Enzymes are catalysts that change the rate of a reaction without being changed themselves. Enzymes are highly specific and their activity can be regulated. Virtually all enzymes are proteins, although some catalytically active RNAs have been identified (Ribozymes).
Enzymes were discovered accidentally when a German scientist Buchner (1897) found that extract from yeast cells could also carry out fermentation. The term Enzyme was coined by W. Kuhne (1898).
Structure and Composition of Enzyme
Enzymes are proteinaceous in nature. They are giant molecules and their molecular weight ranges from 35,000 (pepsin) to 483,000 (urease). Enzyme may entirely consist of protein (e.g. amylase, pepsin) or may contain a non-protein part.
If an enzyme consists only of protein, it is called simple protein enzyme and if it contains another group it is called Conjugated protein enzyme. Conjugated enzymes showing complete activity are called holoenzyme. This consists of two portions:
The protein part of enzyme is called Apoenzyme
Non protein part is called prosthetic group or Cofactor.
Depending upon the nature of prosthetic group, conjugated enzymes are of two categories:
1. The prosthetic group is an inorganic ion- Cofactor. Role of manganese, potassium and calcium in phosphatases, phosphorylases, carboxylases is well known.
2. The prosthetic group is an organic compound- Conezyme. A coenzyme constitutes about 1% of the entire enzyme molecule. This part of enzyme can be separated, usually heat resistant. Eg. NAD, NADP, FMN and FAD etc.
Properties of enzymes:
1. Enzymes are colloidal in nature- thus provide large surface area for reaction to take place, hydrophilic in nature and form hydrosols in free state.
2. Enzymes can react with both acidic and alkaline substances- due to presence of protein (amphoteric in nature).
3. Enzymes show sensitivity- inactivated by all those factors and suctances which can precipitate or denature protein.
4. Enzymes are thermomobile.
5. Enzyme activity can be inhibited or accelerated.
6. Enzymes are organic catalyst.
7. Enzyme molecules are too much greater.
8. Enzymes are specific.
9. To a great extent enzyme activity is controlled by pH.
10. Enzymes lower down the energy of activation.
The two models have been proposed to explain how enzyme binds its substrate. In the Lock and Key model proposed by Emil Fischer in 1894, the shape of the substrate and the active site of the enzyme are thought to fit together like a key into its lock. The two shapes are considered as rigid and fixed.
In the induced fit model proposed in 1985 by Daniel E. Koshland, the binding of substrate induces a conformational change in the active site of the enzyme.
Enzyme Unit
Enzyme activity may be expressed in a number of ways. The commonest way is by the initial rate (VO) of the reaction being catalyzed (eg, �mol of substrate transformed per minute, �mol/min). There are also two standard units of enzymes activity; the enzyme unit (U) and the katal (kat).
An enzyme unit is that amount of enzyme, which will catalyze the transformation of 1 �mol of substrate per minute at 25OC under optimal condition.
The katal is accepted SI unit of enzyme and is defined as that catalytic activity which will raise the rate of a reaction by one mole per second in a specified system.
