seen in Figure 4, at first glance both these compounds look very much the same. Both have a ribose ring, a cyclic phosphate group and an added base unit.

However, the base is different in each and therefore a cell can recognise them as different. Indeed, different enzymes make them, adenylyl cyclase produces cAMP from ATP and guanylyl cyclase makes cGMP from GTP. Downstream they are recognised as separate compounds too. cAMP controls protein kinase A, while cGMP controls amongst other things protein kinase G (Figure 5).

Musical instruments are the same. Take a quick glance at a viola, and then at a violin, and they look the same. They have the same basic shape and the same basic parts. But they are different. In an orchestra they will play different music, at different times perhaps, but what is important here is just like cAMP and cGMP, a violin and a viola have their own distinct roles and parts to play in the construction of the whole, despite the fact that they are outwardly so similar.

There are occasions when cAMP and cGMP can have similar activities, and in some cases both are removed by the same phosphodiesterase. Does this mean that our analogy breaks down? Perhaps not, as if pushed different string players can pick up alternate instruments and allow the orchestra to continue. If a violin player gets ill, a viola player can often step up to the breach to fill the gap.

Fig. 4. Structures of signalling molecules can be very similar and yet unique. Here, the structures of cAMP and cGMP are given as examples, but they have very different signalling roles, controlling different proteins for example, that is, cAMP-dependent protein kinase and cGMP-dependent protein kinase respectively.

### **6. Domains and common features**

In the discussion above it was argued that instruments may be similar but unique. However, there are often quite diverse instruments which share common mechanisms or structures. In signalling many proteins may also share common structures, with those structures having similar roles within the protein. A good example here is the EF hand (Lewitt-Bentley & Rety, 2000), which binds and causes a conformational change in a protein in response to changes in the levels of calcium ions in cells. EF hands can be found in a calcium controlled kinases which are able to phosphorylate downstream proteins, but EF hands are found in a wide range of other proteins too, for example the DUOX proteins involved in reactive oxygen species metabolism (Lambeth *et al*., 2007). Although there might be subtle differences in the EF hands in different proteins the structural domain is still identifiable as being an EF hand, having the same basic function but being involved in a proteins which when taken as a whole have different functions in the cell.

Fig. 5. cAMP and cGMP pathways are very similar. As well as cAMP and cGMP having very similar structures, if drawn in a simplistic view as shown here the signalling pathways in which they are involved is also very similar. Both are produced by cyclases, both are perceived by protein kinase, and in fact both are removed by phosphodiesterases. Both can have other effects and if viewed in more detail there are significant differences in their pathways. Importantly, cAMP and cGMP are involved in specific signalling, despite the similarities. PKA: cAMP-dependent protein kinase; PKG: cGMP-dependent protein kinase.

Therefore many signalling proteins have domains which are similar to each other and to continue our analogy musical instruments are often the same. An idea is repeated, but perhaps has a slightly different role. Consider keyboards on pianos and organs. The idea of having a set of keys which can be pressed can be found on a whole range of instruments, including different types of piano, electric keyboards, organs, harpsichords, accordions, melodicas and many others. Beyond the keyboard the mechanisms may be very different. A piano uses hammers to strike the strings, an organ opens valves to control air into pipes and so on. But the keyboard is a common feature or common structure, like the domain of a protein. And just like the EF hands discussed above, the keyboards in these different instruments may be subtly different, but they are still recognisable as being keyboards despite the overall instruments being quite different, both in shape and the type of music they play.
