Structure and Communication Semiochemicals

In this chapter we have stressed the importance of being able to predict the three-dimensional structure of a molecule. Molecular structure is important because of its effect on chemical reactivity. This is especially true in biological systems, where reactions must be efficient and highly specific. Among the hundreds of types of molecules in the fluids of a typical biological system, the appropriate reactants must find and react only with each other—they must be very discriminating. This specificity depends primarily on structure. The molecules are constructed so that only the appropriate partners can approach each other in a way that allows reaction. 

Molecular structure is also central for those molecules used as a means of communication. Examples of chemical communication occurring in humans are the conduction of nerve impulses across synapses, the control of the manufacture and storage of key chemicals in cells, and the senses of smell and taste. Plants and animals also use chemical communication. For example, ants lay down a chemical trail so that other ants can find a certain food supply. Ants also warn their fellow workers of approaching danger by emitting certain chemicals. 

Molecules convey messages by fitting into appropriate receptor sites in a very specific way, which is determined by their structure. When a molecule occupies a receptor site, chemical processes are stimulated that produce the appropriate response. Sometimes, receptors can be fooled, as in the use of artificial sweeteners—molecules fit the sites on the taste buds that stimulate a sweet response in the brain, but they are not metabolized in the same way as natural sugars. Similar deception is useful in insect 

A semiocbemical is a molecule that delivers a message between members of the same or different species of plant or animal. There are three groups of these chemical messengers allomones, kairomones, and pheromones. Each is of great ecological importance. 

Antibiotics are also allomones, since the microorganisms produce them to inhibit other species from growing near them. 

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Many of the proteins in scent marks are members of the lipocalin superfamily, characterised by their distinctive barrel shaped structure and central ligand binding pocket. Given the well established role of volatile compounds in olfactory communication (Buck and Axel, 1991), the function of semiochemical proteins may be to modulate scent signals by interaction with volatile components. Thus, a further step in characterising a novel scent secretion protein may be to identify and characterise any associated ligands. 

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