The nature of isoenzymes.

Abstract

Many enzymes are known to exist in multiple molecular forms; in fact, it is now believed to be the exception rather than the rule for an enzyme to exist in only one fornm (Kaplan, 1968). The term 'isoenzyme' has been applied to each of these forms and, for the time being, it has been decided to retain a broad definition of isoenzyme such as 'one of a series of different proteins with similar enzymatic activity'. It is likely that eventually it will be established that the multiple forms of enzymes are merely a special example of the structural variations that occur in proteins generally, of which haemoglobin is a well known example, but until more is known of the molecular structure of isoenzymes, it would be unwise to adopt too narrow a definition. Some authorities have limited the term to the multiple forms of an enzyme which are all present in one tissue or organ of an individual plant or animal or in a culture of a unicellular organism. This narrow definition would not embrace the multiple forms of alkaline phosphatase (EC 3.1.3.1) which are found in many different tissues, but which are nevertheless customarily referred to as isoenzymes. The broad definition above also includes such examples as cytoplasmic and mitochondrial malate dehydrogenase (EC 1.1.1.37) although these have been regarded as entirely separate enzymes, each with its own isoenzymes; however, hybrids of these two enzymes have been prepared in vitro (Chilson, Kitto, Pudles, and Kaplan, 1966). The mitochondrial and cytoplasmic forms of aspartate aminotransferase are also very different in their properties. As has already been indicated, however, such difficulties will undoubtedly be resolved when the molecular composition and structure of the various 'isoenzymes' has become known. It is now common knowledge that proteins are composed of one or more polypeptide chains and that their molecular structure can be subdivided, into primary, secondary, tertiary, and quaternary components. The primary is of course the aminoacid sequence, which is also the final arbiter of the eventual shape of the molecule. The secondary structure is the occurrence within the polypeptide chains of alpha helices, the rigid parts of the chain, and the looser bent sequences-largely, but by no means entirely-determined by the presence of proline. The tertiary component, or final shape of a polypeptide, is brought about by the three-dimensional bending of the secondary form by forces such as hydrogen bonding, internal interactions of hydrophobic groupings, electrostatic bonding, van de Waals interactions, disulphide bridges, and the like. In accordance with thermodynamic considerations, there is little doubt that the final form is that structure with minimal energy content. Most proteins contain more than one polypeptide and the combination of these makes up the quaternary component of structure. It is now accepted that each polypeptide is synthesized as a result of a message initiated in a cistron, often loosely referred to as a gene, and thus in one sense the old adage 'one gene, one enzyme' is incorrect, since the quaternary structure of an enzyme is frequently made up of different polypeptides, which individually have no enzymatic activity. Hence genetic considerations are capable of explaining the existence of a number of different types of isoenzymes.