What is enzyme, properties of enzyme, Location of enzyme, Energy of activation, Classification of enzymes, mechanism of action enzyme, factors influencing enzyme action, enzyme inhibition and cofactors of enzyme.
What is enzymes ?
Enzymes are are biological catalysts. They catalyse biochemical reactions in living systems. Almost all enzymes are proteins. There are some nucleic acids that behave like enzymes. These are called ribozymes. An enzyme like any protein has a primary structure that is amino acid sequence of the protein. An enzyme like any protein has the secondary and tertiary structure. When you look at tertiary structure you will notice that the backbone of the protein chain folds upon itself the chain criss-crosses itself and hence , many crevices or pockets are made. One such pocket is the 'active site'. An active site of an enzyme is a crevice Or pocket into which the substrate fits. Thus enzymes through their active site,catalyse reactions at a high rate.
Enzyme catalyst differ from inorganic catalysts in many ways what one major difference needs mention inorganic catalyst work efficiently at high temperature and high pressure. While enzyme get damaged at high temperature at above 40 degree Celsius. However enzyme isolated from organism who normally live under extremely high temperature that is hot vents and Sulphur Springs , are stable and retain their catalytic power even at high temperature up to 80 to 90 degree Celsius. Thermal stability is such important quality of isolated from thermophilic organisms .
Properties of enzymes
• Enzymes are required only in in small amounts .
• They quicken the reactions without being consumed o r lost in the process.
• Enzymes are proteins like all protein enzymes are colloidal in nature and precipitated bi salt solutions.
• They are in activated by heat and alteration of PH .
• enzymes have great specificity. Each enzyme catalyzes one particular reaction.
Location of enzyme
Enzymes maybe present inside the cell o r outside the cell. For example :
• enzymes of citric acid cycle are present in the mitochondria.
• Enzymes of glycolytic pathway are present in the cytoplasm.
• Enzymes for the transport of nutrients are present in the cell membrane.
Energy of activation
Enzymes catalyse various biochemical reactions. Normally, for any reaction to occur, the reacting molecules must come in contact. For this, the molecules must gain minimum amount of energy this is called energy of activation. Normally, the energy of activation can be lowered by increasing the temperature but in human body, the temperature is constant. So enzymes act by lowering the energy of
activation at normal body temperature.
Classification of enzymes
Enzymes are classified according to the type of reaction catalyzed by them. The following are major classes of enzymes
• Oxidoreductases
These enzymes catalyse oxidation and reduction of their substrates.
Examples : Alcohal dehydrogenase , lactate dehydrogenase, glucose -6 -phosphate dehydrogenase .
• Transferases
These enzymes catalyse the transfer of of particular group from one substrate to another .
- Examples :
- Transaminase
- Transacylase
- Transpeptidase
• Hydrolases
These enzymes catalyse hydrolysis of substrates .
• Lyases
They catalyse addition o r removal of groups from the the substrate mechanism other than oxidation reduction and hydrolysis .
Examples : carboxylase ,decarboxylase, aldolase and enolase
• Isomerases
They catalyse the conversion of compound into its isomer .
Examples : Isomerases, racemase and epimerase
• Ligases
They catalyse the linking together of two compounds coupled with breakdown of ATP o r GTP
Examples : DNA ligase, acetyl CoA ligase, succinyl CoA ligase.
Mechanism of enzyme action
Enzyme acts on a substrate and converts it into a product. The enzyme is regenerated at the end of the reaction. The sequence at which this occurs is as follows :
• All enzymes have reactive sites on their surface . The substrate fits on the reactive site of the enzyme. This produces a complex called enzyme substrate complex .
• This complex activates the substrate which has fit on the surface of the enzyme. As a result an activated complex is formed.
Factors influencing enzyme action
• Temperature
The rate of enzyme action increases with increase in temperature. This happens only e up to 40 degree Celsius beyond this temperature the enzyme gets denatured. So the rate of enzyme action comes down film the temperature reaches beyond 40 degree Celsius. The temperature at which enzyme action is maximum is called optimum temperature.
• PH
The rate of enzyme reaction increases with increases in pH. After the maximum rate is reached the activity decreases with further increase in pH. The pH at which enzyme action is maximum is called optimum PH.
• Concentration of enzyme
The rate of enzyme reaction increases with increase in enzyme concentration. This is true only when sufficient substrate molecules are available for combination with enzyme. When all the substrate molecules are saturated with the enzyme further increase in enzyme concentration has no effect.
• concentration of substrate
The rate of enzyme reaction increases with increase in concentration of the substrate. After a certain limit it becomes constant. So further increases concentration has no effect .
ENZYME INHIBITION
Enzyme inhibitors are substances which lower down the rate of enzyme reaction. They produce dear effect by acting on the coenzyme enzyme are prosthetic group. Also , they can act by inhibiting the combination of the substrate with the enzyme.
Enzyme inhibition is classified as follows :
• Competitive inhibition
The inhibitor and the substrate have structural similarity . So they both compete with each other to bind with the enzyme.
The inhibitor is successful in this competition. So an enzyme inhibitor complex is formed. This complex cannot lead to the formation of a product (which can be formed only by enzyme-substrate complex ).
Enzyme + Inhibitor <------------> E - In ------> No
Product
E + S <---------> ES -------------> product
Competitive inhibition is a reversible type of reaction. The inhibition can be reversed by adding and excess of the substrate. The excess substrate will displace the inhibitor from the enzyme.
Example : The conversion of succinic acid to fumaric is catalysed by the enzyme succinic dehydrogenase .
• The reaction is inhibited by malonic acid which has a structural resemblance to succinic acid
• Non -competitive inhibition
This type of inhibition does not involve competition between the substrate and the enzyme. The inhibitor affects the enzyme substrate complex and prevents its dissociation to release the product.
ES + I --------> ESI ----------> No product
Example : enzymes with SH group are Non competitively inhibited by metal ions like
Ag++ and Hg++.
This type of inhibition does not require structural similarity between the the inhibitor and the substrate. Also the inhibition cannot be reversed by excess concentration of the substrate .
•Allosteric inhibition
Allosteric site is a site other than the active site which is present in the enzyme. The inhibitor binds to the allosteric site and produces conformational changes in the enzyme. So the substrate cannot bind with the enzyme and a product cannot be formed .
Examples
• ATP is an allosteric inhibitor of hexokinase.
• ADP is an allosteric inhibitor of pyruvate carboxylase.
CO-FACTORS OF ENZYMES
Enzymes are composed of one or several polypeptide chains. However there are a number of cases in which non protein constituents called co-factors are bound to the enzyme to make the enzyme catalytically active. In the instances, the protein portion of the enzymes is called the apoenzyme.
Three kinds of cofactors may be identified prosthetic groups,coenzymes and metal ions
Prosthetic groups are organic compounds and our distinguished from other core factors in that they are tightly bound to the apoenzyme, heam is the prosthetic group and it is a part of the active site of the enzyme.
For Example, peroxidases and catalase whitch catalyse the breakdown of hydrogen peroxide to water and oxygen.
Coenzymes are also organic compounds but their Association with the apoenzyme is only transient usually occurring during the course of catalysis. Furthermore coenzymes serve as cofactors in a number of different enzyme catalysed reactions .The essential chemical components of many coenzymes are vitamins eg. Coenzyme nicotinamide adenine dinucleotide (NAD) and NADP contain the vitamin niacin.
A number of enzymes require metal ions for their activity which form coordination bonds with side chains at the active site and at the same time form one or more coordination bonds with the substrate eg. zinc is a cofactor for the proteolytic enzyme carboxypeptidase .
Catalytic activity is lost when the cofactor is removed from the enzyme which testifies that they play a crucial role in the catalytic activity of the enzyme.
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