Immunology | Immune system| Immunity

         One of the most unique characters of a living cells is its ability to allow complex reactions to proceed rapidly at the temperature of the surrounding.The principle agents which participate in these transformation belong to a group of proteins called ''Enzymes''. Enzymes are biological catalyst.They increase the rate of chemical reactions by taking place within living cells without themselves undergoing any change.The reactants of enzyme catalyzed reactions are called as substrate and each enzymes is quite specific in character.
          All enzymes are proteins.However most of the enzymes have a non-protein component called co-factor.Without this co-factor these enzymes lack their catalytic activity activity. In such as a case,the inactive protein component is called as 'Apoenzyme' and active enzyme including the co-factor is called as 'Holoenzyme''. The co-factor may be an organic molecule called co-enzyme or it may be a metal ion.Some enzymes bind the co-factors very tightly without damaging the enzymes.Such a co-factor is called as prosthetic group.
           The polypeptide or protein part of the enzyme is called the apoenzyme and may be inactive in its original synthesized structure. The inactive form of the apoenzyme is known as a proenzyme or zymogen. The proenzyme may contain several extra amino acids in the protein which are removed, and allows the final specific tertiary structure to be formed before it is activated as an apoenzyme.

Co-enzyme is a small heat stable organic molecule readily dissociates off an enzyme protein and in fact can be dialised from protein.
A co-enzyme has an important relationship with vitamins play an essential role,because most of co-enzyme show a vitamin as a part of their structure.
There are also organic molecules of small size compared to the enzymatic protein.
These molecules are called co-enzymes for historical reasons.They remained firmly bound to the enzymatic protein throughout the purification procedure.
This designation is extremely misleading.There are two broad categories of co-enzymes.The first category which really deserve to be called co-enzyme,includes the co-enzymes which are really part of active site.In other words,the small co-enzyme organic molecules assists the protein side chains in catalysis.The best example of such a co-enzyme is pyridoxal phosphate.
The second category of molecule called co-enzymes does not really deserve its name.it would be preferably to call themselves substrates.The prototype of this type of co-enzyme is NAD.

 Regulatory enzymes:
 A regulatory enzyme is an enzyme in a biochemical pathway which, through its responses to the presence of certain other biomolecules, regulates the pathway's activity. This is usually done for pathways whose products may be needed in different amounts at different times, such as hormone production. Regulatory enzymes exist at high concentrations (low Vmax) so its activity can be increased or decreased with changes in substrate concentrations.
Regulatory enzymes are of two types:
1. Allosteric enzymes
2. Covalently modulated enzymes.
The allosteric enzymes has two binding sites. One of the binding sites is for the substrate of the enzyme, the other site is for small molecules called effectors which modulates the enzymes activity.effectors are non-covalently linked to the enzyme at its allosteric site (site of enzyme where the effector binds) and its interaction with the enzyme is reversible. Based on modulation, allosteric enzymes can be grouped into two groups:
1.homotropic allosteric enzymes.
2.heterotropic allosteric enzymes.