OCR A Level Biology:复习笔记2.4.8 Coenzymes, Cofactors & Prosthetic Groups

• 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

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 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 and allow them to function properly

Prosthetic groups

• Some cofactors are actually a permanent part of the structure of the enzyme they assist
  • These cofactors are known as prosthetic groups

   

• Prosthetic groups are essential to the enzyme functioning properly, as they help to form the final 3D shape of the enzyme
  • For example, by forming part of the active site of the enzyme, a zinc ion acts as the prosthetic group for carbonic anhydrase (an enzyme found in red blood cells that converts CO₂ and H₂O into carbonic acid, H₂CO₃)

   

Summary

• Cofactors are non-protein substances (i.e. not made from amino acids) that enzymes require in order to function properly. Cofactors can be a temporary part of the enzyme or a permanent part (known as a prosthetic group)
• 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)