In 1966 D. Koshland presented the hypothesis of enzyme mechanism of action. The obsession doctrine can also be called a modified version of the lock-and-key doctrine. In this theory, the structure of the enzyme is variable. Such enzymes are called allosteric. The obsessional doctrine is discussed as follows:
A special type of protein is an enzyme. The site of apoenzyme where the substrate binds and causes the reaction is called the active site or reaction center. Enzymes contain one or more active sites. The size of the substrate and the size of the enzyme’s active site can vary. According to induction theory, the substrate does not require a specific conformation or affinity to bind to the enzyme’s active site. Rather, the binding of the substrate molecule requires some modification of the active site of the enzyme. Such modification makes the active site fit for the substrate. As a result, the substrate binds to the active site of the enzyme. The enzyme assumes its maximal catalytic shape after binding to the substrate. The substrate and enzyme are then joined by hydrogen bonding to form an enzyme-substrate complex. Once the enzyme-substrate complex is formed, the enzyme readily cleaves the substrate. At the end of the reaction, the product is freed from the bond and moved away and the enzyme is released unchanged. The free enzyme takes part in the new reaction.
The obsession theory has gained acceptance among many scientists. In support of this model, X-ray observations of carboxypeptidase-A and several other enzymes are presented
[According to D.Koshland, the active site of an enzyme consists of two parts. buttracing groups and catalytic groups. The buttracing group holds the substrate and the catalytic group helps to weaken various substances in the substrate to reactants]