MECHANISM OF ENZYME ACTION
In this article, we shall learn and understand the mechanism of enzyme action. There are two models that explain the mechanism of enzyme action. 1 Lock and Key model and 2. Induced fit model.
Factors affecting the rate of enzyme action
Enzymes are very sensitive to the environment in which they are performing their functions. The activity of an enzyme is affected by any factor that can change its shape or structure. Some factors that can bring change in the action of enzymes are given below;
- SUBSTRATE CONCENTRATION
- INHIBITORS OR ACTIVATORS
- Substrate concentration
The rate of chemical reaction increases with the increase of enzyme molecules present in a particular reaction if the substrate concentration increases. But if the concentration of substrate increases by keeping the concentration of enzyme constant, a point reaches where any further increase in substrate does not increase the reaction furthermore. The reason is that the active sites of all enzymes are occupied by substrates which may present in very high concentration. In this state, any single substrate molecule does not find free active sites. This state is called saturation of active sites and the reaction rate does not increase.
The rate of enzyme action increases with the increase in temperature but up to a specific limit. Every enzyme works at a maximum rate at a specific temperature which is called the optimum temperature for that enzyme. For example, the optimum temperature for some mammals and birds is 37 and 40 degrees centigrade respectively, regardless of the external environmental temperature.
When the temperature increased to a certain limit, the heat adds in the activation energy and also provides the kinetic energy for the reaction. So reactions are accelerated. But when the temperature of the enzyme increases well above the optimum temperature, heat energy increases the vibration of the molecules of the enzyme. In this condition the molecules of the enzyme will start to leave their fixed position, that’s why the globular shape or structure of the enzyme will destroy. This is called denaturation of the enzyme. It results in a rapid decrease in the rate of enzyme action or it may block the enzyme’s function thoroughly.
Every enzyme performs its maximum function at a narrow range of PH, known as optimum PH. A little change in the PH results in retardation in enzyme action or stops its activity completely. Every enzyme has its specific optimum value. For example, trypsin works in the medium of high PH (alkaline), and pepsin works in the medium of low PH (acidic). Any change in PH can affect the ionization of the amino acids present on the active sites of the enzyme.
- Effect of activators
Some of the enzymes require certain inorganic metallic ions (cations) Such as Mn2+, Mg2+, Co2+, K+, and Cu2+, etc. A few enzymes also require anions like chloride ions or amylase. Changes in these ions can change enzyme activity.
- Effects of inhibitors
Enzyme inhibitors are substances that can alter the catalytic behavior of enzymes and causing a decrease in, or in some cases stop catalysis. Generally, there are three common types of enzyme inhibition. Competitive inhibition, non-competitive inhibition, and substrate inhibition.
This type of inhibition takes place when the substrate and substrate resembling substance are both added to the enzyme. In order to demonstrate why inhibition occurs, a model termed,” Lock and Key” of enzyme action is used. The Lock and Key model normally explains the concept of a particular part of the enzyme “the active site”. It states that such a particular part of the enzyme has a strong affinity for the substrate. The substrate is held in such a way that its conversion to the reaction products is more suitable. When an inhibitor resembles the substrate, it will compete with the substrate for the active site of the enzyme. If the inhibitor excels, it occupies the active site but is unable to open the lock. So the reaction becomes slows down because the active site is occupied by the inhibitor. When a different substance that does not exactly fit the active site is attached, the enzyme does not accept it and reacts with normal substrate and the reaction proceeds.
Non- competitive inhibitors:
Non- competitive inhibitors are assumed to be the substrate which when added to the enzyme alters the enzyme in such a way that it cannot accept the substrate.
Substrate inhibition sometimes occurs when there is a large amount of substrate is present. An excessive amount of substrate added to the reaction mixture after this point generally decreases the rate of reaction. It is due to the fact that there are so many substrates that compete with the active sites on the enzyme surface that cover the site and prevent any further substrate from occupying them.
SPECIFICITY OF ENZYME ACTION
The specificity is the capability of an enzyme to select a correct substrate amongst a group of similar molecules. The specificity is actually a molecule recognition process and it operates through the structural as well as conformational complementarity between substrate and enzyme. For example, the enzyme protease which breaks down peptide bonds in proteins is not able to bind with starch, which is broken down by another enzyme amylase. The specificity of enzymes is generally determined by the shape and structure of their active sites.
- MECHANISM OF ENZYME ACTION
During the mechanism of enzyme action, the enzyme combines with its substrate, so a temporary enzyme-substrate (ES) complex is formed. Enzyme transforms the substrate into products at the end of catalysis. After the reaction, the enzyme-substrate complex breaks and the enzyme and substrate become separate from each other. There are two models which explain the mechanism of enzyme action. (1) Lock and model and (2) Induced fit model.
- Lock and Key Model
This model of the mechanism of enzyme action was presented by German Chemist Emil Fischer in 1894. According to this model, both enzymes and substrate possess specific shapes that fit incorrect orientation into one another. This model demonstrates the specificity of enzyme action.
- Induced Fit Model
This model of the mechanism of enzyme action was demonstrated by American biologist Daniel Koshland in 1958. This model suggests a modification to the lock and key model. Induced fit model explains that the active site of the enzyme is not a solid or rigid structure, but rather it is flexible in nature and has the ability to mold into the required shape, so that it may perform its function properly. This model is more acceptable than the lock and key model.