What is Catalysis and Enzymatic Reaction?

Catalysis is the kind of chemical reaction in which the rate (speed) of a reaction is enhanced by the catalyst which is not consumed during the process of reaction and afterward it is removed when the catalyst is not used to make up the impurity in the product. The enzymatic reaction is the reaction that is catalyzed via enzymes.

Important features of Catalytic Reactions

The following features are usually common for the number of catalytic reactions:

• The catalyst remains unaffected in chemical composition & amount at the end of the reaction.
• Only a minute amount of catalyst is usually required. The concentration of catalyst and the rate of reactions are proportional to each other in a homogeneous reaction. In a heterogeneous reaction, the rate (speed) of the reaction is increased as the portion of the surface of the catalyst.
• The position of equilibrium will not alter by the addition of catalysts in reversible chemical reactions.
• The catalyst will not start the reaction. The reaction is already proceeding but very slowly in non-appearance of catalyst. The catalyst is mainly used to enhance the speed of the reaction.
• Usually, the catalysts are specific in their action.
• The nature of the reaction could not be altered by the introduction of a catalyst.
• The catalyst can be poisoned by certain compounds. This process is known as catalytic poison.

Subdivisions of Catalysis

Catalysis could be categorized into two which are,

• Homogeneous catalysis.
• Heterogeneous catalysis.

Homogeneous catalysis is a reaction in which the catalyst & the reactant produce only one phase, whereas in heterogeneous catalysis, the catalyst produces one phase (generally solid) but the reactant forms a different phase.

Enzymes

Enzymes are made up of proteins (amino acids) which are folded into tangled shapes. Enzymes are produced by the stringing of 100 to 1,000 amino acids. The series of amino acids are then connected into a different shape. Enzymes are substances that are produced by living organisms that could catalyze certain biochemical reactions. In other words, enzymes are the proteins that enhance the rate of the chemical reaction in the case of a living organism. Based on the type of amino acids in the enzymes, the properties of the active site will change. The amino acids could have side chains that are hydrophobic or hydrophilic, acidic or basic, large or small.

Enzymatic Catalysis

Enzyme catalysis belongs to the category of homogeneous catalysis. In these reactions, the enzymes act as a catalyst and, each enzyme could catalyze a particular reaction. Generally, enzyme catalysts are more efficient because one molecule of an enzyme catalyst could be changed into million molecules of reaction in one second. These catalysts are varying to a specific class of reactions and they are identical and these catalysts will not be used in more than a single reaction.

At the optimum temperature, the ability of the catalyst is maximum. The reactivity of this catalyst could be canceled at either part of the optimum temperature. This biochemical catalysis is dependent on pH. This catalyst works well in the pH range of 5-7. The enzymatic activity of the reaction is enhanced when the coenzymes have catalyzed the reaction. The examples for coenzymes are Na²⁺, Ca²⁺. The rate (speed) of the reaction enhances when there is a weak bond that is present between a metal ion and an enzyme.

Mechanism of Enzyme Catalysis

Enzymes contain a lot of cavities on their outer surface. The substituents such as -SH  and-COOH are located in these cavities (active sites). This is termed the active site of the particle. The substrate that possesses a charge which is opposite to the enzyme’s charge can be fit into the cavities similar to a key inserted into the lock.

Because of the presence of active substituents, the complex that is produced will be decomposed into products.

This will occur in two steps which are,

Step 1:

Binding of enzyme & substrate. In this step, the substrate is binding to the active site of the enzyme. It could be depicted as follows:

E+R→ER

Step 2:

The disintegration of complex to forms the product. In this step, the product is detached from the active site of the enzyme. It could be depicted as follows:

ER→E+P

The best-known example of the enzyme kinetic is given by Michaelis-Menten kinetics & the equation could be given as,

ν=V_max[S] / K_m+[S]

Where,

The velocity of the reaction is denoted by ν.

The maximum rate reached by the substance is denoted by V_max.

The concentration of the substrate is denoted by [S].

The Michaelis constant is denoted by K_m.

Induced fit model

In the case of enzyme-substrate reaction, the induced fit model is a classic model. The induced-fit model said that the bonding which initially occurs between the enzyme & substrate is weak. However, such weak interactions are easily induced changes in conformations of enzymes which will strengthen the binding.

This model is more advantageous because of the stabilizing effect of the strong binding of enzymes. The binding of substrates occurs in two mechanisms which are,

Uniform binding: The uniform binding contains the strong substrate binding.

Differential binding: The differential binding contains the strong intermediate state binding.

In the case of uniform binding, the stabilizing effect will increase both transition states & substrate-binding affinity whereas the differential binding will increase only the affinity of intermediate state binding. Both could be utilized by the enzymes, and they are chosen to reduce the energy of activation in the reaction. The saturated enzyme has huge affinity substrate binding needs differential binding to minimize the activation energy while the small substrate unbound enzymes can be used either a uniform or differential binding.

Due to these effects, most of the proteins will use differential binding mechanisms for minimizing the activation energy. Hence, most of the substrates will have a huge affinity for an enzyme while in the intermediate state. Induced fit mechanism happens in the differential binding where the reactant (substrate) weakly binds after which the conformational changes in enzymes will take place which enhances the affinity to the intermediate state & stabilizes it, hence minimizing the energy required for activation to reach it.

Effect of temperature

The reaction rates of the enzymatic reactions usually enhance with the temperature. But for a living cell, this much-elevated temperature is not favorable. Because enzymes are greatly sensitive to elevated temperatures. Due to the proteinous nature of enzymes, the huge temperature will result in the denaturation of the protein enzyme that results in a decrease in the concentration of the enzyme which will lead to a decrease in reaction rates. The figure shown below depicts the effect of temperature on the enzymatic reaction.

Effect of pH

The change in pH will modify the shape of the enzyme’s active site. Every enzyme works well at a particular pH value. The optimum pH of certain enzymes is dependent on where it usually works. The optimum pH of enzymes that are present in the minute intestine is about 7.5. But the enzymes that are located in the stomach have a pH of about 2. The figure shown below depicts the effect of the pH on the enzymatic reaction.

Effect of concentration of substrate

The rate (speed) of the enzyme activity increases with substrate concentration. But the rate of activity of enzymes will not raise forever. This is because that a point will be achieved if the enzymes get saturated and no more reactants (substrates) could be fit at any one time even though there is a lot of substrates available. The optimum rate is achieved at the optimum substrate concentration of the enzyme. The figure shown below depicts the effect of the concentration of a reactant (substrate) on an enzymatic reaction.

Context and Applications

This topic is significant for both Undergraduate and Postgraduate courses, especially for Bachelors's and Master's in Chemistry, Bachelors's, and Masters's in Biochemistry.

Practice Problems

Question 1: How would be the catalyst enhances the speed of the reaction?

1. By producing an intermediate complex
2. By decreasing the activation energy
3. By increasing the activation energy
4. By modifying the equilibrium constant.

Answer: Option 2 is correct.

Explanation: Generally catalysts are used for enhancing the speed of the reaction. The main role of catalyst is to reduce the energy of activation of the reaction which will enhance the rate of reaction. Hence the correct option is by decreasing the activation energy.

Question 2: Among the following, which is not a characteristic of the catalyst?

1. It speeds up the rate of equilibrium
2. It activates equilibrium
3. It participates in the reaction
4. It starts the reaction

Answer: Option 3 is correct.

Explanation: The main use of the catalyst is to enhancing the speed of the reaction, other than that it will not involve or initiate the reaction.

Question 3: The nature of the enzyme is

1. Carbohydrate
2. Protein
3. Lipid
4. Vitamin

Answer: Option 2 is correct

Explanation: Enzymes are  usually made up of proteins which enhances the rate of the chemical reaction

Question 4: The polymer of ------ is called enzymes.

1. Amino acids
2. Fatty acids
3. Hexose sugar
4. Inorganic phosphate

Answer: Option 1 is correct.

Explanation: Enzymes are the polymer of amino acids. It could join prosthetic substituents which involve in enzyme reactions.

Question 5: The Coenzyme is

1. Often a vitamin
2. Often a metal
3. Always a protein
4. Always an inorganic compound

Answer: Option 1 is correct.

Explanation: Coenzymes are often vitamins or their derivatives and are organic compounds.