Mode of Action of Enzymes

Mode of Action of Enzymes:

There are  two views regarding the mode of enzyme action: lock_ and _ key hypothesis and induced_ fit hypothesis. 

1): Lock_ and_Key Hypothesis:

It was suggest  by Emil Fisher in 1894. The enzyme molecule operates by chemically uniting with the substrate molecules. The combination of the two is known as the enzyme_ substrate complex (ES). An enzyme may combine two small substrate molecules into one larger product molecule, or may split a large substrate molecule into two smaller product molecules. 

(i) Combining Reactions:

The catalytic property of the enzymes is located at specific regions on their surface. These regions are called active sites, or catalytic sites , or active centres. The latter are small pockets or grooves formed as the enzyme molecules fold up to acquire 3_ dimensional forms. The size,shape and electrical charge of amino acid R groups at the active site determine which substrates can fit there. The substrates have on their surface specificae can fit there. The substrates have on their surface specific areas, the reactive sites, to precisely fit the active sites of enzymes. The active and the reactive sites fit like a lock_ and _ key.The enzyme molecule holds the substrate molecules close together in such a way that their reactive sites are juxtaposed, thereby lowering the activation energy and facilitating their union to form a single large molecule. The enzyme molecules bring the reactant molecules into proper contract far faster than chance collisions at that temperature. The enzymes also pull on the bonds of the substrates and loosen them. The reactions are, therefore, accelerated. 

A single molecule of the fastest known enzyme, carbonic anhydrase found in red blood cells, hydrates 36 million ( 36,000,000) molecules of CO2 to carbonic acid and H+ in just a minute.

                 Carbonic Anhydrase
CO2 + H2O➡️➡️➡️➡️➡️➡️➡️➡️HCO‐ + H+

An enzyme catalysed reaction is some 10 million times faster than the noncatalysed one.

The enzyme molecule itself remains unaffected. It only provides a uniquely structured platform or template on which the specific substrate molecules can join and interact. For a short period, the product remains joined to the enzyme, forming an enzyme _ product complex. Soon the product leaves the enzyme surface. The enzyme molecule is now free to receive a new set of starting molecules. Thus, the same enzyme is being used again and again. This is why the enzymes are needed in very small amounts.

  The scheme of lock_ and_ key mode of enzyme action is summarised below__ 

Enzyme+ Substrate ➡️Enzyme _substrate complex 

Enzyme_ substrate complex ➡️Enzyme- product complex 

Enzyme-product complex ➡️Enzyme+ product

(ii) Splitting Reactions:

Enzymes also split large molecules into smaller ones. The enzyme molecule unites with the substrate molecule at two or more places, and hold the substrate molecule in a way that strains its molecular bonds and facilitates their break. After the break,the products leave the enzyme which is reused.

2): Induced Fit Hypothesis:

This is the current hypothesis for enzymatic catalysis. It was suggested by Daniel E. Koshland and others in 1959. It includes the following particulars__

(a): There is an intermediate condition, called transition state, between the substrate and the products. In this state, the " old" chemical bonds break, atoms assume new positions, and " new" bonds are formed . The transition state is highly unstable and persists for a very short period.

(b): The active site does not have a rigid lock_ and _ key conformation for the substrates. As the substrates enter the active sites, they induce the enzyme to change its shape slightly so that the active sites fit more closely around the substrate. This induced fit is like a clasping handshake.

(c): The induced fit is possible because of the flexibility of the protein molecules. The tight fit conformation holds the substrates at the correct and for the reaction to occur. The enzyme also pulls on the substrates' bonds and " loosens" them. This brings the substrates into the transition state, and also lowers the activation energy to let the reaction occur readily.

(d) In the unstable transition state, the enzyme pull atoms off each substrate. The substrates bind to each other, forming the product.

(e) The product that fits the last conformation of the active site leaves the enzyme molecule.

(f) Then the detached atoms combine and leave the enzyme. The latter is now free to receive the new set of substrate molecules. 

Isoenzymes ( Isozymes):

There are certain enzymes which have slightly different structure but  have similar catalytic function.Such enzymes are called isoenzymes, or simply isozymes. More than 100 enzymes are known to have isozymes. A good example of isoenzymes is lactic dehydrogenase ( LDH). It catalyze change of pyruvate to lactate. There are five LDH isoenzymes in humans. Alcohol dehydrogenase has four isoenzymes in maize. The isoenzymes differ in optimum activity and enable the organism to adapt to varied environmental condition. It is held that the isoenzymes are produced by genetic changes during evolution. 


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