Enzyme Structure & Function (SQA National 5 Biology): Revision Note

Enzyme structure

• Enzymes are proteins made by all living cells; they can be described as:

biological catalysts that speed up the rate of cellular reactions without being changed or used up

• Enzymes maintain the speeds of all metabolic reactions at a rate that can sustain life

  • Enzymes lower the energy needed for reactions to begin, so allowing them to occur at a higher rate

  • If we did not produce digestive enzymes, it would take around 2 - 3 weeks to digest one meal; with enzymes it takes around 4 hours

• Enzymes are specific to one particular substrate

  • This is because of the shape of the active site; the region of the enzyme that binds to the substrate

  • The active site of an enzyme molecule is complementary to its specific substrate

Enzyme function

• During a chemical reaction, enzymes convert substrate molecules into products

• This occurs during, e.g.:

  • degradation reactions

  • synthesis reactions

• As part of a reaction, an enzyme-substrate complex forms temporarily, which facilitates the reaction

Degradation reactions

• Degradation reactions break large, complex molecules down into smaller, simpler molecules

  • A large substrate molecule is broken down into smaller product molecules

• Examples include:

  • the breakdown of protein into amino acids by protease enzymes

  • the breakdown of starch into glucose by carbohydrase enzymes

Synthesis reactions

• During synthesis reactions, smaller molecules are joined together to make larger, more complex molecules

  • Multiple substrate molecules are joined to give a single product molecule

• Enzymes help by holding the molecules in place so they can bond

• Examples include:

  • joining of DNA bases to build a DNA strand

  • joining amino acids during protein synthesis