Definition of Enzyme:

Catalytically active protein of biological origin or organic catalyst produced by living cells

Substrate: Substance upon which an enzyme acts.

Classification of Enzyme (IUD System, 1961):

Based on Type of Reaction Enzyme is classified as below:
1) Oxidoreducctase (Dehydrogenases):

Reaction catalyzed – Biological oxidation- reduction- ex. Pyruvie oxireductase, Ascorbic oxidase, succinic dehydrogenase, cytochrome oxidase.

2) Transferases (Transaminase):

Ex. Transketolase, Transmethylase,Reaction catalyzed: Transfer of groups.

3) Hydrolases: Ex- Lipases, urease, pepsin, amylase, cholinesterase.Reaction catalyzed – Hydrolysis

4) Lyases (Decarboxylases):  Ex. carboxylases, aldolases, fumarase.Reaction catalyzed – catalyzed removal of groups

5) Isomerases: Ex. triose phosphate isomerase, epimerase, alanine racemase.Reaction catalyzed – Catalyse conversion to other isomeric form.
6) Ligases or synthetases: Ex. Succinic thiokinase, Glutamine synthetase.Reaction catalyzed – catalyse linking together of two compounds.

Mechanism of Enzyme Action:

The enzyme combines with the substance on which it acts (substrate) to form an enzyme substrate complex called Michaelis complex

E+S          → ES

Enzyme is liberated and the substrate is broken into the products of the reaction

ES            →   E + P (products)

Active Site: The region of an enzyme surface that binds the substrate molecule and catalytically transforms it.

Functional Groups Present at the Active Site:

Amino acids such as aspartic acid, glutamic acid, lysine, serine etc. the side chain groups – COOH, – NH2, -CH2OH etc. serve as catalytic groups at active site.

Fischer’s Lock and Key Model:

Activation energy with and. without enzyme

Chemical Nature of Enzyme:

All enzymes are protein in nature without any exception. Most enzymes contain protein and non-protein groups.
Enzyme → Protein + Non Protein
Holoenzyme → Apoenzyme + Cofactor

Cofactors may be divided into three groups

1) Prosthetic Group e.g. Porphyrin
2) Coenzyme e.g. NAD; NADP +
3) Metal Activators e.g. K+, Mn2+, Mg+, Zn2+ etc.

Factors Affecting Enzyme Activity:

1) PH
2) Temperature
3) Substrate Concentration
4) Enzyme Concentration
5) Concentration of any Activator Present
6) Concentration of any Inhibitor Present
7) Ionic Strength
8) Redox Potential
9) Concentration or Reaction Products


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