Top 50 Biology Project Topics for Class 12 Students [2025 Updated List]

Enzymes are biological catalysts, primarily made up of proteins, and in some instances ribonucleic acid (RNA), that increase the velocity of biochemical reactions without being consumed. Enzymes are highly specific and operate under room physiological conditions of temperature and pH, which is why they are essential for life. 

Enzymes serve a critical role in a variety of biological processes like food digestion, cellular respiration, nutrient metabolism, biomolecules synthesis, nucleic acid replication, and defense against genetic damage. Without enzymes, most of those reactions would take too long to happen to support life.

The central function of enzyme action is to control and regulate biochemical reactions within the cell. Enzymes are important for lowering the activation energy of reactions so a series of metabolic pathways can happen effectively, quickly, and in an organized fashion. This is essential for the homeostasis and proper functioning of any living organism.

Definition of Enzyme Action

Enzyme action is defined as the successive molecular events involved in substrate recognition, substrate binding, catalytic modifications to generate product(s) and regeneration of the functional enzyme that can then catalyze multiple cycles of reactions.

Enzyme action refers to the process by which biological catalysts known as enzymes increase the rate of specific chemical reactions in living systems, by lowering the activation energy and stabilizing the reaction transition state.

Theories of Enzyme Action

There are generally two theories of the Mechanism of enzyme action: the Lock and Key model and the Induced fit Model.

Image Source: scienceinfo.com

1. Lock and Key Model

• The lock and key model is used to explain how enzymes interact with substrates to catalyze biochemical reactions.
• The lock and key hypothesis was proposed by Emil Fischer in 1894 and remains an important concept in biochemistry today. 
• The 'lock and key hypothesis' mechanism is related to enzyme specificity. 
• According to this hypothesis, enzymes have a specific three-dimensional structure with an active site that is complementary in shape and chemical properties to a specific substrate molecule, like a lock and key.
• Where the active site of an enzyme is like a lock, and the substrate is like a key that fits into the lock. 
• The active site is a three-dimensional region on the enzyme's surface that has a specific shape, charge distribution, and chemical environment that complements the shape and chemical properties of the substrate molecule.
• When the substrate binds to the active site of the enzyme, it undergoes a conformational change, forming an enzyme-substrate complex. 
• This complex undergoes chemical reactions to produce a product, which is then released from the active site, and the enzyme returns to its original state, ready to catalyze another reaction.
• The lock and key hypothesis suggests that enzymes are particular in their catalytic activity and that only substrates with the correct shape and chemical properties can bind to the active site and undergo catalysis. 
• This specificity is important for the efficient functioning of enzymes in the body, as it allows them to selectively catalyze specific reactions and avoid unwanted side reactions.

2. Induced Fit Model

• In 1958, Daniel Koshland published the induced-fit model, which describes enzyme action better than the previous lock-and-key model. 
• This model incorporates flexibility in the active site rather than holding a completely rigid structure. When the substrate approaches the active site, the enzyme undergoes conformational changes to increase complementarity between the enzyme and substrate. 
• The changes will align the catalytic residues and stabilize the transition state. The conformational change will lower the activation energy by either straining some bonds in the substrate or stabilizing the energy-rich transition state. 
• After the substrate has completed the reaction, the product(s) is released, and the enzyme returns to its starting, or original, conformational state. 
• The induced-fit model provides several benefits relative to the lock-and-key theory. It explains how enzymes can have broad substrate specificity while also performing reactions without incorrect reactions taking place. 
• It also describes how catalytic efficiency is regulated as a function of flexibility in the enzyme. 
• This flexibility will also allow an enzyme to co-opt previously unrelated biochemical functions, fit effectively even as bacterial cells and populations undergo wide-range evolutionary pressures. 
• Because of these traits, the induced-fit hypothesis is accepted as a modern day depiction of enzyme action.
• The induced-fit model ensures specificity, regulation, and efficiency, making it a key concept in understanding enzyme mechanisms.

Mechanism of Enzyme Action (How Enzymes Catalyse the Reaction)

Enzymes are biological catalysts that increase the velocity of biochemical reactions by lowering the energy of activation. All chemical reactions must overcome an energy barrier before the reactant molecules can be converted into products. The energy barrier to be overcome by the reactant molecules to form products is referred to as the energy of activation.

If the energy of activation is high, the reaction will proceed slower and if the energy of activation is low, the reaction will proceed faster. Enzymes catalyze biochemical reactions by lowering the activation energy of the reaction so that it can proceed faster.

In enzyme catalysis, the active site of the enzyme interacts with substrate molecules to form an enzyme–substrate (ES) complex. In the formation of an ES complex some binding energy is released (the amount of binding energy is the energy lost by the substrate) and this energy is used to activate the substrate and form produced, thus as the energy of activation is reduced, the others approach maximum velocity. The amount of energy of activation that is removed from the substrate is equal to the amount of binding energy released in the enzyme–substrate interaction.

The interaction of an enzyme with a substrate can be divided into three stages:

1. Attachment of a substrate to an enzyme macromolecule.
2. Direct enzymatic reaction.
3. Separation of the products of the transformation of the substrate from the enzyme.

1. Attachment of a substrate to an enzyme:

• The first stage, the fastest, is the limiting stage of the catalytic process as a whole. 
• Its rate depends on the structures of the enzyme and substrate, the nature of the environment in which the enzymatic reaction is carried out, pH, and temperature. 
• Enzymes are characterized by specificity concerning substrates and high binding energy with them. 
• This energy is partly used to deform the substrate and reduce the activation energy of the subsequent chemical reaction.

2. Direct enzymatic reaction:

• The interaction of the enzyme with the substrate is preceded by the approach and orientation of the substrate concerning the enzyme's active center. 
• Then enzyme-substrate complexes are formed, the real existence of which can be fixed in various ways. 
• The most obvious and effective method is X-ray diffraction analysis. 
• An example is the identification of the enzyme-substrate complex of carboxypeptidase A and its substrate glycyl-L-tyrosine. 
• The method makes it possible not only to establish the very fact of complex formation but also to determine the types of bonds. 
• A simpler but sufficiently effective method is the spectral analysis of the enzyme and the corresponding enzyme-substrate complex. 
• Thus, in particular, enzyme-substrate complexes were identified for several flavin enzymes.

3. Separation of the product:

• The interaction of the enzyme with the substrate causes a local conformational change in some sites of the protein macromolecule of the enzyme, 
• As a result the complementarity of its active center to the substrate sharply increases and makes it possible to carry out the catalytic process. 
• A change in the conformation of an enzyme under the action of a substrate was first shown by D. Koshland and is called induced conformity.

Examples of Enzyme Action

1. Serine Proteases (Chymotrypsin): 

Chymotrypsin is a digestive enzyme in the serine protease family. Chymotrypsin hydrolyzes (or breaks down) proteins into smaller peptides by cleaving peptide bonds. Chymotrypsin employs a catalytic triad of amino acids- serine, histidine, and aspartate to provide this function. In the mechanism, Serine acts as a nucleophile, forming a temporary covalent bond with the substrate while the histidine and aspartate help facilitate the proton transfer. The enzyme mechanism is characterized by an acyl–enzyme intermediate which is then hydrolyzed to give the product.

2. Lysozyme

Lysozyme is an enzyme found in tears, saliva, and other bodily secretions to protect against bacterial infection, and it catalyzes the hydrolysis of glycosidic bonds in the peptidoglycan layer of bacterial cell walls. The enzymatic mechanism involves distorting the substrate into a transition state form and using acid–base catalysis to cleave the glycosidic bond. It should be noted that enzymatic hydrolysis weakens bacterial cell walls ultimately leading to a destruction mechanism and therefore an antibacterial defensive mechanism.

Factors Affecting Enzyme Action

The common factors that influence enzyme action

1. Temperature affects motion of molecules and activation energy, and since most enzymes have an optimal temperature above which thermal denaturation prevents their activity.
2. pH affects the the ionization states corresponding to both amino acid side chain and cofactor. Each enzyme has pH operating range- where the active catalytic residues have the correct protonation for activity.
3. Substrate concentration affects the reaction velocity in that reaction velocity increases with substrate until the enzyme is saturated and has Vmax.
4. Enzyme concentration affects the reaction rate directly under substrate saturation conditions.
5. Enzyme inhibitors can be competitive, noncompetitive, uncompetitive, or mixed and they influence enzyme action by binding to the active site or other sites and altering reaction kinetics.
6. Allosteric activators of inhibitors bind at regulatory sites and change activity by either of stabilizing an active or inactive conformational state.
7. Ionic strength and salt composition can affect electrostatic interactions in the active site and thus, influence binding and catalysis.
8. Post-translational modifications, i.e. phosphorylation or proteolytic activation can turn an enzyme on or off and/or change the enzyme cellular location.