Mechanism of enzyme action and Enzyme kinetics

Mechanism of enzyme action:

Mechanism of enzyme action means that,how does the enzyme do its job or work in the cell. Enzymes work on starting materials called reactants and convert them into end products.Every mechanism that occurs in a body is complete due to the action of enzymes.For example, in the process of glycolysis, the starting material glucose is converted into acid and alcohol giving energy ATP.

Enzymes are called biological catalyst that speeds up the chemical reaction that occurs in the body faster than the normal reaction occur . But the most interesting things is that how the enzyme works and carry out reaction faster than as usual.We have already studied in chemistry also every reaction has an energy barrier behind this product is obtain. To convert the reactant into product reaction must cross that barrier.This energy barrier is called activation energy.The minimum amount of energy required to cross the energy barrier is called activation energy.

Enzymes accelerate the rate of reaction by decreasing the amount of activation energy of the reaction.Every reaction has its energy barrier to converting substrate into product.The extra energy that must be supplied from outside to cross this energy barrier is called activation energy.Enzymes increase the rate of reaction by decreasing the energy needed for crossing the energy barrier. Substrate molecule binds to the activate site of the enzyme. This binding energy which is used to decrease the activation energy. Two methods are described for binding of enzyme and substrate

The two methods for binding of enzyme and substrate are :

1) Lock and key model

2) Induced fit model

1) Lock and key model:

In lock and key model enzymes and substrate binds together in which the enzyme is considered as the lock and the right key the substrate that fit in it perfectly.To make enzyme specific each enzyme is specific to only one or two substrates.While binding the enzyme with substrate there is the slight change in the shape of the enzyme.We can say that according to this model ,shape of the active site of the enzyme is complementary to that of a substrate ,same as a shape of key complementary to a groove of a lock. Therefore the substrate binds to active site of the enzyme are similar to key binds to lock. The specificity of the enzyme-catalyzed reaction is determined by a shape of activating site and shape of substrate molecules.

In this model following steps occur:

• The metabolite or the substance in which the enzyme act is called substrate,
• When the enzyme combines with substrate molecules then enzyme-substrate complex is formed,
• This, in turn, dissociates releasing products and free enzyme are released for further binding and again the same process occurs,

2) Induced fit model:

To overcome the limitation of lock and key model ,induced fit model is proposed.The main limitation or drawback of lock and key model is that it behaves enzyme molecule as a solid structure.However, enzymes are flexible globular types of protein and do not have any fixed shape . The induced fit model assumes that the substrate plays the main role in the determination of the final shape of the enzyme as the enzymes are partially flexible.This is the only reason why the certain compounds that bind to the enzymes but do not react because enzymes have been distorted too much and other molecule become too small to induced the proper setup and therefore can not react. Only the suitable substrate is capable of the proper alignment of the active site. It is concluded that the shape of an active site of an enzyme is not already complementary shape of the substrate . The complementary shape is induced in active site during binding to substrate

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Nature of active site and substrate interaction:

We already know about the substrate and active site

Substrate: The reactant in the chemical reaction that acted upon the enzyme is called substrate.

Active site: The part of an enzyme to which substrate binds and where the reaction is catalyzed is called active site.

Depending on the particular chemical reaction,for each type of enzyme, there may be one or more substrate ,for some reaction a single reactant substrate is broken down into many products and for some reaction two substrates may come closer to creat one larger molecule.There is also the possibility to enter the both two reactants in a reaction,they become modified and leave the reaction as two products.

The enzymes active site is directly affected by the environmental conditions, as a result, the rate of chemical reaction altered. The increase in environmental temperature the rate of reaction also increases.because the molecules move quickly and more likely to come in contact with each other.However, the increase and decrease in temperature may affect the chemical bond within the enzyme and the shape may also change. If such condition comes ie;the shape of enzymes change ,the active site may no longer bind to the appropriate substrate and subsequently the rate of reaction decreases.The change in temperature and pH may cause the denaturation of the enzyme.

Enzyme inhibition:

Molecules or substances that inhibit an activity of the enzyme called inhibitors and the process is known as enzyme inhibition.There are two types of enzyme inhibition.

A)Reversible enzyme inhibition

Reversible inhibitor inhibits the activity of the enzyme when hey bind to enzyme but enzyme activity can be restored when inhibitors leave the enzyme.There are two types of reversible inhibition

1)Competitive inhibition :

Competitive inhibitors are similar in structure to the normal substrate.The inhibitor binds to active site of the enzyme to which substrate molecule binds.Therefore the inhibitors or substances complete with each other for binding to the same active site.Hence the process is known as competitive inhibitors.

This process is also medically important that can be used in the treatment of various health effect.Eg ethanol which is the competitive inhibitor of methanol is used in the treatment of methanol poisoning.

2)Noncompetitive inhibition:

Noncompetitive inhibition inhibitors do not bind to an active site rather they bind to Allosteric site (extra).Therefore there is no competitive between inhibitor and substrate and the phenomena is called noncompetitive inhibition. Some enzymes contain an extra site other than active site known as an allosteric site.Such enzymes are known as allosteric enzymes. Binding of inhibitor to allosteric site changes the configuration of an active site of enzymes. As a result, a substrate can not bind with enzymes. With the removal of inhibitors, an activity of the enzyme is restored. Hence,this type of mechanism is reversible.

B) Irreversible enzyme inhibition:

Irreversible inhibitors bind covalently with the functional group of active site. Such inhibitors may denature proteins structure of enzyme and damage the function of enzyme permanently. Therefore, an activity of an enzyme can not be restored. Such inhibitors include heavy metal reactive chemical,acids,base ,detergent etc.