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How are enzymes classified?

Metabolism influences building or replacement of tissue, conversion of food to energy, disposal of waste materials, reproduction etc. “Catalysis” is defined as the acceleration of a chemical reaction by a substance which itself undergoes no permanent chemical change. Most biochemical reactions do not take place spontaneously and enzyme catalysis plays an important role in biochemical reactions necessary for all life processes. Without enzymes, these reactions would take place at a rate far too slow for effective metabolism.

Enzymes can be classified by the kind of chemical reaction they catalyze. One such scheme of enzyme classification is defined by IUBMB.

The IUBMB assigns a 4-digit code to each enzyme. Each enzyme is prefixed by EC, followed by the digits.

For exampleoxidoreductases EC

1.     The first digit denotes “Class” of the enzyme

2.     The second digit indicates, “Sub-class” of the enzyme

3.     The third digit gives “Sub sub-class” of the enzyme

4.     The fourth digit in the code is “Serial number” of the enzyme

The classification is as follows:

Group Name Type of Reaction Catalysed Example
Oxidoreductases Oxidation-reduction reactions Alcohol oxidoreductase (EC 1.1)
Transferases Transfer of functional groups Methyltransferase (EC 2.1)
Hydrolases Hydrolysis reactions Lipase (EC 3.1)
Lyases Addition to double bonds or single bonds Decarboxylases (EC 4.1)
Isomerases Isomerization reactions Epimerases and Racemases (EC 5.1)
Ligases Formation of bonds with ATP cleavage Enzymes forming carbon-oxygen bonds (EC 6.1)

b) How can I find similar enzymes?

Any similarity search is based on the presence of similar patterns (similar bond changes and/or small molecules) shared between query and target reactions. A large number of shared patterns results in higher similarity score or lesser distance score. In Bioinformatics, the concept of similarity or distance is used to find similar sequences based on amino acid similarity, structural topology, etc. In Chemoinformatics similarity between small molecules/drug molecules (i.e. based on Tanimoto score) is based on the presence of similar bonds and atoms between query and target molecules.

c) Literature

  1. Automatic Assignment of EC Numbers.
  2. Computational assignment of the EC numbers for genomic-scale analysis of enzymatic reactions.
  3. Automatic Determination of Reaction Mappings and Reaction Center Information. 2. Validation on a Biochemical Reaction Database.
  4. Genome-scale classification of metabolic reactions and assignment of EC numbers with self-organizing maps.
  5. Chemical similarity searching.
  6. Quantitative comparison of catalytic mechanisms and overall reactions in convergently evolved enzymes: implications for classification of enzyme function.
  7. Using Reaction Mechanism to Measure Enzyme Similarity
  8. etc.

I reckon in the near future we might see such concepts being adapted by IUBMB itself to annotate and classify enzymes.

This would be vital in the study of the interactions between the components of biological systems (metabolites, enzymes and metabolic pathways), and how these interactions give rise to the function and behavior of that system.

As always, thoughts/suggestions are welcome!