Chemical composition and International classification of enzymes

Source: 2006 Pearson Education Inc.
  • 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.


  • 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.
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
8.SeGlutathione peroxidase
9.Zn2+Carbonic anhydrase, alcohol dehydrogenase, carboxypeptidases A and B


  • 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.NCo-enzymeChemical groups transferredDietary precursors in mammals
2.Co-enzyme AAcyl groupsPantothenic acid and other compounds
3.5’-deoxyadenosylcobalamin (co-enzyme B12)H atoms and alkyl groupsVitamin B12
4.Flavin adenine dinucleotideElectronsRiboflavin (vitamin B2)
5.LipoateElectrons and acyl groupsNot required in diet
6.Nocotinamide adenine dinucleotide (NAD)Hydride ion (H)Nicotinic acid (Niacin)
7.Pyridoxal phosphateAmino groupsPyridoxine (vitamin B6)
8.TetrahydrofolateOne-carbon groupsFolate or Folic acid
9.Thiamine pyrophosphateAldehydesThiamine (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 NameType of reaction catalyzed
1.OxidoreductasesTransfer of electrons (hydride ions or H atoms)
2.TransferasesGroup transfer reactions
3.HydrolasesHydrolysis of reactions (transfer of functional groups to water)
4.LyasesAddition of groups to double bonds, or formation of double bonds by  removal of groups
5.IsomerasesTransfer of groups within molecules to yield isomeric forms
6.LigasesFormation 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