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Coenzymes & Cofactors

• There are substances other than substrates and inhibitors that interact with enzymes

  • Some enzymes can only function properly if another non-protein substance is present

  • For example, some enzymes are inactive until they combine with a non-protein substance that changes their tertiary structure (allowing the active site to bind correctly with the substrate)

• These substances are broadly known as cofactors

Cofactors

• Some enzymes require inorganic ions to function properly

• Particular inorganic ions may help to stabilise the structure of the enzyme or may actually take part in the reaction at the active site

  • For example, chloride ions act as a cofactor for amylase

  • This means that in order for amylase to be able to digest starch into maltose, chloride ions must be present

• The inorganic ions that an enzyme requires in order to function are known as inorganic cofactors

Prosthetic groups

• Some cofactors are tightly and permanently bound to the enzyme

• These are called prosthetic groups

• Unlike temporary cofactors, they form a permanent feature of the enzyme’s structure and are essential for its function

  • For example, carbonic anhydrase contains a zinc-based prosthetic group that plays a crucial role in the rapid conversion of carbon dioxide and water into carbonic acid

Coenzymes

• Larger organic (carbon-containing) cofactors are known as coenzymes

  • Some coenzymes are permanently bound to the enzyme they assist, often in or near the active site

  • Some coenzymes only bind temporarily during the reaction

  • Coenzymes are involved in carrying electrons or chemical groups between enzymes, aiding in catalysis 

• Coenzymes link different enzyme-catalysed reactions into a sequence during metabolic processes, such as photosynthesis and respiration

• Vitamins are an important source of coenzymes. For example, many vitamins in the B vitamin group are used in the production of important coenzymes, including:

  • Pantothenic acid, a key component of coenzyme A (a coenzyme required for the oxidation of pyruvate during the link reaction that occurs between the glycolysis and Krebs cycle stages of respiration)

  • Nicotinic acid, used to produce the coenzymes NAD and NADP (coenzymes required in many different metabolic reactions, including many of the reactions that take place during photosynthesis and respiration)

  • Vitamin B₁ (riboflavin), used to produce the coenzyme FAD (a coenzyme required in the Krebs cycle during respiration)

Examples of coenzyme functions

• During many of the reactions in respiration, the coenzymes NAD and FAD are alternately reduced and oxidised, transferring energy in the form of hydrogen ions

• The coenzyme NADP fulfils this same role in chloroplasts during photosynthesis

• The coenzymes ATP and coenzyme A act in a different way, by transferring chemical groups. For example:

  • ATP is responsible for the transfer of phosphate groups between respiration and energy-consuming processes in cells

  • Coenzyme A is responsible for the transfer of an acetyl group (-CH₃CO) from fatty acids and glucose during respiration

Coenzymes are a type of cofactor that interact with enzymes. They are involved in carrying electrons or chemical groups between enzymes

Summary

• Cofactors are non-protein substances (i.e. not made from amino acids) that enzymes require in order to function properly

• Coenzymes are organic non-protein cofactors. Coenzymes contribute to enzyme-catalysed reactions by accepting or donating hydrogen ions or chemical groups (e.g. phosphate groups)