Enzymes site of Action


What is the Enzymes site of Action ?

All enzymes are produced in the living cells
 About 2,000 enzymes have  been recorded.These are of two types with regard to the site where they act: intracellular and extracellular. 

1): Intracellular Enzymes:

Most of the enzymes remain and function inside the cells. They are called the intracellular enzymes, or endoenzymes.Some occur dissolved in the cytoplasmic matrix. A water extract of ground up liver cells contains all the eleven enzymes necessary to change glucose to lactic acid. Certain enzymes are bound to particles, such as ribosomes, mitochondria and chloroplasts, in the cell. The respiratory enzymes needed to convert lactic acid to carbon dioxide and water are found in the mitochondria. 

2): Extracellular Enzymes: 

Certain enzymes leave the cells and function outside them. They are called the extracellular Enzymes , or enoenzymes. They mainly include the digestive enzymes, e.g., salivary amylase, gastric pepsin, pancreatic lipase secreted by the cells of the salivary glands, gastric glands and pancreas respectively, lysozyme present in tears and nasal secretion.

The enzymes retain their catalytic action even after extracellular from the cells. Rennet tablets, containing the enzyme rennin from the celf's stomach, are used to coagulate milk protein caseinogen for cheese ( casein) formation. 

How do enzymes bring about such High Rates of chemical Conversion?

                        Or                           What is the active site of enzymes chemical?

To understand this we should study enzymes a little more. We have already understood the idea of an ' active site'. The chemical or metabolic conversion refers to a reaction. The chemical which is converted into a product is called a ' substrate'. Hence enzymes, i.e. proteins with three dimensional structure including an ' active site', convert a substrate ( S) into a product ( P) . Symbolically, this can be depicted as:

 It is now understood that the substrate 'S' has to bind the enzyme at its ' active site'within a given cleft or pocket. The substrate has to diffuse towards the ' Active site '. There is thus, an obligatory formation of an ' ES' complex formation is a transient phenomenon. During the state where substrate is bound to the enzyme active site, a new structure of the substrate called transition state structure is formed. Very soon, after the expected bond breaking/ making is completed, the product is released from the active site. In other words, the structure of substrate site. In other words, the structure of substrate gets transformed into the structure of products(s).The pathway of this transformation must go through the so _ called transition state structure. There could  be many more ' altered structural states'between the stable substrate and the product. Implicit in this statement is the fact that all other intermediate structural states are unstable . Stability is something related to energy status of the molecule or the structure. Hence, when we look at this pictorially through a graph it  looks like something.

The Y_ axis represents the potential energy  content. The X_ axis represents the progression of the structural transformation or states through the ' transition state' . You would notice two things. The energy level difference between S and P
 If 'P' is at a lower level than ' S' , the reaction is an exothermic reaction. One need not supply energy ( by heating) in order to form the product. However, whether it is an exothermic or spontaneous reaction or an endothermic or energy imbalance requiring reaction, the ' S' has to go through a much higher energy state or transition state.The difference in average energy content of ' S' from that of this transition state is called ' activation energy'.

Enzymes eventually bring down this energy barrier making the transition of ' S'to ' P' more easy.

Nature of Enzymes Action:

 Each enzyme( E) has a substrate (S) binding site in its molecule so that a highly reactive enzyme__ substrate complex ( ES) is produced. This complex is short _ lived and dissociates into its products(s) P and the unchanged enzyme with an intermediate formation of the enzymes _ product complex ( EP).

The formation of the ES complex is essential for catalysis .

E+S ↔️↔️ES➡️➡️ EP➡️➡️E+P

The catalytic cycle of an enzyme  action can be described in the following steps:

1): First, the substrate binds to the active site of the enzyme, fitting into the active site.

2): The binding of the substrate induces the enzyme to alter its shape, fitting more tight around the substrate. 

3): The active site of  the enzyme, now in close proximity of the substrate breaks the  chemical bonds of the substrate and the new enzyme_ product complex is formed .

4): The enzyme released the products of the reaction and the  free enzyme is ready to bind to another molecule of the  substrate and run through the catalytic cycle once again.

Factors Affecting Enzymes Activity:

The activity of an enzyme can be affected by a change in the conditions which can alter the tertiary structure of the protein. These include temperature, pH, change in substrate concentration or binding of specific chemical that regulate its activity.

Temperature and pH

Enzymes generally function in a narrow range of temperature and pH. Each enzyme shows its highest activity  at a particular temperature and pH called the optimum temperature and optimum pH. Activity decline both below and above the optimum value. Low temperature preserves the enzyme in a temporarily inactive state whereas high temperature destroys enzymatic activity because proteins are denatured by heat.

Concentration of Substrate:

With the increase in substrate concentration, the velocity of the enzymatic reaction rises at first. The reaction ultimately reaches a maximum velocity ( Vmax) which is not exceeded by any further rise in concentration of the substrate. This is because the enzyme molecules are fewer than the substrate molecules and after saturation of these molecules, there are no free enzyme molecules to bind with the additional substrate molecules. 

The activity of an enzyme is also sensitive to the presence of  specific chemicals that bind to the enzyme. When the binding of the chemical shuts off enzyme activity, the process is called Inhibition and the chemical is called an inhibitor. 

When the Inhibitor closely resembles the substrate in its molecular structure and inhibits the activity of the enzyme, it is known as competitive Inhibitor. Due to it's close structural similarity with the substrate, the Inhibitor competes with the substrate cannot bind and as a result, the enzyme action declines, e.g., inhibition of succinic dehydrogenase by malonate which closely resembles the substrate succinate in structure. Such competitive inhibitors are often used in the control of bacterial pathogens. 


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