3 Types of Cofactors:

• Prosthetic Groups

• Cofactors

• Coenzymes

Prosthetic Groups: (eg. Zn²⁺ in Carbonic Anhydrase)

• Part of the enzyme structure, this is a cofactor permanently bound to the enzyme via covalent bonds.

• A key eg. of this is Zinc ions (Zn²⁺), which are permanently bound to Carbonic anhydrase, and form part of its active site.

• Carbonic anhydrase is found in erythrocytes and catalyses the interconversion of CO₂ + H₂O into Carbonic acid (H₂CO₃) or vice versa.

Cofactors: (eg. Cl^- and amylase)

• Presence of certain ions, that temporarily bind to either the substrate or enzyme, can aid the formation of ESC’s, thus increasing the rate of reaction.

• Some act as co-substrates; temporarily bind to active site ensuring it’s complementary to the substrate.

• Some temporarily bind to enzyme molecule, stabilising the charges of either the substrate or active site, allowing temporary bonds to form easier, between substrate and enzyme molecules, thus aiding the formation of ESC’s, increasing rate of reaction.

• 2 types:

  • Coenzymes: organic molecules (carbon-containing), usually act as co-substrates and are chemically altered during the reaction, and must be regenerated back into its original form, in the other reactions within the series of enzyme-catalysed reactions. This allows for them to be continually recycled. Vitamins are usually sources of coenzymes.

  • Cofactors: inorganic molecules (do not contain carbon), these are usually metallic ions but can also be non-metallic. (eg. Cl^- which is a cofactor, that must be present, for amylase to catalyse the breakdown of starch into maltose.)

Precursor Activation

• Some enzymes are produced in an inactive precursor form, which is useful as it prevents the enzyme digesting cell’s molecules, causing damage within cells, and also, ensuring they are only used when needed.

• These inactive enzymes, known as apoenzymes, must be activated in order for them to catalyse their specific reactions, which is done by the binding of a cofactor. This causes a change in the enzymes 3^o causing the active site to change and become complementary to its substrate, and the activated enzyme is known as a holoenzyme

• An eg. of this is the enzyme Pepsin, which is secreted as an apoenzyme Pepsinogen, however is converted to the holoenzyme Pepsin, by the action of both the H⁺ and Cl^- present in HCl acid within the stomach.