Publisher Summary

The aims of biochemistry are to describe the nature of living forms and living processes in terms of chemistry and physics. Biochemists believe that the existence and activities of living organisms are explained on the basis of the interaction of their component molecules. These are divided very broadly into two groups, namely, small molecules and macromolecules. The main types of macromolecule are proteins, nucleic acids, and polysaccharides, all of which are long, chain-like molecules built up from a large number of linked subunits. These biopolymers tend to associate into still larger complexes with other molecules that may or may not be of the same type. The small molecules act not only as the building units from which macromolecules are synthesized but also as sources of energy, messengers, and regulators. Macromolecules are the essential basis of the elaborate structures in and around which the life processes occur; they also control and regulate these processes. Thus, macromolecules are responsible for the energy exchanges and chemical reactions that comprise metabolism for the irritability that enables the organism to respond to changes in its environment, for mobility, and for reproduction. The aims, attitudes, and techniques of biochemistry are as relevant to dentistry as to medicine or any other aspect of biology. Only when the normal structures of the mouth and their development and reactions are understood is it possible to appreciate the true nature of dental disease. All disease has a biochemical basis regardless of whether its origin is nutritional or genetic or it is caused by an infectious or toxic agent.

The scope of biochemistry

The aims of biochemistry are to describe the nature of living forms and living processes in terms of chemistry and physics. Biochemists believe that the existence and activities of living organisms can be explained on the basis of the interaction of their component molecules. These may be divided very broadly into two groups, namely small molecules and macromolecules.

The main types of macromolecule are proteins, nucleic acids and polysaccharides, all of which are long, chain-like molecules built up from a large number of linked subunits. These biopolymers tend to associate into still larger complexes with other molecules which may or may not be of the same type. The small molecules act not only as the building units from which macromolecules are synthesized but also as sources of energy, messengers and regulators. Macromolecules are the essential basis of the elaborate structures in and around which the life processes occur; they also control and regulate these processes. Thus macromolecules are responsible for the energy exchanges and chemical reactions that comprise metabolism, for the irritability which enables the organism to respond to changes in its environment, for mobility and for reproduction.

Since molecules do not function alone but by interaction with other molecules, living processes require specific arrangements of molecules and ‘life’ resolves itself into a question of molecular organization.

This organization is based on certain general principles, e.g.:

In the higher forms of life biological organization falls into a series of levels arranged in a discontinuous order. At each level there appear to be units of a fairly definite size which become associated to form a unit at the next level, and as each successive level of organization is achieved new properties emerge. The levels may be listed as follows:

For the most part, biochemical studies are concentrated at the levels of molecules, subcellular structures and cells, but, in order to explain ‘life’ in chemical and physicochemical terms every level of organization must be studied and it is where discontinuities exist that the most challenging problems occur. For example, how do cells differentiate? What makes cells of the same and different types associate to form tissues? Why do cells, organs and organisms grow to a certain size and then stop growing? Why are partially constructed subcellular components, e.g. mitochondria, membranes, cilia etc., never seen within cells? Such structures are either present in their completed form or not present at all. This suggests that assembly of the components into the single appropriate configuration depends on the existence of a precise set of conditions under which they fall into their proper place in the right numbers and orientation for the assembly of the complete structure.

Biological organization therefore seems to require the provision of a suitable environment where the individual molecules can interact in such a way that they become specifically orientated. The mitotic spindle is formed in just this way since it appears spontaneously in response to a set of critically determined environmental conditions. Another example is seen in the formation of collagen fibres, where a self-ordered fabric is produced by the mutual interaction of collagen molecules (Chapter 27). Furthermore molecular organization depends on appropriate molecular architecture since, for molecules to interact, they must be suitably designed. In fact, natural selection has ensured that biological macromolecules are uniquely fitted for their functional role. The simplest function of a macromolecule is perhaps the storage of energy. A substance which fulfils this role in the animal world is glycogen, a biopolymer of glucose. To serve its purpose the structure of glycogen does not need to be very precisely defined provided that it can be degraded rapidly when extra energy is required. It is significant, therefore, that the molecular weight of glycogen is variable and that it possesses a highly branched structure, which means that the molecule is vulnerable to enzymic attack at many points simultaneously (Figure 7.2).

Proteins, the most ubiquitous of the biological macromolecules, perform an enormous variety of functions, and have structures which are not only complicated but also highly specific. They are composed of a large but definite number of amino acid units selected from twenty or so different types joined together in a specific sequence. Although the protein molecules are linear and unbranched the properties of their amino acid side chains are such that highly specific interactions occur. These take place between different protein molecules so that they are able to associate to form sheets and bundles and also between different parts of the same molecule so that it may assume an elaborately folded configuration. As a consequence, protein molecules have well-defined geometrical shapes, and as a result of ionization effects they also have characteristic patterns of electric charge. Such patterns are believed to play an important part in their interaction with other molecules.

Proteins fulfil both structural and metabolic functions within the organism. The requirements for structural proteins are that they should be insoluble, chemically stable and possess a rigid structure capable of orientation. The long chain-like molecules of structural proteins are usually only slightly folded and have specific bonding sites so that the molecules can join together both in series and in parallel to form molecular aggregations with great tensile strength and varying elasticity. Muscle proteins are able to slide over one another so that the fibres can shorten or lengthen in response to appropriate stimuli.

Even more complex and varied in their structure and functions are the enzymic proteins which are designed for the accomplishment of molecular changes. To achieve these they must have structures that are closely related to those of the molecules with which they interact. Here, where very delicate mechanisms are involved, large stable aggregations of protein molecules are not appropriate, but the association of a small number of subunits may give the molecule flexibility and enable it to react to its environment.

Molecular biology defines the area where structure and function meet since it includes the enzymic, aggregational or genetic information which macromolecules contain by virtue of their structure. It is in the preservation of genetic information that the nucleic acids come into the picture since it is they that carry the coded information which ensures the supply of specifically designed macromolecules needed for the organization of subcellular structures, cells, tissues, organs and organisms which enable cells to replicate.

Biochemistry in relation to dentistry

The aims, attitudes and techniques of biochemistry are as relevant to dentistry as to medicine or any other aspect of biology. Only when...