Enzymes


What are Enzymes?

Enzymes are biological catalysts which accelerate the rate of biochemical reactions. They regulate the speed and specificity of reactions without being used. They are produced by living cells only. They integrate cellular chemical reactions and provide the order without which complex processes of life would not be possible.



Enzymes are Protins

The following factors prove that enzymes are proteins.
(1) When an enzyme is acted upon by pepsin (a protein digesting enzyme), the crystalline composition of enzyme disappears with a decrease in its catalytic activity.
(2) Enzymes have complex structure like proteins which are unique complex molecules.
(3) Heat, acid, alkali and other agents which denatured proteins also destroy the activity of enzymes.
(4) Enzymes show color reactions and ultraviolet absorption spectrum, a characteristic of proteins,
(5) Enzymes are themselves subject to a variety of cellular controls and their rate of synthesis as well as their final concentration is under genetic control.




Molecular Structure of Enzymes

In general, enzymes are protein molecules of high molecular weight. They are amphoteric and colloidal in nature. There are two general types of structures of enzyme molecules. Those are as follows:

Simple protein molecules

They are proteins containing only amino acids.

Conjugated protein molecules

They are proteins with attached non-protein groups. A conjugated enzyme thus consists of two parts : (i) apoenzyme, which is composed of amino acids only, and (ii) prosthetic group which is a non-protein group. The enzyme molecule consisting of protein and prosthetic group is known as holoenzyme.
The prosthetic group may be an inorganic cofactor or an organic coenzyme. The inorganic factor is usually a metal associated with the catalytic properties of the enzyme. Separation of apoenzyme from its metal component usually results in complete loss of activity. The activity is resumed with re-establishment of the metal to apoenzyme. Many enzymes associated with the glycolysis require metal cofactors. The metals .known to be cofactors of enzyme systems are copper, iron, manganese, zinc, calcium, potassium, and cobalt. An organic coenzyme generally acts as a donor or acceptor of atoms that are added to or removed from the substrate. Many coenzymes play a very important role in oxidation-reduction reactions.

Enzymes and Inorganic Catalysts

Enzymes are organic biocatalysts and they show many similarities with inorganic catalysts on the one hand and dissimilarities on the other.

Similarities

(1) Both enzymes and inorganic catalysts remain unchanged chemically and quantitatively after the reaction and can be used repeatedly.
(2) Both are required in very small quantities as compared to substrate.
(3) Both do not initiate a reaction, but only enhance its rate by lowering the activation energy.
(4) Both do not alter the equilibrium of a reversible reaction.
(5) Both form intermediate, short-lived complexes with the substrate.

Dissimilarities

(1) Enzymes are specific protein molecules with complex 3-dimensional structure and inorganic catalyst are small inorganic molecules, usually metal ions.
(2) Enzymes can catalyse only specific reaction of a single or structurally related substrates whereas the inorganic catalyst can catalyse a wide range of reactions.
(3) Catalytic action of enzymes can be regulated by specific regulating molecules which change their conformation whereas catalytic action of inorganic cataltst can not be regulated by any regulating molecule.
(4) The enzymes are very sensitive to change in pH and temperature and the inorganic catalysts are less sensitive to pH and temperature changes.




Properties of Enzymes

The general properties of enzymes are given below:
(1) They are active in extremely small amounts. The number of moles of substrate converted per minute by one mole of enzyme is called turnover number of the enzyme.
(2) Under stable conditions enzymes remain unaffected by the reaction they catalyze.
(3) An enzyme though considerably hastens completion of a reaction, it does not affect the equilibrium of the reaction.
(4) They are very specific in nature, i.e., an enzyme that catalyzes one reaction may not catalyze another.
(5) Being proteinaceous in nature, enzymes are thermolabile in nature, i.e., they generally work over a narrow range of temperature.


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