Why are the Lock and Key Model Important? 

The lock-and-key model is used to describe the catalytic enzyme activity, based on the interaction between enzyme and substrate. This model considers the lock as an enzyme and the key as a substrate to explain this model. The concept of how a unique distinct key only can have the access to open a particular lock resembles how the specific substrate can only fit into the particular active site of the enzyme. This is significant in understanding the intermolecular interaction between proteins and plays a vital role in drug interaction. 

Enzymes 

Any substance that elevates the speed or rate of a chemical reaction without modifying the reaction itself is termed a catalyst. Enzymes are referred to as biological catalysts. Most of these biological catalysts belong to the category of protein biomolecules. Due to their presence, the reaction rate gets elevated to a rate that is 10⁶ or more times quicker compared to the reaction rate when the enzymes are absent. They are capable of performing their catalysis reaction at the normal body temperature which is ~37°C and normal physiological pH~7. Also, it exhibits the property of high specificity in its action. This defines how each enzyme catalyzes only one particular type of reaction in a particular compound or group of compounds that are structurally related. The particular compound or the group of compounds on which exhibit its action is termed substrates. 

Classification of Enzymes 

Enzymes are generally purified and investigated as a study to understand how these biological catalysts effectively work with such kind of specificity. As the discovery of different enzyme types occurs, there is a need for chemists to come up with an identification scheme that is chemically informative and highly systematic. As per the current scheme, enzymes are categorized into  6 significant classes based on the general reaction type that they catalyze. These classes are categorized along with their subgroups and secondary subgroups that are significant in specifying the reaction in a more precise manner. The 6 significant classes are namely: 

• Transferases 
•  Oxidoreductases 
•  Hydrolases 
•   Isomerases 
•   Ligases 
•  Lyases

These names for classes are given consisting of the root of the substrate or substrates name along with the suffix -ase. This can be explained with an example of urease. Urease is the enzyme that is responsible for catalyzing the hydrolysis reaction of urea. 

Enzyme Action 

Enzyme-catalyzed reaction occurs in at least 2 steps.  In the 1^st step, enzymes and substrates have collided for the formation of an intermediate compound termed enzyme-substrate complex. This is represented as the E-S complex. This is achieved specifically with the help of the active site. Once the complex is formed, the enzyme has the capability to catalyse the product formation which is then released from the surface of the enzyme. This is represented as 

S + E → E–S 

E–S → P + E 

In the above equation, E represents enzymes, S for substrate, and P for product. 

An active site is defined as the region in the enzyme where the substrate molecule binds. This active site is composed of the residues of amino acids that are vital for the temporary bond formation with the substrate’s binding site and certain residues that are required for catalyzing a reaction of that particular substrate which is otherwise known as the catalytic site. Though the active site occupies only about 10 to 20 percent of an enzyme, the active site is considered to be the most important part in directly catalyzing the reaction.

Factors Affecting Enzymatic Activity 

The factors that impact the enzymatic activity of the enzyme are listed and explained as follows: 

• pH: Every enzyme has a certain distinct range of pH. Modifying the pH beyond its certain range can result in a decrease in the reaction rate. Extreme alteration can also cause the enzyme to get denatured. 
• Temperature: When the elevation of temperature occurs, the reaction rate also gets elevated, and when the factor temperature gets decreased, the reaction rate also tends to decrease. In cases when the temperature is too high, the enzymes losses their shape, and the catalytic reaction cannot occur. 

• Substrate: Concentration: When the concentration of the substrate increases, the reaction rate also increases, assuming that the enzyme is present. If the enzyme is all utilized, then there will be no modification in the reaction rate as the enzyme that is available is already functioning at its maximum rate. 
• Enzyme-Concentration: When the concentration of an enzyme gets increased, the reaction rate also gets increased, assuming that the substrate is required to bind the enzyme’s increased concentration. When there is no substrate, then the reaction rate will not elevate. 

Models of Action 

There are 2 models that are used to describe the interaction of enzymes with their substrates. They are: 

• Lock and key model 
• Induced fit model 

Lock and Key Model 

Emil Fischer in the year 1890 proposed a theory to describe the reaction of enzymes and substrates. In this particular theory, he draws a parallel between the lock-and-key function and enzyme-substrate reaction. He referred to the reaction of enzyme and substrate by explaining how the right key is required to open a particular lock, similarly, the right substrate in terms of size and shape is required to initiate the reaction at the active site of the enzyme in order to produce a product.  

As per this model, the enzyme and the substrate have a definite distinct molecular structure and shape, and the enzyme does not modify its structure for substrate accommodation. This also states that this biological catalyst is highly specific, which ultimately conveys that the substrate that is characterized to bind with the particular enzyme should also be highly specific to catalyze the particular reaction. 

Induced Fit Model 

An attempt to describe the flaws in the lock and key model is the induced fit model. It is to denote that not every enzyme is a perfect fit for its specific substrates. This model also exhibits how an active site of the particular enzyme will function with specific substrates. This is possible only because those substrates have the ability to cause a slight alteration in the enzyme’s shape for creating a good bond. Substrates that lack the proper distinct size or shape will not be able to induce the fit, and thus the enzyme will not be able to react with them. 

Common Mistakes 

It is important to distinguish the difference between the 2 models to avoid confusion. Also, get familiarized with the enzyme catalysis with the help of a diagram to completely understand various related concepts.  

Context and Applications 

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelors and Masters Chemistry, Biochemistry and Molecular Biology, Biological science and Masters in Biotechnology.  

Practice Problems 

Q. Which determines the shape of the active site? 

Answer: The specificity of the enzyme determines the shape of the active site.