Enzymes: Definition, Functions, Classification

Enzymes: The human body is composed of different, complex molecules, organs, tissues, cells, and proteins. Enzymes are the proteins that help our body in boosting metabolism. One of the functions is to build the substances while tearing down others at the same time. Enzymes are available in all living organisms, and not just in living things; they are also found in food products, beverages, and other materials. These proteins also act as biological catalysts. 

The molecules upon which the enzymes work are known as ‘substrates’. What enzymes do is that they convert the substrates into different substances known as products. To sustain a healthy life, the human body releases some chemicals and needs some protein for smooth functioning, enzymes are considered in those. We have added detailed analysis of enzymes in this comprehensive article for all students trying to understand it in an easy manner. Continue reading to find out more.

What Are Enzymes?

Enzymes are substances that act as catalysts in living organisms. They help in regulating the rate of a chemical reaction without itself being altered in the process. Enzymes can be defined as biological polymers that catalyse biochemical reactions. Enzymes play an important role in every function of the human body. Without enzymes, we cannot even think of survival!

1. Enzymes are defined as complex nitrogenous organic compounds produced by living plants and animals. 
2. They are formed when a large chain of one or more amino acids are linked with the help of amide or peptide bonds. 
3. They are proteins of high molecular mass responsible for catalysing natural processes prevailing in the bodies of animals and plants, also known as polypeptides. Based on their structure and properties, they are classified into different types of enzymes. Enzymes work in a specific mechanism (Lock-and-Key mechanism and Enzyme Fit Hypothesis).

Structure of Enzyme

1. Enzymes are a type of protein made up of many polypeptide chains, also known as amino acids, which are folded and coiled many times. 
2. They have three-dimensional structures that are linear chains of amino acids. 
3. The catalytic activity of the enzyme is dependent on the sequence of amino acids. In the entire structure of enzymes, only a small section of the structure takes part in catalysis and is situated next to the binding sites. 
4. They have different sites; the catalytic site and binding site together constitute the enzyme’s active site.

Classification of Enzymes

Based on the type of reaction in which a particular enzyme is used to catalyse, they are classified into six classes by the International Union of Biochemists (I U B). The six kinds of enzymes are oxidoreductases, transferases, hydrolases, lyases, ligases, and isomerases. Learn their functions below:

Study about Digestive Enzymes

1. Oxidoreductases: The enzyme Oxidoreductase catalyses the oxidation and reduction reaction where the electrons transfer takes place from one form of a molecule (electron donor) to the other (electron acceptor). For example, pyruvate dehydrogenase. Oxidoreductase enzymes usually utilize NADP+ or NAD+ as cofactors.

\(A{H_2} + B \to A + B{H_2}\)

2. Transferases: These catalyses transfer the chemical group (functional group) from one compound (called the donor) to another compound (called the acceptor). An example is a transaminase, which transfers an amino group from one molecule to another.

\(A – X + B \leftrightarrow B – X + A\)

3. Hydrolases: They are hydrolytic enzymes, which catalyses the hydrolysis reaction by using water molecules to cleave the bond and hydrolyze it, i.e., they catalyse the hydrolysis of a bond. For example, the enzyme pepsin hydrolyzes peptide bonds in proteins.

\(A – X + {H_2}O \to X – OH + A – H\)

4. Lyases: They are enzymes that catalyze bodywork, breaking a chemical bond through not involving hydrolysis or oxidation and forming a double bond or adding a group to a double bond. For example, aldolase (an enzyme in glycolysis) catalyses the splitting of fructose-1, 6-bisphosphate to glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.

\(A – X + B – Y \to A = B + X – Y\)

5. Isomerases: They are a class of enzymes that convert a molecule from one isomer to another. Isomerases facilitate intramolecular rearrangements in which bonds are broken as well as formed. Example: phosphoglucomutase catalyses the conversion of glucose-1-phosphate to glucose-6-phosphate (phosphate group is transferred from one to another position in the same compound) in glycogenolysis. Glycogen is converted to glucose for energy to be released quickly.

\({A_{{\rm{Cis}}}} \to A_{{\rm{Trans }}}^\prime \)

6. Ligases: Ligase catalyses the ligation or joining of two large molecules by forming a new chemical bond between them. For example, DNA ligase catalyses the joining of two fragments of DNA by forming a phosphodiester bond.

\(A + B \to AB\)

Enzyme Cofactor

Cofactors are non-protein chemical compounds that are associated with enzymes. A cofactor acts as a catalyst and regulates the functioning of an enzyme. An enzyme without a cofactor is called an apoenzyme. An enzyme and its cofactor together constitute the holoenzyme.

Three Kinds of Cofactors Present in Enzymes:

1. Prosthetic groups: These are cofactors covalently or permanently bound to an enzyme at all times. A FAD (Flavin Adenine Dinucleotide) is a prosthetic group present in many enzymes.
2. Coenzyme: A coenzyme is an organic, non-protein compound that binds to an enzyme only during catalysis. At all other times, it is detached from the enzyme. NAD⁺ is a common coenzyme.
3. Metal ions: For the catalysis of certain enzymes, a metal ion is required at the active site to form coordinate bonds. Zn²⁺ is a metal ion cofactor used by a number of enzymes.

Mechanism of Enzyme Action

An enzyme’s active site attracts substrates towards it, catalyses the chemical reaction by which products are formed. After product formation, allows the products to dissociate or separate from the enzyme’s surface. The combination formed by an enzyme and its substrates is called the enzyme-substrate complex.

\({E_{{\rm{Enzyme }}}} + {S_{{\rm{Substrate }}}} \mathbin{\lower.3ex\hbox{$\buildrel\textstyle\leftharpoonup\over {\smash{\rightharpoondown}}$}} {[ES]_{{\rm{Enzyme – SubstrateComplex }}}} \to {E_{{\rm{Enzyme }}}} + {P_{{\rm{Product }}}}\)

Collision of any two molecules along with the right orientation and a satisfactory amount of energy is needed for the reaction to occur. This energy between these molecules needs to overcome the barrier in the reaction, which is known as Activation Energy. The substrate and the enzyme form an intermediate reaction with low activation energy without any catalysts.

Among the various mechanisms of enzyme action, two of them are very famous, i.e., Induced Fit Hypothesis and Lock and Key Mechanism. Let us discuss them in detail:

Induced Fit Hypothesis: Daniel Koshland suggested the induced fit model in \(1958.\) This is one of the main models, describing the enzyme-substrate interaction. According to the hypothesis, the active site of the enzyme does not have a rigid conformation.

Therefore, the substrate does not completely fit into the active site of the enzyme. Hence, the enzyme’s active site modifies its shape upon the binding of the substrate, becoming complementary to the shape of the substrate. Significantly, this conformational change is possible due to the flexibility of the protein.

Lock and Key Mechanism: The lock and key model was suggested by Emil Fischer in \(1894\) and is hence known as Fisher’s theory and it describes the enzyme-substrate interaction.

The lock and key model suggested by Emil Fischer in \(1894.\) Therefore, it is also known as Fisher’s theory. This is the second model, which describes the enzyme-substrate interaction.

1. According to the lock and key model, the active site of the enzymes serves as the ‘lock’ while its substrate serves as the ‘key’. On that basis, the shape of the active site of the enzyme is complementary to the shape of the substrate. Thereby, the active site of the enzyme can hold the substrate closer to the enzyme by forming an unusable intermediate compound, which is the enzyme-substrate complex.

Enzymes as Biochemical Catalysts

Enzymes are also termed biochemical catalysts, and the phenomenon is termed biochemical catalysis. Enzymes are commonly added in beverages, chocolates, curd, predigested baby food, washing powders, etc., to aid or accelerate their efficient preparation and effect.

Examples of Enzyme Catalysis

1. Inversion of cane sugar: The enzyme invertase converts cane sugar into glucose and fructose.

\({{\rm{C}}_{12}}{{\rm{H}}_{22}}{{\rm{O}}_{11}}({\rm{aq}}) + {{\rm{H}}_2}{\rm{O}}(1)\mathop \to \limits^{{\rm{ Invertase }}} {{\rm{C}}_6}{{\rm{H}}_{12}}{{\rm{O}}_6}({\rm{aq}}) + \mathop {{{\rm{C}}_6}{{\rm{H}}_{12}}{{\rm{O}}_6}}\limits_{{\rm{ Glucose }}} ({\rm{aq}})\)

2. Conversion of milk into curd: The enzyme lactase secreted from lactobacilli is responsible for converting milk into curd.

3. Conversion of glucose into ethyl alcohol: The zymase enzyme converts glucose into ethyl alcohol and carbon dioxide.

\(\mathop {{{\rm{C}}_6}{{\rm{H}}_{12}}{{\rm{O}}_6}}\limits_{{\rm{ Gilucose }}} ({\rm{aq}})\mathop \to \limits^{{\rm{ zymase }}} \mathop {2{{\rm{C}}_2}{{\rm{H}}_5}{\rm{OH}}}\limits_{{\rm{ Ethyl akohol }}} ({\rm{aq}}) + 2{\rm{C}}{{\rm{O}}_2}({\rm{aq}})\)

4. Conversion of starch into maltose: The diastase enzyme converts starch into maltose.

Factors Affecting Enzyme Catalysis

As we know, the function of enzymes is to increase the rate of enzyme catalysis. There are various factors that play a vital role in altering the rate of enzyme catalysis in a chemical reaction. These factors are pH, temperature, and the concentration of an enzyme and its substrate.

1. Concentration of Substrate

The rate of a chemical reaction in the presence of an enzyme increases as the substrate concentration increases until a limiting rate is reached, after which further increase in the substrate concentration produces no significant change in the reaction. At this point, the enzyme molecules are saturated with the substrate. The excess substrate molecules cannot react until the substrate already bound to the enzymes has reacted and been released.

2. Concentration of Enzyme

When the concentration of the enzyme is significantly lower than the concentration of the substrate, the rate of an enzyme-catalysed reaction is directly dependent on the enzyme concentration. This is true for any catalyst; the reaction rate increases as the concentration of the catalyst is increased.

3. Temperature

A famous rule of thumb for most chemical reactions is that a temperature rise of 10°C approximately doubles the reaction rate. To some extent, this rule holds for all enzymatic reactions. After a certain point, even an increase in temperature causes a decrease in the reaction rate due to denaturation of the protein structure and disruption of the active site.

4. Hydrogen Ion Concentration (pH)

As we know, most enzymes are proteins and are sensitive to changes in pH or hydrogen ion concentration. Any change in pH alters the degree of ionization of an enzyme’s acidic and basic side groups and the substrate components as well. Neutralization of even one of these charges alters an enzyme’s catalytic activity. An enzyme exhibits maximum activity over the narrow pH range. The median value of this pH range is called the optimum pH of the enzyme.

5. Inhibition of Enzymes

To help and ensure that our body’s systems work correctly and efficiently, sometimes enzymes need to be slowed down. For instance, if an enzyme is making too much of a product, there needs to be a way to reduce or stop production. In such cases, inhibitors are required.

Enzymes Inhibition

1. Competitive inhibitor: A molecule blocks the active site so that the substrate competes with the inhibitor to attach to the enzyme.
2. Non-competitive inhibitors: A molecule binds to an enzyme somewhere other than the active site and reduces how effectively it works.
3. Uncompetitive inhibitors: The inhibitor binds to the enzyme-substrate complex. The products leave the active site less easily, and the reaction is slowed down.
4. Irreversible inhibitors: An irreversible inhibitor binds to an enzyme and permanently inactivates it.

Drug Action of Enzymes

Enzyme action can be regulated, i.e., inhibited or promoted by the drugs as they act on the active sites of enzymes. Most of the drugs which act on enzymes act as inhibitors, and most of these are competitive inhibitors, i.e., they compete for binding with the enzyme’s substrate. For example, the majority of the original (first generation) kinase inhibitors bind to the ATP pocket of the enzyme.

Examples of Enzymes

1. Lipases: These are a group of enzymes that help digest fats in the gut.
2. Amylase: Amylase helps change starches into sugars. This enzyme is found in saliva.
3. Maltase: Maltase is also found in saliva; it breaks the sugar maltose into glucose. Maltose is found in foods such as potatoes, pasta, and beer.
4. Trypsin: Trypsin is found in the small intestine, breaks proteins down into amino acids.
5. Lactase: This enzyme is also found in the small intestine, and helps break lactose, the sugar in milk, into glucose and galactose.
6. Helicase: Helicase enzyme unravels DNA.
7. DNA Polymerase: They synthesize DNA from deoxyribonucleotides.

Summary

Hence, it can be concluded that enzymes are the most essential protein in the human body for metabolism activities. They increase the rate of a chemical reaction in our body hence are known as biological catalysts. When an enzyme binds its substrate, it forms an enzyme-substrate complex.

One of the important properties of enzymes is that they remain ultimately unchanged by the reactions they catalyse. In this article, you will learn the entire functions of enzymes along with their classification, examples, and many more points.

PRACTICE QUESTIONS RELATED TO ENZYMES

FAQs on Enzymes

Here are some of the facts about Enzymes:

1. What is the difference between a protein and an enzyme?
Answer: Though both enzymes and proteins are made up of amino acids. But the basic difference between them is that proteins are involved in the formation of structures, transportation and regulation of biological processes. Enzymes act as biological catalysts to speed up the chemical reactions in the body

2. What is the function of all enzymes?
Answer: The primary function of all enzymes is to speed up the rate of a chemical reaction to help support life. Without enzymes, all the biochemical reactions in our body would be very slow and our survival would be tough.

3. How do enzymes work?
Answer: Enzymes work by binding to reactant molecules and holding them in such a way that the chemical bond-breaking and bond-forming processes take place more readily. Enzymes work as biological catalysts. They bring down the Activation Energy of a reaction coordinate that helps in increasing the rate of the reaction.

4. Are Enzymes reusable?
Answer: Yes, Enzymes are reusable. Once an enzyme binds to a substrate and catalyses the reaction, the enzyme is released unchanged and can be used for another reaction.

5. What is an enzyme? State its properties.
Answer: Enzymes are proteinaceous biocatalysts, which accelerate the rate of biochemical reactions but do not affect the nature of the final product. Like catalysts, the enzymes regulate the speed and specificity of reaction without being used up.

6. What is the difference between an Enzyme and a catalyst?
Answer: Unlike catalysts, enzymes are produced by living cells only. While catalysts are inorganic compounds, enzymes are largely organic in nature and are bio-catalysts.
1. Amylase enzyme is produced in the mouth for the digestion of starch.
2. Pepsin is an enzyme produced in the stomach that helps in the digestion of protein in food.
3. Trypsin, produced in the pancreas, also helps in further digestion of protein.
4. Pancreatic lipase, produced in the pancreas, helps in the digestion of fats. It hydrolyses triglycerides to fatty acids and glycerol.
5. Deoxyribonuclease and ribonuclease, produced in the pancreas, catalyses the degradation of both single and double-stranded DNA and DNA-RNA hybrids.

7. What are the 5 Enzymes?
Answer: The five major enzymes used mainly by our digestive system are:

8. What are 3 facts about enzymes?
Answer: (i) Enzymes play an important role in every function of the human body.
(ii) Several parts of our digestive system secrete enzymes.
(iii) Enzymes help with nutrient absorption in our bodies.

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