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The catalytic activity of many enzymes depends on the presence of small molecules termed cofactors, although the precise role varies with the cofactor and the enzyme. Such an enzyme without its cofactor is referred to as an apoenzyme; the complete, catalytically active enzyme is called a holoenzyme.


Cofactors can be subdivided into two groups: metals and small organic molecules

  •  Most common cofactor are metal ions . (Some sources also limit the use of the term “cofactor” to inorganic substances).
  • Cofactors that are small organic molecules are called coenzymes.
  •  If tightly bound, the cofactors are called prosthetic groups
  •  Loosely bound cofactors serve functions similar to those of prosthetic groups but bind in a transient, dissociable manner either to the enzyme or to a substrate.
  • They are more like co substrates because they bind to and are released from the enzyme just as substrates and products are.

 Prosthetic Group

 Tightly integrated into the enzyme structure by covalent or non-covalent forces. It can be organic or inorganic (metal ions) e.g.

 a) Organic

◦     Pyridoxal phosphate

◦     Flavin mononucleotide( FMN)

◦     Flavin adenine dinucleotide(FAD)

◦     Thiamin pyrophosphate (TPP)

◦     Biotin

b) Inorganic

 Metals are the most common prosthetic groups

◦       Metal ions – Co, Cu, Mg, Mn, Zn, Fe

Role of metal ions

  •  Enzymes that contain tightly bound metal ions are termed – Metalloenzymes.
  • Enzymes that require metal ions as loosely bound cofactors are termed as metal-activated enzymes

Metal ions facilitate

◦       Binding and orientation of the substrate

◦       Formation of covalent bonds with reaction intermediates

◦       Interact with substrate to render them more electrophilic or nucleophilic

 Examples of Metallo enzymes- (Table-1)




 Carbonic anhydrase


 Alcohol dehydrogenase



 Fe+++ or Fe++


 Cu++ or Cu+

 Cytochrome oxidase


Propionyl CoA carboxylase




Superoxide dismutase


Glutathione peroxidase


Xanthine oxidase



 Metal activated /Ion activated enzymes

 In a few enzyme-controlled reactions, it is the presence of certain ions that can increase the reaction rate. Ions may combine with the enzyme or the substrate. The ion binding makes the formation of an enzyme-substrate complex happen more easily, because it can affect the charge distribution or the end shape of the complex.

Amylase catalyses the breakdown of maltose molecules. This enzyme will function properly only if chloride ions are present. Without the chloride ions, amylase cannot catalyse the reaction


Co-enzymes serve as recyclable shuttles—or group transfer agents—that transport many substrates from their point of generation to their point of utilization.

  • The water-soluble B vitamins supply important components of numerous coenzymes
  • Chemical moieties transported by coenzymes include hydrogen atoms or hydride ions, methyl groups (folates), acyl groups (coenzyme A), and oligosaccharides (dolichol).

 Examples of Coenzymes- (Table-2)

Coenzyme  Abbreviation Group transferred Enzyme
 Nicotine adenine dinucleotide  NAD+ – Derived from niacin  Electron (hydrogen atom) Lactate dehydrogenase
 Nicotine adenine dinucleotide phosphate  NADP+ – niacin derivative  Electron (hydrogen atom) Glutamate dehydrogenase
 Flavin adenine dinucleotide  FAD – riboflavin (vit. B2) derivative  electron (hydrogen atom) Monoamine oxidase
 Coenzyme A  CoA  Acyl groups  Acetyl CoA carboxylase
 Thiamine pyrophosphate  Thiamine (vit. B1)  Aldehydes Pyruvate dehydrogenase Complex
 Pyridoxal phosphate  Pyridoxine (vit B6)  amino and many other Transaminases, Decarboxylases, Glycogen phosphorylase
 Biotin  Biotin  Carboxyl Pyruvate carboxylase
 5′-Deoxyadenosyl cobalamine  vit. B12  alkyl groups Methylmalonyl mutase
Tetrahydrofolate Folic acid One carbon compounds Thymidylate synthase

 The water-soluble B vitamins supply important components of numerous coenzymes. Several coenzymes contain, in addition, the adenine, ribose, and phosphoryl moieties of AMP or ADP (Table–2). Nicotinamide is a component of the redox coenzymes NAD and NADP, whereas riboflavin is a component of the redox coenzymes FMN and FAD. Pantothenic acid is a component of the acyl group carrier coenzyme A. As its pyrophosphate, thiamin participates in decarboxylation of α-keto acids and folic acid and cobamide coenzymes function in one-carbon metabolism. 

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