Cells are not only the “smallest units of life” but they also depict “the best machineries in the world”. Cells not only work exceptionally well within their own systems but coordinate efficiently with other cells in order to ensure their survival. Reactivity, which refers to the living thing’s ability to respond to signals from the environment; enables the organism to react to various stimuli, either behaviorally or physiologically (The Characteristics of Life).
Signals come in various forms such as changes in temperature, illumination, pressure and even intercellular chemical concentration. Other signals which take subtler forms are those which signal either cell division or cell differentiation. Protective responses are also carried out by most cells whenever foreign substances intrude the organism’s body. Such feats are only possible through signaling systems which are either intracellular or intercellular (The Cell Signaling Test) and are generally referred to as signal transduction pathways.Signal transduction figures in information metabolism. Information metabolism refers to the cell’s reception, processing and consequent response to information from the environment (Signal Transduction Pathways: An Introduction to Information Metabolism). At the cellular level, signal transduction refers to the movement of signals from the outside to the inside of the cell. The process through which signal transduction occurs varies depending upon the nature of chemicals and their associated receptors.
Signal movement can be as simple as those which involve the receptor molecules of the acetylcholine class or can be as complex as those which involve combinations of ligand – receptor interactions and several intracellular events. In the case of simple signal transduction, signals pass in or out of the cell through receptors which are mainly composed of channels. The process is effected out by ligand interaction through the form of small ion movements. These ion movements in turn result to changes in the cell’s electrical potential and consequently, to the propagation of the said signal along the cell (Mechanisms of Signal Transduction). Complex signal transduction, as mentioned before, does not only involve ligand interactions but also several intracellular events. These events include phosphorylations induced by the enzymes tyrosine kinases and/or serine/threonine kinases. Protein phosphorylations are specifically helpful in understanding the process of gene expression since they influence, to a large extent, enzymatic activities and protein conformations (Mechanisms of Signal Transduction).
Pathways of signal transduction could therefore be depicted as molecular circuits. A pathway typically begins with the transfer of information from the environment towards the cell’s internal system. Nonpolar molecules (steroid hormones such as estrogen) could easily penetrate the cell membrane and could therefore, enter the cell. Once inside the cell, these molecules bind to proteins and interact with the cell’s DNA.
The consequence of this interaction is that the molecules’ directly influence gene transcription. Other molecules, such as those which are too large or too polar, are not able to pass through the cell’s membrane. The information that these molecules carry must then be transmitted to the cell’s interior through another means. Such means is accomplished through the action of a membrane – associated receptor protein. A receptor of such type has often two domains: an extracellular and an intracellular domain. The former contains a binding site to which a ligand or a signal molecule is recognized. The interaction between the receptor and ligand consequently alters the receptor’s tertiary or quaternary structure as well its intracellular domain.
Though such structural changes have been effected, the small number of receptor molecules in the cell membrane limits, or to a certain extent, hinders the yielding of an appropriate response. Ligands or primary messengers therefore, carry information which is transduced into forms which can either modify or influence the cell’s biochemistry.Second messengers comprise the next step in the transduction pathway. These messengers relay information from the receptor – ligand complex through changes in their concentration. Changes in concentration amplify the signal’s effect and influence, to a large extent, intracellular signal and response. Protein phosphorylation is another means of information transfer.
Responses are elicited by the activation of enzymes such as protein kinases. These enzymes transfer phosphoryl groups from ATP to certain protein residues such as serine, tyrosine and threonine through the process of phosphorylation. cAMP-dependent protein kinase as well as other protein kinases are links that transduce changes in the free second messenger concentration into alterations in protein covalent structures. These changes are less transient compared to secondary – messenger concentrations but the results of such process are reversible (Signal – Transduction Pathways: An Introduction to Information Metabolism).Once the signal has been initiated and has been transduced to affect other cellular processes, it is equally important that the signal is terminated effectively. Signal termination is mostly carried out by the action of protein phosphatases. Whereas protein kinases are responsible for phosphorylation (Signal – Transduction Pathways: An Introduction to Information Metabolism), protein phosphatases are responsible for dephosphorylation (Protein Phosphatases).
Dephosphorylation refers to the removal of a phosphate group (Dephosphorylation). The importance of this phase in the signal transduction pathway is clearly manifested when the process goes awry. If the effects of a previous signal are still in effect, the cell could not respond to new and incoming signals. Cancers and uncontrolled cell growth are often the offshoot of the failure in this phase of the pathway (Signal – Transduction Pathways: An Introduction to Information Metabolism).