Mechanism of Enzyme Action


Enzymes are proteinaceous molecules that help in catalyzing the biochemical reactions in our body. Due to this property, they are also known as biocatalysts. As they are proteinaceous in nature, they also possess secondary and tertiary structures. When the enzymes are present in their tertiary structure, their protein chains get folded upon themselves and due to this many crevices are formed that are termed as active. These active sites are mainly responsible for the mechanism of enzyme action and also the mechanism of enzyme catalysis. 

Enzyme Action

To understand the enzyme mechanism and enzyme action two hypotheses were proposed. They were:

1. Lock and Key Hypothesis: This hypothesis was proposed by Emil Fischer in 1894. This hypothesis helps us to understand the mechanism of action of enzymes. According to this hypothesis, the enzyme and substrate molecules exhibit various geometrical shapes and these shapes are very specific. Just as the lock and key model, this hypothesis states that the active sites of enzymes act as a lock that has specific molecules such as -COOH, -SH. These enzyme molecules can only be opened with the help of specific substrate complexes. This hypothesis explains the specificity of enzymes and also the mode of action of enzymes. The substrate comes in contact with the active site of the enzyme complex and then forms an enzyme-substrate complex. When this complex is formed, then it undergoes chemical changes, and then eventually a product is formed. When this product is formed, it no longer fits into the active site and then escapes out into the surrounding. In this way, the active site is available again for fresh substrates. By this hypothesis, it can be concluded that a very small amount of enzyme can act upon large substrate molecules. It also explains how the enzymes are not used in the reaction and can be used again and again. Moreover, it helps us to understand the mechanism of competitive inhibition. 

2. Induced Fit Hypothesis: Koshland proposed this hypothesis in the year 1960. This hypothesis is actually quite different from the previous hypothesis. It states that the active site of the enzyme is flexible in shape and can change its shape according to the nature of the substrate, which means that it can form its active site complementary to the substrate. It is easy to understand it in a way that how a hand induces a change in the glove, that is the same way an active site induces a change in the chemical substrate. The substrate gets into the active site of the enzyme. According to this, the structure of the active site of an enzyme is flexible. There are two types of groups that are present in the active site of the enzyme. One is a buttressing group and the other is a catalytic group. The buttressing group helps in supporting the substrate whereas the catalytic group helps to explain the mechanism of enzyme catalyzes. When the buttressing group comes in contact with the substrate, changes take place in the active site and these changes help to bring the catalytic group opposite to the substrate bonds that are needed to be broken. The above two models help us to deeply understand and describe the mechanism of enzyme action.


Mechanism of Enzyme Catalysis

Enzymes are responsible for bringing out the high rate of chemical conversions. The substrates are converted into products. The substrates get bound to the active site of the enzyme and then there are changes in the enzyme complex which brings about changes in the enzyme-substrate complex and thus a product is formed from the substrate. This enzyme-substrate complex is formed for a very short time and is thus known as a transient phenomenon. The transient state structure is formed when the substrate gets bound to the active site of the enzyme. Then some making/breaking of bonds takes place and a final product is formed. Activation energy is the energy that is required to start the reaction. Enzymes work by lowering these high activation energies of the reactions. 

Nature of Enzyme Action

There is a presence of an active site on each enzyme molecule. Substrate comes in contact with the enzyme and then an enzyme-substrate complex is formed. After a while, the enzyme-substrate complex changes to enzyme-product complex, and then the product gets separated from it. This way the enzyme is not used up in the reaction. The catalytic cycle helps to explain the mechanism of enzyme action through the following points:

  • The substrate gets bound to the active site.

  • This induces an alteration in the shape of the enzyme.

  • The enzyme-product complex is formed by making and breaking bonds. 

  • Enzyme releases away from the product and it available again for a fresh batch of substrates. 

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                                                     Figure: Mechanism of enzyme action

Factors Affecting Mechanism of Enzyme Catalysis

Three factors are responsible for affecting the mechanism of enzyme catalysis:

  • Temperature: Enzyme catalysis works in a narrow range of temperature. Optimum temperature is the temperature at which the enzymes show the highest catalytic activity. Anything above and below the optimum temperature declines the enzyme activity. Low temperature makes the enzymes inactive whereas high temperature denatures the structure of enzymes. 

  • Hydrogen Ion Concentration: As there is an optimum temperature required for the enzyme to function, there is also an optimum pH concentration. Any fall or rise in pH reduces the activity of enzymes. Some enzymes show good catalytic activity in acidic medium whereas some show good activity in alkaline medium. Every enzyme has an optimum pH where their activity is maximum.

  • Substrate Concentration: Substrates act on enzymes and then are changed to products. An increase in the concentration of substrate results in increasing the velocity of enzymes. 

Mechanism of Enzyme Action in Biochemistry

There are two types of changes observed in chemical compounds that are physical change and chemical change. Physical changes take place by only changing the shape of the compound. No bonds are broken in physical changes. Whereas in chemical changes new bonds are formed and broken during the transformation process. When the ice melts into water, it is termed a physical change as there is a change of state. Hydrolysis of starch into glucose is a type of chemical change. The rate of both these physical and chemical processes is the amount of product formed per unit of time. When a direction is specified to rate it is called velocity. Temperature influences the rate of chemical and physical processes. Various types of enzymes are present and each has a unique catalytic activity or a unique chemical or metabolic reaction. A metabolic pathway is a pathway when one enzyme catalyzes the reaction at different steps. When there are one or two additional reactions in the metabolic pathway then it gives rise to a variety of end products. This, thus, helps us to understand the mechanism of enzyme action in biochemistry. 


From the above discussion we can conclude that enzymes are proteinaceous in nature and are involved in catalyzing various biological and biochemical processes. Different models are proposed to understand the mechanism of enzyme action such as the Lock and key hypothesis and Induced fit model. A basic understanding is that enzymes have an active site on them and the substrates get bound to it and then change into the product. In this way, the enzyme can be used again and again as it is not used in the reaction. Temperature, pH, and substrate concentration are the factors that can affect the rate of enzyme action. 

FAQs (Frequently Asked Questions)

1. Name Two Enzymes that are Not Composed of Protein.

Answer: The two enzymes are Ribozyme which is isolated from Tetrahymena and Ribonuclease which was discovered by Altman from bacteria.

2. What are Inorganic Catalysts?

Answer: Inorganic catalysts do not occur in living cells. They are small and simple molecules. For example nickel, platinum, etc. They show very little efficiency as compared to enzymes and work efficiently at very high temperatures and pressures. Also, they are not specific in nature.