Lock vs Key vs Induced Fit
Enzymes are known as biological catalysts, which are used in almost every cellular reaction, in organisms. They can increase the rate of a biochemical reaction, without the enzyme being changed itself by the reaction. Due to its reusability, even a small concentration of an enzyme can be very effective. All the enzymes are proteins and globular in shape. However, like all other catalyst, these biological catalysts do not alter the final amount of products, and they cannot make reactions to be occurred. Unlike the other normal catalyst, enzymes catalyze only one type of reversible reaction, so called reaction specific. Since, the enzymes are proteins; they can work within a certain temperature, pressure and pH range. Most enzymes catalyze reactions by making a series of ‘enzyme- substrate complexes’. In these complexes, the substrate binds most tightly to enzymes correspond to the transition state. This state has the lowest energy; hence it is more stable than the transition state of an uncatalysed reaction. Consequently, an enzyme reduces the activation energy of biological reaction, which it catalyzes. Two main theories are used to explain how enzyme-substrate complexes form. They are lock-and-key theory and induced-fit theory.
Lock-and-Key Model
Enzymes have very precise shape, which includes a cleft or pocket called active sites. In this theory, the substrate fits into an active site like a key into a lock. Mainly ionic bonds and hydrogen bonds hold the substrate in the active sites to form the enzyme- substrate complex. Once it is formed, enzyme catalyzes the reaction by helping to change the substrate, either splitting it apart or lining pieces together. This theory depends on the precise contact being made between the active sites and substrate. Therefore, this theory may not be totally correct, especially when the random motion of substrate molecules is involved.
Induced-Fit Model
In this theory, the active site changes its shape to enfold a substrate molecule. The enzyme, after binding with a particular substrate, takes up its most effective shape. Therefore, the shape of the enzyme is affected by the substrate like the shape of a glove affected by the hand wearing it. Then in turn the enzyme molecule distorts the substrate molecule, straining the bonds, and makes the substrate less stable, thus lowers the activation energy of the reaction. Since, the activation energy is low, the reaction occurs at a great speed forming the products. After the products are released, the activation site of the enzyme then returns to its original shape and, binds the next substrate molecule.
What is the difference between Lock-and-Key and Induced- Fit?
• Induced- fit theory is a modified version of lock-and-key theory.
• Unlike the Lock-and-key theory, induced-fit theory does not depend on the precise contact being made between the active site and substrate.
• In Induced- fit theory, the enzyme shape is affected by the substrate whereas, in Lock-and-key theory, the substrate shape is affected by the enzyme.
• In Lock-and-key theory, the actives sites have precise shape, whereas in Induced- fit theory, the active site initially do not have a precise shape, but later the site shape is formed according to the substrate, which is going to bind.