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

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)

Exam Tip

For exam questions on this topic, you do not need to learn the names of the various coenzymes described above (although these will need to be learnt for later A level topics on respiration and photosynthesis)! In addition, you do not need to learn the names of the specific vitamins here, just that vitamins of the B group are used in the production of several important coenzymes.

 

转载自savemyexams

 

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