Chemistry and Biology of Proteins - F. Haurowitz 1953

Proteins with Enzymatic Properties
Functions of Proteins as Enzymes and Apoenzymes

It is not the purpose of this book to provide a comprehensive Overview of all Enzymes or to cover the mechanisms of catalysis and the Kinetics of Enzymatic reactions in detail. The subject of enzymes is touched upon here solely because all enzymes are proteinaceous substances. This fact was definitively established only recently. For a long time, it was believed that enzymes were merely adsorbed onto Proteins. Willstätter and his coworkers spent a long time attempting, unsuccessfully, to separate enzymes from proteins. Later, Sumner [1] and Northrop [2] demonstrated that urease and Pepsin (two typical enzymes) are true proteins; they succeeded in crystallizing both enzymes and proving that their catalytic activity is inseparable from the protein matrix. Since then, many Other Enzymes have been obtained in crystalline form, all of which have likewise proven to be proteins [3].

Enzymes may be defined as macromolecular catalysts of biological origin. This definition does not explicitly mention the protein nature of enzymes, and it is indeed conceivable that some enzyme, not yet isolated in pure form, might turn out to be non-proteinaceous. To date, however, no such enzymes have been discovered, and we are therefore justified in defining enzymes also as catalytically active proteins.

It has long been known that certain enzymes contain catalytically active prosthetic groups. The activity of these enzymes, however, also depends on their protein components, as evidenced by the sharp drop in their catalytic activity upon heating or exposure to other factors that induce Protein Denaturation. Certain enzymes (such as Ribonuclease) are relatively heat-resistant. This is because their protein component resists denaturation [4]. Since most Globular proteins are readily denatured, the thermal stability of ribonuclease and certain other similar enzymes indicates that their Cell/13.html">Protein Structure is more rigid and stable than that of ordinary globular proteins. It is sometimes very difficult to determine whether a protein has been denatured or not. It is well known that most proteins become insoluble upon denaturation. However, insolubility is not always a reliable indicator of denaturation, as denatured proteins are frequently kept in solution by other native proteins present in the medium, protective colloids (such as gelatin), or certain organic anions. On the other hand, The addition of ethyl alcohol can render certain proteins insoluble without impairing their enzymatic activity; for instance, urease treated with alcohol becomes irreversibly insoluble yet retains its catalytic activity [5]. Similarly, The formation of monomolecular films does not affect the enzymatic activity of Proteolytic Enzymes [6], urease [7], and invertase [8]. Evidently, the active sites of these enzymes remain unaltered when the protein forms a film at the Water's surface. Since enzymes are themselves proteins, it is only natural that their enzymatic activity should be diminished by the action of proteolytic enzymes [9, 10]. In this case, only the undigested portion of the enzyme retains its catalytic activity.

According to some researchers, the inhibitory effect of monochloroacetic acid or other halogenated acetic acid derivatives on enzymes suggests that their catalytic Functions are associated with The sulfhydryl groups of the protein component, since sulfhydryl groups are known to react with halogenated acetic derivatives. It must be noted, however, that amino groups and possibly hydroxyl groups also react with these same acetic acid derivatives, In addition to sulfhydryl groups [11]. Therefore, these experiments do not warrant any definitive Conclusions regarding The Nature of the groups responsible for catalytic activity.