Enzymes are organic catalysts which catalyze biochemical reactions without being utilized themselves.
|Enzymes: Structure and Functions|
Enzymes occur in a colloidal state and are often produced in inactive form called proenzymes or zymogens. These are converted to active enzymes in the presence of specific factor like pH, substrate, etc. with few exceptions.
All known enzymes are proteins but all proteins are not enzymes. Ribozyme, ribonuclease-P and peptidyl transferase are three non-protein enzymes.
Lysozymes have slightly different molecular structures but exert similar catalytic action, e.g., lactic dehydrogenase (LDH).
Components of enzyme: –
Cofactors: – Cofactors are non-protein substances which associate with enzymes. It is only in the presence of cofactorsthat some enzymes can exhibit their catalytic activities e.g., Mg++ions.
Apoenzyme: – An enzyme which functions only in the presence of a cofactor is called apoenzyme.
Coenzyme: – If the cofactor is an organic compound which can be easily separated from the apoenzyme is called a coenzyme.
Prosthetic group: – If the co-factor is firmly bound with the enzyme it is called prosthetic group.
Holoenzyme: – A working combination of an apoenzyme and its co-factor is called as holoenzyme.
Broadly there are six classes of enzyme:
1. Oxidoreductases/dehydrogenases: catalyse oxidoreduction between two substrates S and S`.
2. Transferases: catalyse transfer of a group, G (other than hydrogen) between a pair of substrates S and S`).
3. Hydrolases: catalyse hydrolysis of ester, ether, peptide, glycosidic, C-C, C-halide or P-N bonds.
4. Lyases: catalyse removal of groups from substrates by mechanisms other than hydrolysis leaving double bonds.
5. Isomerases: catalyse inter-conversion of optical, geometrical or positional isomers.
6. Ligases: catalyse the linking together of two compounds, e.g., enzymes which catalyse joining of C-O, C-S, C-N, P-O, etc. bonds.
Turn over number: – The number of substrate molecules converted to products per minute by enzyme molecule is called the turn over number. This exhibits the efficiencyof an enzyme.
All enzyme have a specific three-dimensional structure.
Active site: – A part of an enzyme into which substrate fits, is known as active site. The point where substrate is bound on the active site is known as the substrate binding site.
When a substrate binds into active site of an enzyme then, there is an obligatory formation of an ‘ES’(enzyme substrate) complex. This complex formation is a transient phenomenon.
During the state where substrate is bound to the enzyme active site, a new structure of the substrate called transition state structure is formed. Very soon, the product is released form the active site.
If ‘P’ is at a lower level than ‘S’, the reaction is an exothermic reaction and is not supplied energy (by heating) in order to form the product.
However, whether is an exothermicor spontaneous reaction or an endothermic or energy requiring reaction, the ‘S’ has to go through a much higher energy state or transition state. The difference in average energy content of ‘S’ form that of this transition state is called ‘activation energy’.
Enzymes eventually brings down this energy barrier making the transition of ‘S’ to ‘P’ more easy.