General Nature and Basic Structure of Enzyme

 A living cell is capable of performing a multitude of  biochemical reactions in order to survive grow and multiply. This involves

  • Degradation of complex nutrients into simpler forms so that they can be absorbed by the cell.
  • Uptake of these simple nutrients.
  • Chemical transformation of these simple nutrient molecules into various precursor metabolites, so that they are available for biosynthesis.
  • Generation of ATP and other bio reactive molecules, which co-operate in cellular biochemical reactions.
  • Biosynthesis of cellular molecules and structural components of the cell etc.

All these diverse types of chemical reactions can occur with a high degree of specificity and rate, under normal growth conditions for the organism. These chemical changes are brought about by the living cell through the role and participation of a special class of molecules known as enzymes . Enzymes function in the cell by acting as a biocatalyst.

Discovery of Enzymes

The term enzyme was coined by Kuhne in 1878, suggesting in yeast (en = in, zyme = yeast). It was examined that cell free extract from yeasts is capable of causing conversion of sugar into alcohol. This lead to the understanding that the molecules present inside the cells of yeasts are responsible for these chemical transformations. Kuhne called them as enzymes.

Later on, it was established that all biochemical activities of a living cell are attributed to these magic molecules called enzymes. The first enzyme to be discovered was 'amylase'. Its presence in malt extract was detected in 1833 by two French chemists Pain and Persuses.

General Nature of Enzymes

Enzymes are regarded as organic biocatalysts which are capable of functioning both extra cellular and intra cellular. Following are the major characteristics of enzymes. 

1] With exception of a few catalytic RNA molecules or ribozymes, all enzymes are protein in nature. Over 90 % enzymes are globular proteins. Many of them are conjugated proteins, having non protein component. 

Enzymes may be monomeric or multimeric proteins. As they are proteins, they share all major characteristic of proteins .

  • They are macromolecules with high MW.
  • They are non dialyzable and are unable to pass through semi permeable membrane.
  • They are amphoteric in nature. i.e. they possess both types of lonizable groups : -NH₂ and -COOH. which on ionization . yield positively charged ammonium (NH4+) ion and negatively charged carboxyl (COO-) ion.
  • As they are amphoteric, they possess specific isoelectric pH. At this pH, both -NH₂ and -COOH groups get equally ionized such that their net electrical charge becomes minimum.
  • They show electrophoretic mobility. If the enzyme solution is at pH, above isoelectric value, they acquire negative charge and hence move towards anode and vice versa.
  • They are colloidal in nature.
  • They can be salted out by salts like ammonium sulfate
  • They can be precipitated out by protein denaturing solvents like acetone and alcohol.
  • They can absorb maximum UV light at 280 um wavelength.

2]. Enzymes are mostly thermo labile and get denatured at high temperature, usually above 60°C. However, some enzymes are found thermo stable and can withstand high temperature up to 70°C - 80°C or even more.

3]. Enzymes are highly specific in action. They can act on a specific substrate molecule to bring about specific biochemical reaction .

Catalytic RNA - Ribozyme

  Apart from proteins, a few RNA have also been recognized to have catalytic activity. They are called ribozymes or catalytic RNA. They were first recognized by Thomas Cech in 1982. The ribozymes have two types of common roles :
  1. RNA processing, where the RNA is involved in RNA splicing, RNA ligation and RNA replication.  
  2. Peptide bond formation during protein synthesis. In ribosomes, they function as a part of rRNA and participate in peptide bond formation.

Basic structure of Enzymes

In general, enzyme molecule consists of two components.
  1. Apo enzyme and
  2. Prosthetic group or cofactor

Apo enzyme is protein part of enzyme.
Prosthetic group or cofactors are non protein part of enzymes. They consist of vitamin or their derivatives or metal lons. (If these non protein components are bound loosely to protein, they are called cofactors) and if they are bound firmly (covalently), are called prosthetic groups. Apo enzyme and prosthetic group form complex to form active form of enzyme, known as Holoenzyme.

Classification of Enzymes