Enzyme Catalysis and its Mechanism

Introduction

Catalysis is basically a phenomenon in which the reaction rate gets altered with the assistance of a substance known as a catalyst. As the catalyst does not participate in the reaction, there is no change in its concentration and composition. Catalyst is the substance used to change the rate of reaction. Enzymes are a class of catalysts that tend to be responsible for increasing and facilitating the rate of multiple critical biochemical reactions in animals and plants. Enzyme catalysis basically refers to how enzymes act as a catalyst.

What are Enzymes?

Enzymes are types of complex compounds that contain nitrogen. Plants and animals both produce these compounds naturally in their bodies. Enzymes are proteins that have high molecular mass and tend to form a heterogeneous mixture if it is dissolved in water. Such proteins act quite differently and are responsible for varied reactions that take place in the body of any living being. 

An enzyme tends to attract substrates to the site where it is active, forming products by catalyzing the chemical reaction. It then helps the products to dissociate or separate from the enzyme surface. The combination formed by an enzyme and its substrates is called the enzyme-substrate complex. In case two substrates and a single enzyme are involved, the complex is referred to as a ternary complex. A single enzyme and single substrate, on the other hand, is known as a binary complex. Substrates are attracted to the active site by hydrophobic and electrostatic forces, which are known as noncovalent bonds as they are not chemical bonds but have physical attractions.

Synthetic catalysts are generally used for the purpose of accelerating a lot of industrial processes and tend to be crucial to the chemical manufacturing industry. Catalysts are, however, found in nature as well as in the form of enzymes. 

Characteristics of Enzyme Catalysis

Single molecules of enzyme catalysis are able to transform up to a million molecules at the reactant per second. As a result, enzyme catalysts are known to be pretty efficient.

Such biochemical catalysts are known to be unique to particular types of reactions. This basically implies the fact that the same catalyst cannot be used in more than just a single reaction.

The overall efficiency of a catalyst generally is maximum at its optimum temperature, and the activity of biochemical catalysts usually goes down at either side of the optimum temperature.

Biochemical catalysts generally depend upon the pH of the solution and work best at an optimum pH that ranges between pH values of 5-7.

The activity of enzymes generally goes up in the presence of an activator or coenzyme like Na+, . The rate of reaction goes up owing to the presence of a weak bond that exists between a metal ion and an enzyme.

Mechanism of an Enzyme Catalyst

Enzyme catalyzed reaction generally proceed in two ways: 

Binding of the enzyme to the substrate to form an activated complex

E + S → ES*

Decomposition of the activated complex

ES* →E +P

Here: E- Enzyme, S- Substrate,  ES*- Activated complex,  P- Product

Lock and Key Mechanism: There are multiple cavities present on the enzyme’s surface. Cavities are known to have active groups like -OH, -NH2, -SH and -COOH, along with a characteristic shape. These basically are the active centers on the surface of the enzyme. Reactant modules having complementary shapes can fit into these cavities in a manner similar to an essential fitting into a lock. Owing to the active centers or groups present, an activated complex is known as the enzyme-substrate complex. This complex decomposes to give products and the enzyme.

Ways by Which Enzyme Catalyzed Reaction Occur

Enzyme catalyzed reaction occurs through diverse mechanisms, such as: 

Bond strain: Enzymes may destabilize bonds within the substrate. 

Orientation and proximity: Conformational changes in enzyme upon substrate binding might bring reactive groups closer, or it can end up orienting them so that they are able to react.

Acceptors and proton donors: The presence of primary or acidic groups might end up affecting reaction speed and bond polarization.

Electrostatic catalysis: Electrostatic attractions taking place between substrate and enzyme might end up stabilizing the activated complex. 

Covalent catalysis: The energy of the transition state can get lowered due to covalent bonding to side chains or cofactors.

Enzymes and catalysts show that evolutionary biology has managed to produce highly effective catalysts.

Rate of Enzymatic Reactions

In case the velocity of an enzyme-catalyzed reaction is represented graphically as a substrate concentration function, the curve obtained in the majority of the situations would be a hyperbola.

The shape of the curve is due to the logical consequence of the active-site concept. This basically means that the curve flattens at the maximum velocity or VM, which takes place when all the active sites of enzymes are filled with substrate. Velocity approaches a high substrate concentration to allow it to facilitate the assumption that an intermediate enzyme, substrate complex forms. The concentration of substrate in moles per litre (M) is equivalent to the Michaelis constant at the point of half the maximum velocity. This Michaelis constant is a rough measure of the affinity of the substrate molecule when it comes to the surface of the enzyme. Usually, KM values vary from around 10−8 to 10−2 M, and VM, as well as from 105 to 109 molecules of product formed per molecule of enzyme per second. The value of VM here implies the turnover number when expressed as moles of product created per mole of enzyme per minute. Binding of molecules that generally activate or inhibit the protein surface generally leads to similar types. 

Conclusion

Enzyme catalysis is basically the process of increasing the rate of almost all chemical reactions within cells by the active site of the protein. Enzymes could be a part of a multi-subunit complex, as well as permanently or transiently conjugate with a cofactor. Theoretically speaking, the mechanism of enzyme biocatalysis is pretty similar to other types of chemicals; catalysis in which an enzyme is recycled and not consumed so that a single enzyme executes several rounds of catalysis. Moreover, reaction rates are known to be enhanced by enzymes without any alteration to the chemical equilibrium between products and reactants. From the energetic point of view, the reason why an enzyme can accelerate a reaction is due to the fact that it can provide a proper environment to lower the energy barrier or activation energy that is needed to reach the highest energy transition state of the reaction. In most cases, only a few molecules have adequate energy for the reaction.