1.5. Bioreceptor Molecules
Enzymes have been the most widely used bioreceptor molecules in biosensor applications. Recently, antibodies and protein receptor molecules are increasingly incorporated in biosensors. The specificity of a biosensor comes from the specificity of the bioreceptor molecule used. An enzyme is a good example. It has a three dimensional structure that fits only a particular substrate (Fig. 1.9a). An enzyme is a protein synthesized in the cell from amino acids according to the codings written in DNA. Enzymes act as catalysts for biochemical reactions occurring in the cell. To maintain high enzyme activity, the temperature and pH of the environment have to be maintained at proper levels.
Antibody. Antibodies represent one of the major class of protein; they constitute about 20% of the total plasma protein and are collectively called immunoglobulins (Ig). The simplest antibodies are usually described as Y-shaped molecules with two identical binding sites for antigen. An antigen can be almost any macromolecule that is capable of inducing an immune response. The antibody has a basic structural unit consisting of four polypeptide chains - two light chains and two heavy chains (Fig. 1.9b). The antibody binds reversibly with a specific antigen. Unlike the enzyme proteins, the antibody do not act as catalysts. Their purpose is to bind foreign substances - antigens - so as to remove them from the system.
Receptor Protein. Receptor proteins are protein molecules having specific affinity for hormones, antibodies, enzymes, and other biologically active compound. These proteins are mostly bound to membrane (Fig. 1.9c). There are hormone receptors, taste receptors, olfactory receptors for smelling, photoreceptors for eyes, etc. Receptor proteins are responsible for opening and closing of membrane channels for transport of specific metabolites.
Other Possibilities. In principle, any biomolecules and molecular assembly that have the capability of recognizing a target substrate (= the analyte) can be used as a bioreceptor. In fact, membrane slices or whole cells have been used in biosensors. Fig. 1.10 summarizes possible bioreceptors that can be utilized in a biosensor. Note that the bioreceptors require suitable environment for maintaining their structural integrity and biorecognition activity. These requirements are described in Fig. 1.10 along with the type of signal generated as a result of the biorecognition activity. The transducer in a biosensor has to be capable of measuring this signal.