Any molecule that increases the rate of a chemical reaction without being used up during that reaction is catalyst. Biological catalysts are primarily protein molecules known as enzymes. Enzymes control reactions in living cells.
An active enzyme may speed up a particular reaction, but living organisms do not need all reactions to be going at the maximum rate all of the time. Actually enzymes organize the biological systems.
Many of the complex chemicals that living organisms need cannot be made in a single reaction. Instead, a series of simpler reactions occurs, one reaction after another, forming a metabolic pathway. For example different pathways in Photosynthesis & Respiration.
MODELS TO EXPLAIN HOW ENZYMES WORK
The Lock and Key theory:
Enzyme function depends on an area on the molecule known as the active site.
The active site is a groove or pocket in the surface of the enzyme into which the substrate molecule fits. Typically, the active site is formed by 3 to 12 amino acids.
The size, shape and chemical nature of the active site matches with the substrate molecule.
According to this theorythe active site is a rigid structure. There is no modification or flexibility in the active site before, during or after the enzyme action and it is used only as a template. This theory /model does not support all reactions.
Induce Fit theory:
On the basis of new evidences Koshland (1959) proposed this theory /model.
The active site in many enzymes is not exactly the same shape as the substrate, but moulds itself around the substrate as the enzyme-substrate complex is formed. Only when the substrate binds to the enzyme in the active site the correct shape is developed. It means that when a substrate combines with an enzyme, it induces changes in the enzyme structure. This change allows the enzyme to do its catalytic activity more effectively.
How Fast Do Enzymes Work?
The speed at which an enzyme works is expressed as its turnover number. This is the number of substrate molecules turned into product in one minute by one molecule of enzyme. Values range from less than a hundred to many millions.
Naming and Classifying Enzymes:
International Union of Biochemistry has developed a scheme for naming and classifying enzymes. Enzymes are mostly named by adding the suffix -ase to the name of their substrate. The rest of the name indicates the nature of the reaction. For example Alcohol dehydrogenase catalysis the removal of hydrogen from alcohol (ethanol).
Six Main Categories of Enzymes:
Oxidoreductases: These are involved in oxidation and reduction (redox) reactions. In aerobic respiration, most of the cell’s ATP is generated by redox reactions.
Transferases: These catalyze the transfer of a chemical group from one compound to another. Transfer of an amino group from an amino acid to another organic acid in the process of transamination.
Hydrolases: These catalyze hydrolyses (splitting by use of water) reactions. Most digestive enzymes are hydrolases.
Ligases: These catalyze the breakdown of molecules by reactions that do not involved hydrolysis.
Isomerases: These catalyze the transformation of one isomer into another, for instance the conversion of glucose 1, 6 bisphosphate into fructose 1, 6 bisphosphate.
Ligases: These forms bonds between compounds using ATP. For example DNA ligase is involved in the synthesis of DNA.
Enzymes and Activation Energy:
To start chemical reactions some energy is required. This energy is called activation energy. The way enzymes operate is by lowering the amount of activation energy required for a chemical reaction to start. This happens because enzymes weaken a covalent bond within a substrate molecule. In other cases this lowering of activation energy happens because the enzyme holds the substrate molecules in a particular way.