Enzymes are specialized protein complexes that serves as catalysts and are employed to accelerate the rate of reactions. They carry out chemical reactions within the mild conditions of temperature, pH, and pressure of the cells.
Catalyst is a substance that increases the velocity or rate of a chemical reaction without itself undergoing any change in overall process. Thus, enzymes accelerates the rate of reaction without any change upon themselves.
These are biocatalysts synthesized by living cells. . They are colloidal and thermolabile in character, and specific in their action.
Enzymes speeds up reactions by as much as a million times or more than that of uncatalyzed one and are central to every biochemical process. Indeed, most reactions in biological systems do not take place at perceptible rates in the absence of enzymes.
They are remarkable molecular devices that determine patterns of chemical transformations and also mediate transformation of one form of energy into another.
Enzymes are highly specific both in reactions that they catalyze and in their choice of reactants i.e. substrates. An enzyme usually catalyzes a single chemical reaction or a set of closely related reactions. Side reactions leading to the wasteful formation of by-products are rare in enzyme-catalyzed reactions, in contrast with uncatalyzed ones.
Some enzymes require no chemical groups for activity other than their amino acid residues while others require an additional chemical component called a cofactor i.e. either one or more inorganic ions, such as Mg2+, Zn2+ or a complex organic or metallo-organic molecule called coenzyme. Some of them require both coenzyme and cofactor. Catalysis takes place at a particular site on the enzyme called the active site.
A coenzyme or metal ion that is very tightly or even covalently bound to the enzyme protein is called a prosthetic group. A complete, catalytically active enzyme together with its bound coenzyme and/or metal ions is called a holoenzyme. The protein part of such an enzyme is called the apoenzyme or apoprotein.
Holoenzyme = Apoenzyme + Coenzyme and/or Cofactor
Apoenzyme is polypeptide or protein part and may be inactive in its primary synthesized structure. The inactive form of the apoenzyme is known as a proenzyme or zymogen.
Coenzymes act as transient carriers of specific functional groups. Most of them are derived from vitamins, organic nutrients required in small amounts in diet.
Features of Enzyme
The most striking characteristics of enzymes are their catalytic power and specificity.
- Enzymes are protein macro-molecules.
- They have a defined amino acid sequence and are typically 100-500 amino acids long.
- They have a defined three-dimensional structure.
- Mostly enzymes are proteinaceous in nature except a small group of catalytic RNA molecule (ribozymes).
- Enzymes are catalysts
- They act as a catalyst to a chemical or biochemical reaction, with a defined mechanism.
- They increase the speed of that reaction, typically by 106-1014 times faster than the rate of the uncatalyzed reaction by lowering the activation energy.
- They are selective for a single substrate (substrate specificity).
- They are stereospecific, i.e. reaction produces a single product.
Classification of enzymes can be based on the reactions they catalyze, their site of occurence and action as well as their composition etc.