Catalysis : Some chemicals alter the rate of chemical reaction without undergoing any change, usually hasten the rate of reaction, such substances are known as catalysts and the phenomenon is known as catalysis.
Enzymes : We may call enzymes as organic catalysts. Enzymes increase the rate and efficiency of biochemical reactions in cells.
Simple Enzyme : Enzymes which contain only proteins and are without non-protein part, are called simple enzymes.
Conjugate Enzymes : Enzymes which need non-protein part for their efficient working, are called conjugated enzymes.
Nature of Enzymes : Most enzymes are proteins and they are composed of derivative proteins. The proteinaceous part is called apoenzyme.
Non proteinaceous part is called Co–enzyme.
The apoenzyme and co-enzyme together are called holoenzyme.
Some terms used in enzymes—
Metallo enzyme : If a co–enzyme is metallic cation, it is called metalloenzyme.
Substrate : Substrates are those substances on which enzymes act.
End product : Substances produced from substrate in an enzyme reaction are called enzymatic end product.
Turn Over Activity : Small amount of enzyme convert a large amount of substrate in the product. This is called turn over ability.
Nomenclature of Enzymes : Enzymes are named on the basis of substrate and type of reaction.
On the basis of substrate the suffix, ‘ase’ is added in the name of the substance, e.g. Maltose becomes maltase, sucrose becomes sucrase.
Nature of reaction means the type of reaction catalysed by enzyme, e.g. oxidase is responsible for oxidizing reaction. Invertase brings changes in optical rotation.
Major Groups of Enzymes
I. Hydrolases
1. Carbohydrases : Enzymes hydrolysing carbohydrates.
2. Esterases : Enzymes hydrolysing esters.
3. Proteases : Enzymes hydrolysing peptide bonds.
4. Amidases : It is an enzyme that hydrolyzes amides with the liberation of ammonia and aminoacids.
II. Desmolases
1. Dehydrogenases (Redoxases) : Enzymes which transfer hydrogen between two substrates.
2. Oxidases : Enzymes which produce water by adding electrons (released from electron carries due to oxidation) and protons with O2.
3. Transaminase : Enzymes which transfer amino (NH2) group from amino acid to carboxylic acid.
4. Carboxylases : Enzymes which catalyse carboxylation or decarboxylation reactions.
5. Peroxidases : Enzymes that catalyze the oxidation of various substances by peroxides.
6. Hydrases : They catalyse reactions in which water is added to or removed from substrate.
7. Transphosphorylases : These enzymes catalyse transfer of phosphate from one substrate to another.
8. Isomerases : They catalyse isomerisation reactions.
IUB Classification
1. Oxydoreductases : Those enzymes which catalyse oxido-reduction reactions (i.e.) transfer of H and O atoms or electrons from one substrate to another.
2. Transferases : Those enzymes which catalyse group transfers except H2 between two substrates. (Transfer of methyl-, acyl-, amino-, or phosphate groups).
3. Hydrolases : These enzymes hydrolyse a variety of components.
4. Lyases : These enzymes catalyse non-hydrolytic removal or addition of group from substrate. They act on C—C, C—O, C—N, C—S and C—halide bonds and form compounds with C = O, C = C, C = N like bonds.
5. Isomerases : They catalyse isomerisation reactions. They catalyse the rearrangement of molecular structure to form isomers. (Dextrose to Levo, cis to trans, Aldose to ketose).
6. Ligases and synthetases : These enzymes catalyse joining of two molecules by synthesis of new C—O, C—S, C—N or C—C bonds with simultaneous breakdown of ATP.
Activation Energy : The energy that is required for a substrate to react in order to get converted into end products is called the activation energy.
Characteristics of Enzymes
(i) Molecular Weight : Molecular weights of enzymes are high because they are made of proteins, e.g. urease has molecular weight of 4,83,000.
(ii) Amphoteric in nature : Enzymatic proteins are amphoteric in nature, i.e. capable of ionising either as an acid or as a base depending upon the pH value of the external solution.
(iii) Colloidal nature : Enzymes are colloidal in nature and this presents a large surface area for reactions to take place.
(iv) Specificity : All enzymes have high degree of specificity and can catalyse only specific types of reactions.
(v) Heat sensitivity : Most enzymes get inactivated or destroyed at a temperature below the boiling point of water. Most enzymes in liquid medium become inactivated at 50OC temperature.
(vi) Catalytic property : In biochemical reactions, extremely small amount of enzyme is necessary to convert a large amount of substrate into product.
(vii) Maintaining equilibrium : Although enzymes considerably hasten the completion of the reactions, they do not change the equilibrium of the reaction. Like a true catalyst enzymes accelerate the reaction forward or backward so as to maintain an equilibrium between substrate and product.
(viii) Inhibition : Enzyme activity can be inhibited in three ways—
(a) Competitive inhibition : Substances having identical structure to the substrate combine at the active site of the enzyme are called competitive inhibitors and such mechanism is known as competitive inhibition.
(b) Non-competitive inhibition : These substances do not combine with the enzyme at their active sites but get attached anywhere else on the enzyme and destroy its activity, and such mechanism is known as non-competitive inhibition.
(c) Feedback inhibition : If end product of any enzymatic reaction inhibits the enzyme catalysing the first step, such regulatory mechanism is called feedback inhibition.
Mechanism of enzyme action : In enzyme reactions, the enzyme combines with the substrate and forms an enzyme substrate intermediate complex. Intermediate complex requires less energy than it does to active molecules by heat, so we can say that enzyme lowers the energy of activation required by the substance, thus increases the rate of the unstable intermediate formed and hence the final product.
Two theories have been put forward to explain the mechanism of enzymatic actio—
(a) Lock and key hypothesis : Fischer in 1894 suggested that the enzyme and its substrate have a complementary shape. Specific substrate molecules can be attached to the active site of a specific enzyme forming enzyme substrate complex.
(b) Induced fit hypothesis : Koshland in 1960 said that active site of the enzyme does not exist at the initial stage. The active site and substrate initially have different shapes but become complementary upon substrate binding.
Factors affecting enzyme activity
Like chemical catalytic reactions, enzymatic reactions are also susceptible to external conditions.
(i) Substrate concentration : At a constant enzyme concentrate, increase in the substrate concentration increases the rate of enzymatic activity.
(ii) Enzyme concentration : Increase in the enzyme concentration would have the same effect on the rate of reaction as would be of increasing the substrate concentration.
(iii) Temperature : The reaction rate increases on an average 2.5 times for every 10OC rise in temperature upto 25OC. Enzyme has a optimum temperature which is usually somewhere between 30OC to 50OC. But above 50OC,enzymatic activity is reduced.
(iv) Hydrogen ion concentration : Different enzymes have different optimum levels of pH generally ranging between 2 to 10.
(v) Concentration of end products : Accumulation of the end products decreases the rate of enzyme activity.
(vi) Hydration : Enzyme activity is very low in dry conditions, such as dry viable seeds with the imbibition of water during germination, the activity of enzyme increases.
(vii) Heavy metals : Many metals like Ag, Cu, Cd, Pb and Zn can inactivate enzymatic action.
(viii) Ultraviolet light : Ultraviolet light has inactivating effect on enzymatic activity.
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