Classification of Enzymes | 6 Important Classes of Enzymes and their Biochemistry with examples

What are Enzymes?

Enzymes are biological catalysts that speed up the reaction rate by lowering activation energy without undergoing any change by itself.

Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (NC-IUBMB) proposed a system for Classification and Naming of Enzyme-Catalyzed Reactions.

The 6 main classes are

• Class 1: Oxidoreductases: all enzymes catalyzing oxidoreduction reactions
• Class 2: transferases: Enzymes catalyzing transfer of functional group from one compound to another
• Class 3: Hydrolases: Enzymes catalyzing the hydrolytic cleavage of C-O, C-N, C-C and some other bonds.
• Class 4: Lyases: enzymes cleaving C-C, C-O, C-N, and other bonds by forming double bonds or rings other than hydrolysis or oxidation
• Class 5: Isomerases: Enzymes catalyzing rearrangement of atoms within a molecule
• Class 6: Ligases: catalyzing the joining together of two molecules coupled with the hydrolysis of a diphosphate bond in ATP or a similar triphosphate.

Class 1: Oxidoreductases: all enzymes catalyzing oxidoreduction reactions

Examples: Glucose oxidase, Succinate dehydrogenase

Step 6 of Krebs cycle Explanation of the reaction:

• Succinate dehydrogenase that catalyzes the reaction or oxidation of succinate to Fumarate.
• Hydrogen is donated by the succinate so it become oxidized this hydrogen is received by this fad that becomes fadh2 here fad is reduced to fadh2 a reduction reaction has happened.
• In this reaction, hydrogen is donated by succinate; that is succinate is oxidized to form the Fumarate so oxidation and reduction reaction is coupled and the enzyme is called dehydrogenase as hydrogen is removed from succinate.
• Oxidase is only used in cases where O2 is the acceptor.

Case 2: Xanthine oxidase:

xanthine oxidase catalyzes the oxidation of xanthine to form uric acid.

Here oxygen is the acceptor therefore the term oxidase should be used as per the nomenclature committee. So if an enzyme is named as oxidase it suggests that oxygen is the acceptor and often H2O2 or H2O is produced in the reaction.

Other examples: glucose oxidase, cytochrome oxidase, monoamine oxidase

Class 2: Transferases: Enzymes catalyzing transfer of functional group from one compound to another.

Step 1 of Glycolysis

Explanation of the reaction: Hexokinase, a transferase enzyme

Glucose becomes glucose 6-phosphate; catalyzed by enzyme hexokinase. Glucose receives a phosphate from ATP; therefore, ATP becomes ADP and the phosphate is donated to this glucose. Glucose becomes glucose 6-phosphate as a phosphate group is transferred from one molecule; that is ATP to glucose forming glucose 6-phosphate. The enzyme is kinase. It is called as hexokinase as glucose is a six-carbon compound.

Class 3: Hydrolases: Enzymes catalyzing the hydrolytic cleavage

Protease

Explanation of the reaction: Protease, a hydrolase enzyme

Proteases catalyze hydrolytic reactions that degrade protein molecules down to peptides and eventually to free amino acids. These class of enzymes often ends with ’ase’.

Other examples: Lipase, Protease, Nuclease, Amylase, phosphatase.

Class 4: Lyases: enzymes cleaving C-C, C-O, C-N, and other bonds by forming double bonds or rings other than hydrolysis or oxidation

Step 4 of glycolysis

Explanation of the reaction: Aldolase, a Lyase

In glycolysis, the lyase called aldolase catalyses the readily reversible splitting of fructose 1,6-bisphosphate (F-1,6-BP),into the products glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP). This is an example of a lyase that helps to cleave carbon-carbon bonds. The splitting is not a hydrolysis or an oxidation. Here a double bond is formed as you see in the aldehyde group of GAP and ketone group of DHAP.

Other Examples: Citrate lyase, Isocitrate lyase, Pectate lyase

Class 5: Isomerases: Enzymes catalyzing rearrangement of atoms within a molecule

Step 2 of Glycolysis

Explanation of the reaction: Phosphoglucose Isomerase

Glucose 6-phosphate becomes fructose 6-phosphate, catalysed by the enzyme Phosphoglucose Isomerase. The structural arrangement of atoms has changed. Glucose and fructose are isomers with the same molecular formula C₆H₁₂O₆ but differ in structure. Glucose is an aldose with aldehyde group and fructose is a ketose with ketone group.

Other Examples: 

• Triose phosphate isomerase
• Glucose isomerase
• Protein disulfide-isomerase

Class 6: Ligases: Joining Enzymes

Enzymes joining together two molecules with the hydrolysis of a diphosphate bond in ATP or a similar triphosphate.

Explanation of the reaction: aminoacyl-tRNA synthetase in Protein translation

An aminoacyl-tRNA synthetase, also called tRNA-ligase, is an enzyme that attaches the appropriate amino acid onto its corresponding tRNA.

Here amino acid, methionine is joined to its corresponding tRNA by the enzyme tRNA-ligase for methionine along with the hydrolysis of one molecule of ATP, yielding aminoacyl-tRNA (Charged tRNA-Met), AMP, and PPi.

Example 2

Glutamate cysteine ligase (GCL) catalyzes the first and rate-limiting step in the production of the cellular antioxidant glutathione (GSH). GCL joins Glutamate and cysteine to form Glutamyl Cysteine.

Other Examples:

• Ubiquitin Ligases (C-N bond)
• Glutamate–cysteine ligase (C-N bond)
• Aminoacyl tRNA synthetase (C-O bond)
• Succinyl coenzyme A synthetase (C-S bond)
• DNA ligase

These are the six major classes of enzymes. Thank you so much for your support.