Enzymes biomolecules class 11th

At some point in our academic year we must have all heard about enzymes and know that they are basically proteins, but let me tell you one thing that not all proteins are enzymes and that also doesn't mean that enzymes just act as proteins. They have many functions which are important for all living organisms. To know about this important part of life keep reading further. 

What are enzymes ?

There are several metabolic processes taking place in living organisms having one cell or a number of cells. These metabolic processes involves many metabolic pathways which includes many chemical reactions. These reactions can be anabolic, synthetic, catabolic or break down reactions which takes place at normal body temperature. They require high temperature, pressure, alkalinity, acidity, etc. These reactions can take place without any interruption due to the presence of enzymes

In short, enzymes are proteinaceous, specialized macromolecular substances which helps in conducting metabolic reactions or chemical reactions in the body by speeding up the process. 

Enzymes are also involved in bioprocesses such as digestion, respiration, stepwise breaking down and building up and converting carbohydrates, fats, proteins and nucleic acids into each other which is called interconversion.  

Enzymes are organic compounds synthesized by living cells. They are also known as biocatalyst which helps them in speeding up the biochemical processes without themselves being used. This catalytic action is active in enzymes even if they are extracted from the cells.  

The term enzyme was coined by William Kuhne and the study of enzymes is known as enzymology. The substances acquired at the end of the reaction are called as products or end products. It normally acts as a single substance (substrate) or as a group of closely related substances. 

The enzymes are said to be globular proteins having higher molecular weight and the reactions controlled by them can be reversed. They are heat sensitive or thermolabile, each functioning at a particular pH. 

The enzymes which are produced inside the cell, which only carry out metabolic activities are called endoenzymes or intracellular enzymes and the ones which are away from the site of where synthesis takes place is called endoenzyme or extracellular enzymes

There are two types or enzymes - simple and conjugate. Simple enzymes are only made up of proteins and they don't have an additional non-protein part of group whereas, conjugate enzymes contains many non-protein parts called the cofactor. These proteins are called apoenzyme. Complete conjugate protein is when it has a cofactor and apoenzyme and this whole is called as holoenzyme

A cofactor can be organic or inorganic in nature which is a heat stable part of this conjugate enzyme. The energy which is required to start a chemical reaction is called activation energy which gives a boost to the kinetic energy of the system making precise and forceful collisions possible between the reactants. 

Evidences have showed that enzymes combine with substrates to form a complex which is called an enzyme-substrate complex. Pavlov defined enzymes as "activators and key of life".

Structure of enzymes:

Enzymes are made up of amino acids and there are three structures that an enzyme forms. Those structures are as follows:

1. Primary structure:

To make the primary structure, the amino acids are linked together in a liner chain by amide (peptide) bonds. This formed chain of amino acid is called a polypeptide chain or protein. Thus, now we know why enzymes are known as proteins. 

Primary structure of enzyme 

Now, the animo acids won't form by themselves in a linear chain instead it is converted by DNA sequence of a gene which is similar.

2. Secondary structure:

To form the secondary structure, hydrogen present in the amino acid (NH2) and oxygen present in the carboxyl group (COOH) are bond together by hydrogen bonds. That means the amino acids present in the same chain can mix with each other. 

There are two forms in the secondary structure - α-helix and β-sheet. α-helix is formed when the chain keeps wrapping around itself again and again in horizontal direction. β-sheet is formed when the chain fold itself vertically in the upwards direction. 

3. Tertiary structure: 

Since the chain has an ability to keep folding upwards it starts to form a 2D-linear chain in the secondary structure. It keeps on folding up further giving rise to three-dimensional (3D) structure. This make up the tertiary structure

Properties of enzymes:

Enzymes show number of properties and for that reason I have divided the properties of enzymes in three parts - physical properties, chemical properties and general properties. 

1. Physical properties: 

Enzymes behave as colloids. Colloids are substances which have higher molecular weight, that means enzymes have a high molecular weight. They diffuse slowly or they just don't pass through collodion or a similar membrane. 

According to Knitz and Northros in 1934 said that, the activities of enzyme is destroyed if it is heated at higher temperature but can be recovered once it cools down. 

2. Chemical properties:

There are two types of chemical properties, which are as follows:

a. Enzymes as catalyst:

As you know enzymes acts like catalyst in a reaction. A catalyst is a substance which does not use itself in a reaction or make any permanent changes instead it shortens the rate of reaction. As enzymes are have higher molecular weight, just a small quantity can catalyse larger quantities of substrates. 

b. Specificity of enzymes:

Enzymes are very specific in nature. There is different configuration for every enzyme which can only catalyse a certain substrate which fits the enzyme. Enzymes catalyse one particular kind of reaction but some are so specific that they catalyse one particular chemical reaction which includes a particular reactant or substrate. 

This specificity can be explained by lock and key model. Just like how an appropriate key fits inside a lock the same way an appropriate substrate fits inside the enzyme. 

3. General properties:

Following are the basic properties that an enzymes exhibits:
  1. Enzymes are more powerful than inorganic catalyst. 
  2. Enzymes starts and speeds up a chemical reaction. 
  3. The reactions carried out by enzymes are reversible. 
  4. These are unstable compounds which are soluble in water, dilute glycerol, sodium chloride solution and dilute alcohol. 
  5. Enzymes work effectively at room temperature. If their temperature is too high their activity is destroyed and if the temperature is too low their activity decreases. 
  6. Besides carrying out a reaction they can also make changes in their end product either by reversible synthesis or specific synthesis which is taken place by a specific enzyme. 
  7. Each enzyme is most active at a particular pH and temperature. 
  8. Enzymes are amphoteric in nature and can react with acidic substances as well as alkaline substances. 

Types of enzymes: 

The International Commission in 1961 classified enzymes into 6 main categories. These classifications were done on the basis of the type of bond that was affected and the reactions which were catalysed. The 6 categories are as follows:
  1. Oxido-reductase - These are also known as dehydrogenases. These type of enzymes catalyse oxidation and reduction type of reactions by removing or adding electrons from the substrates. Example, the enzyme cytochrome oxidase, oxidises cytochrome
  2. Transferases - This type of enzyme catalyses the transfer of specific group from one substrate to the other. Example, the enzyme glutamate pyruvate transminase shifts an amino group from glutamate to pyruvate
  3. Hydrolases - This type of enzyme catalyses splitting of larger molecules to make smaller ones by adding water and breaking the bonds of ester, ether, peptide, glycosidic, C-C, C-halide, P-N, etc.. 
  4. Isomerases - This type of enzyme catalyses a reaction in which an organic molecule is transferred into isomeric form by rearranging its molecular structure. Example, the enzyme phosphohexoisomerase converts glucose-6-phosphate into fructose-6-phosphate. 
  5. Lyases - This type of enzyme catalyses a specific covalent bonds and removes groups without the need of hydrolysis. Example, the enzyme histidine decarboxylase breaks the covalent bonds between carbon atoms in histidine in order to obtain carbon dioxide and histamine
  6. Ligases - This type of enzyme is also known as synthetases. These help in catalyzing two covalent bond between two substrates to form a larger molecule. Example, the enzyme pyruvate carboxylase bonds pyruvate and carbon dioxide to form oxaloacetate

Mechanism of enzyme action:

The mechanism of enzyme action is based on the "lock and key" model, this is the easiest way one can memorise the mechanism. This was suggested by Emil Fisher. It is still acceptable but the modified name of this hypothesis now is "induced fit" hypothesis. 

The substrate molecule fits inside the active site which is present on the surface of the enzyme just the way a particular type of key fits inside the matching lock. After it fits inside the active site it forms an intermediate compound which is called enzyme-substrate complex. This complex is formed by reversible reaction

It is believed that, this intermediate compound is less stable than the original substance and they tend to break all of a sudden and then the enzyme is free from the substrate. Fruton and Simmonde demonstrated that formation of enzyme-substrate complex is a main and important step in enzyme action. 

As activation energy is needed to carry out a reaction. To reduce this activation energy, enzyme plays a role. It reduces the need of activation energy and carries out biochemical reactions at normal body temperature. 

Non-catalysed reactions need more activation energy than catalysed reaction. Enzyme carbonic anhydrase is the fastest known molecule to hydrate 36 million molecules of carbon dioxide per minute. This is 10 million times faster than non-catalysed reaction.  

Factors affecting enzyme activity:

There are various factors which influence the enzyme activity, some of them are listed below:

1. Temperature:

As mentioned earlier, temperature affects the rate of reaction when enzyme activity is taking place. Enzymes are easily affected by change in temperature. Every enzyme has a particular temperature at which it will work to its fullest, this is called as optimum temperature

Enzyme activity takes place at normal temperature (37-45℃ based on the enzyme), if this temperature increases above 60℃ the enzyme breaks down or even undergoes denaturation. If the temperature is increased to freezing point or above freezing point the enzyme activity will be stopped but not destroyed at it happens when it is exposed to higher temperature. 

2. pH:

Enzyme activity is also influenced by pH of the surrounding. If it is exposed to strong base or strong acid it causes damage that cannot be repaired later in the process. This can take place due to hydrolysis or denaturation of the protein part present in the molecule. If it is exposed to weak base or weak acid, the enzyme can be inactive in some cases. 

Just like temperature, each enzyme works at its fullest at a particular pH, this is called as optimum pH. For example, pepsin which is present in gastric juice acts as an acid and has a pH of 2.0 whereas most intracellular enzymes have a pH near neutrality

3. Substrate concentration:

The rate of reaction increases as the there is an increase in the substrate concentration up to a certain limit. Once it crosses this certain limit, there are no free enzyme molecules left in order to bind with the additional substrate molecules. Hence, the reaction reaches a maximum velocity. 

Maximum velocity is when a reaction stops gaining speed because it has no potential to gain anymore speed.

4. Inhibitors:

Enzymes are sensitive towards presence of a specific chemical that binds to the enzyme. When the reason for the enzyme activity to stop is due to the presence of this chemical, the chemical is known as inhibitor. There are two kinds of inhibitors - competitive inhibitors or non-competitive inhibitors.

The inhibitors that almost resembles the molecular structure of a substrate are called as competitive inhibitors. They are mostly used to control bacterial pathogens. The inhibitors that binds at some other site instead of the substrate binding site and carries on the activity of an enzyme are called as non-competitive inhibitors. Example, cyanide

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