Special Features of Biochemical Reactions

Although biochemical reactions are only a small part of the countless variants of chemical reactions, their special features are unique enough to discuss at least some outstanding aspects. Otherwise, it would be difficult to understand the functionality of modern biosensors. 

A characteristic feature of a bio chemical reaction is the participation of large organic molecules, polyelectrolytes in most cases. Preferably such molecules belong to the group of proteins, but other molecules also play an important role, e.g. the nucleic acids. These natmal molecules are not randomly formed heaps of molecular building blocks but weU-structmed, highly complex molecules which perform many complicated operations very precisely. Some of these functions are of particular interest for biosensors. They will be discussed briefly in the following sections. 

Enzymes are biocatalysts with an extremely high selectivity. Their molecules are protein molecules with a molecular mass between 10 to 10 Da. Enzymes work under mild conditions, i.e. at room temperature or slightly above and at near-neutral pH. Biosensors with enzymes generally contain a layer of enzyme molecules immobilized at the sensor surface. This layer is able to catalyse just one reaction with a definite biologically active substance. The latter is recognized and determined specifically in this way. 

The most important feature of enzyme molecules is their specific three-dimensional configuration with a molecular cavity including an active site which is suitable for a special sort of substrate molecule. At this site, the reaction of the substrate molecule during the formation of a product takes place. The enzyme molecule recognizes the substrate sterically, i.e. by following the lock-and-key principle. The active site makes up only a small part of the overall molecular volume. Its primary function is to stabilize the activated complex, i.e. the molecular transition state which is formed between enzyme and substrate 

Enzyme-catalysed reactions follow the general scheme 

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