Classification of Enzymes

 

There are approximately 3000 enzymes which have been characterized.  These are grouped into six main classes

according to the type of reaction catalysed.  At present, only a limited number are used in enzyme electrodes or

for other analytical purposes.
 

1. Oxidoreductases

These enzymes catalyse oxidation and reduction reactions involving the transfer of hydrogen atoms or electrons. 

The following are of particular importance in the design of enzyme electrodes. This group can be further divided

into 4 main classes.
 

    a. dehydrogenases catalyse hydrogen transfer from the substrate to a nicotinamide adenine dinucleotide

        cofactor (NAD+).   An example of this is lactate dehydrogenase which catalyses the following reaction:

Lactate + NAD+ --> Pyruvate + NADH + H+

    b. oxidases catalyse hydrogen transfer from the substrate to molecular oxygen producing hydrogen peroxide

        as a by-product.   An example of this is FAD dependent glucose oxidase which catalyses the following reaction:

 b-D-glucose + O2 --> gluconolactone + H2O2

 

   c. peroxidases catalyse oxidation of a substrate by hydrogen peroxide.  An example of this type of enzyme

        is horseradish peroxidase which catalyses the oxidation of a number of different reducing substances

        (dyes, amines, hydroquinones etc.) and the concomitant reduction of hydrogen peroxide. The reaction

        below illustrates the oxidation of neutral ferrocene to ferricinium in the presence of hydrogen peroxide:

2[Fe(Cp)2] + H2O2 + 2H+--> 2[Fe(Cp)2]+ + 2 H2O  

   

   d. oxygenases catalyse substrate oxidation by molecular oxygen.  The reduced product of the reaction in

        this case is water and not hydrogen peroxide.  An example of this is the oxidation of lactate to acetate

        catalysed by lactate-2-monooxygenase.

 lactate + O2 --> acetate + CO2 + H2O

 

2. Transferases  These enzymes transfer C, N, P or S containing groups (alkyl, acyl, aldehyde, amino,

   phosphate or glucosyl) from one substrate to another.  Transaminases, transketolases, transaldolases and

   transmethylases belong to this group.

 
3. Hydrolases  These enzymes catalyse cleavage reactions or the reverse fragment condensations.  According

   to the type of bond cleaved, a distinction is made between peptidases, esterases, lipases, glycosidases,

   phosphatases and so on.  Examples of this class of enzyme include:  cholesterol esterase, alkaline phosphatase

   and glucoamylase.

 

4. Lyases These enzymes non-hydrolytically remove groups from their substrates with the concomitant formation

   of double bonds or alternatively add new groups across double bonds.
 
5. Isomerases  These enzymes catalyse intramolecular rearrangements and are subdivided into;

   a. racemases

    b. epimerases

    c. mutases

    d. cis-trans-isomerases

    An example of this class of enzyme is glucose isomerase which catalyses the isomerisation of glucose to fructose.

 

6. Ligases    Ligases catalyze the formation of C-C, C-O, C-N, C-S and C-halogen bonds without hydrolysis or oxidation. 

    The reaction is  usually accompanied by the consumption of a high energy compound such as ATP and other nucleoside

    triphosphates. An example of this type of enzyme is pyruvate carboxylase which catalyses the following reaction:
 

pyruvate + HCO3- + ATP --> Oxaloacetate + ADP + Pi