Chemical composition and International classification of enzymes

• Except for a small group of RNA molecules that act as catalyst, all enzymes are proteins.
• Catalytic activity of enzymes depends on the integrity of their native protein conformation.
• An enzyme loses its catalytic activity if it is denatured or dissociated into its subunits.
• The molecular weight of enzymes ranges from 12,000 to more than a million like other proteins.
• Some enzymes require no chemical groups for activity other than their amino acid residues, whereas others require additional chemical components called a co-factor.
• A co-factor may either be one or more inorganic ions or a complex organic or metallo-organic molecule called a co-enzyme.
• Some enzymes require both a co-enzyme and one or more metal ions for activity.
• The protein part of such enzyme is called apo-protein or apo-enzyme.
• A co-enzyme or metal ion that is very tightly or even covalently bound to the enzyme protein (apo-protein or apo-enzyme) is called a prosthetic group.
• A complete, catalytically active enzyme together with its bound co-enzyme and/or metal ions is called a holoenzyme.
• Enzyme proteins or apo-proteins are modified covalently by phosphorylation, glycosylation and other processes to regulate the enzyme activities.

Co-factor:

• A co-factor is a non-protein chemical substance or inorganic metallic ion that is required for the catalytic activity of an enzyme.

• There can be either one or more inorganic ions such as Fe²⁺, Mg ²⁺, Mn²⁺, Zn²⁺ etc. for the enzyme to properly act on the substrate.

• Some inorganic metal ions that serve as co-factors for the enzymes are given below.

S.N.	Ions	Enzymes
1.	Cu2+	Cytochrome oxidase
2.	Fe2+ or Fe3+	Cytochrome oxidase, catalase, peroxidase
3.	K+	Pyruvate kinase
4.	Mg2+	Hexokinase, glucose-6-phosphatase, pyruvate kinase
5.	Mn2+	Arginase, ribonucleotide reductase
6.	Mo	Dinitrogenase
7.	Ni2+	Urease
8.	Se	Glutathione peroxidase
9.	Zn2+	Carbonic anhydrase, alcohol dehydrogenase, carboxypeptidases A and B

Co-enzyme:

• A co-enzyme is a complex non-protein organic or metallo-organic molecule that binds with an enzyme to catalyze a reaction.

• Co-enzymes act as transient carriers of specific functional groups.

• Most of the co-enzymes are derived from vitamins included in our diet.

• Some co-enzymes that serve as transient carries of specific atoms or functional groups are given below.

S.N	Co-enzyme	Chemical groups transferred	Dietary precursors in mammals
1.	Biocytin	CO2	Biotin
2.	Co-enzyme A	Acyl groups	Pantothenic acid and other compounds
3.	5’-deoxyadenosylcobalamin (co-enzyme B12)	H atoms and alkyl groups	Vitamin B12
4.	Flavin adenine dinucleotide	Electrons	Riboflavin (vitamin B2)
5.	Lipoate	Electrons and acyl groups	Not required in diet
6.	Nocotinamide adenine dinucleotide (NAD)	Hydride ion (H–)	Nicotinic acid (Niacin)
7.	Pyridoxal phosphate	Amino groups	Pyridoxine (vitamin B6)
8.	Tetrahydrofolate	One-carbon groups	Folate or Folic acid
9.	Thiamine pyrophosphate	Aldehydes	Thiamine (vitamin B1)

International classification of enzymes:

• Many enzymes have been named by adding the suffix “ase” to the name of their substrate of to a word or phrase describing their activity.
• For example, urease catalyzes the hydrolysis of urea, and DNA polymerase catalyzes the polymerization of nucleotides to form DNA.
• The major groups of enzymes and the type of reaction they catalyze are given below.

Class No.	Class Name	Type of reaction catalyzed
1.	Oxidoreductases	Transfer of electrons (hydride ions or H atoms)
2.	Transferases	Group transfer reactions
3.	Hydrolases	Hydrolysis of reactions (transfer of functional groups to water)
4.	Lyases	Addition of groups to double bonds, or formation of double bonds by  removal of groups
5.	Isomerases	Transfer of groups within molecules to yield isomeric forms
6.	Ligases	Formation of C-C, C-S, C-O, and C-N bonds by condensation reactions coupled to cleavage of ATP or similar co-factor

Chemical composition and International classification of enzymes