Enzyme

History

Enzyme = En(In) + Zyme(Yeast)
Almost all enzymes are proteinaceous in nature except some nucleic acids that behave like enzymes, called ribozymes. Buchner discovered and isolated the enzyme zymase from yeast cells. 
Kuhne coined the term enzyme. J.B. Sumner purified and crystalized urease enzyme from Canavalia/Jack bean/Lobia plant. L19 RNAase (a ribozyme) was discovered by T. Cech from the rRNA of a protozoan (Tetrahymena thermophila). 
RNAase P or Ribonuclease P (a ribozyme) was discovered by Altman from a prokaryotic cell.  

Characteristics of Enzymes  

Enzymes are colloidal substances, which are macromolecules of amino acids and are synthesized by ribosomes under genetic control. The enzyme can be depicted by a line diagram. An enzyme like any protein has:
  • Primary structure: 
The amino acid sequence of the protein. It lacks an active site. 
A A A A A A A……n 
Peptide bond
  • Secondary structure: 
It is the helical structure that also lacks active sites. 

  • Tertiary structure: 
In this structure backbone of the protein, chain folds upon itself, the chain crisscrosses itself and hence many crevices or pockets are made such pockets represent
  • Active site
An active site of the enzyme is a crevice of pocket into which the substrate fits. Thus enzymes through their active site, catalyze reactions at a high rate.



  • Quaternary structure
Represented by isoenzymes (Multiple molecular forms of an enzyme which has different structures but similar functions. 
Ex. Lactate dehydrogenase (LDH) 





However, enzymes isolated from organisms that normally live under extremely high temperatures (eg. hot vents and sulfur springs), are stable and retain their catalytic power even at high temperatures. Thermal stability is thus an important quality of such enzymes isolated from thermophilic organisms. e.g. Taq polymerase.



Classification And Nomenclature of Enzyme 

Thousands of enzymes have been discovered, isolated, and studied. Most of these enzymes have been classified into different groups based on the type of reactions they catalyze. Enzymes are divided into 6 classes

Oxidoreductases/dehydrogenases

Enzymes that catalyze oxide reduction between two substrates e.g., 

S-H/ereduced + S’ oxidized⟶S oxidized + S’ -H/ereduced

Transferases

 Enzymes catalyze a transfer of a group, G (other than hydrogen) between a pair of substrate S and S’. 
 E.g.


Hydrolases

Enzymes catalyze the hydrolysis of ester, ether, peptide, glycosidic, C-C, C-halide, or P-N bonds.


Lyases

Enzymes that catalyze the removal of groups from substrates by mechanisms other than hydrolysis leave double bonds.



Isomerases

Includes all enzymes catalyzing the interconversion of optical, geometric, or positional isomers.

Ligases

 Enzymes catalyze the linking together of 2 compounds, e.g., enzymes that catalyze joining of C-O, C-S, C-N, P-O, etc. bonds.


  • Enzymes are divided into 6 classes each with 4-13 subclasses and named accordingly by a four-digit number. 
Example: Hexokinase 
  • Class 2 
  • Subclass 7  
  • Sub sub class 1  
  • Reaction/Enzyme 1


Co-enzyme 

  1. Organic Compound 
  2. Loosely bound to the apoenzyme
  3. Association with apoenzyme is only transient (usually occurring during catalysis)
  4. Most of them are vitamins derivatives
  5. E.g. : NAD (Nicotinamide adenine dinucleotide) and NADP contain the vitamin niacin

Prosthetic Group 

  1. Organic or inorganic compound
  2. Tightly bound to the apoenzyme.
  3. E.g.: in peroxidase and catalase, Haem is the prosthetic group and it is the part of the active site of the enzyme

Metal -Ion Activator

  1. Inorganic co-factor
  2. Loosely attached
  3. They form coordination bonds with the side chain at the active site and at the same time form one or more coordination bonds with the substrate.
  4. Zn is the activator of Carboxypeptidase.

Turn Over Number  

It is the number of substrate molecules converted into products per unit time by the molecule of enzyme. Thus catalytic power is directly proportional to turn over number. The catalytic power of an enzyme remains the same even outside the living system.

Note: Enzymes when not in use, represent inactive form, called zymogen or pro-enzyme.
Examples: Pepsinogen is the inactive form of pepsin, a Trypsinogen is an inactive form of trypsin.  


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