L1 - enzymes

• Enzyme action many be intracellular or extracellular
• Enzymes are globular proteins with a specific tertiary structure, which catalyse metabolic reactions in living organisms
• Enzyme action:

  -metabolic pathways are controlled by enzymes in a biochemical cascade of reactions

  -virtually every metabolic reaction within living organisms is catalysed by an enzyme

  -enzymes are therefore essential for life to exist

  
• Enzyme action:

  -all enzymes are proteins that are produced via the process of protein synthesis inside cells

  -some enzymes remain inside cells, whilst others are secreted to work outside of cells
• Intracellular enzymes are produced and function inside the cell (eg. catalase which breaks down hydrogen peroxide in cells)
• Extracellular enzymes are secreted by cells and catalyse reactions outside cells (eg. digestive enzymes in the gut)
• Enzyme action:

  -enzymes have an active site where specific substrates bind forming an enzyme substrate complex

  -the active site of an enzyme has a specific shape to fit a specific substrate

  -substrates collide with the enzymes active site and this must happen at the correct orientation and speed in order for a reaction to occur

  -extremes of heat or pH can change the shape of the active site, preventing substrate binding (denaturation)
• Enzyme-substrate complex:

  -forms when an enzyme and substrate join together

  -only formed temporarily before the enzyme catalyses the reaction and the products are released
• What can enzymes do:

  -build molecules (anabolic enzymes) or break down molecules (catabolic enzymes)

  -responsible for structural and functional activity

  -can be used again and again because they are not used up in a reaction
• Structural (anabolic) enzymes are used when building muscle or bone; synthesis of DNA and protein
• Functional (catabolic) enzymes are used when digesting food (eg. starch broken down to make glucose)
• Enzymes are globular proteins: means their shape (as well as active site) is determined by the complex tertiary structure of the protein that makes up the enzyme and is therefore highly specific
• Enzyme specificity:

  -the specificity of an enzyme is a result of the complementary nature between the shape of the active site on the enzyme and its substrate(s)

  -the active site’s shape and enzyme specificity is determined by the complex tertiary structure of the protein
• Remember:

  -proteins are formed from chains of amino acids held together by peptide bonds

  -the order of amino acids determines the shape of the enzyme

  -if the order is altered, the resulting three dimensional shape changes
• Lock and key:

  -in the 1890s the first model of enzyme activity was described by Emil Fischer

  -he suggested that both enzymes and substrates were rigid structures that locked into each other precisely, much like a key going into a lock

  -this is known as the ‘lock and key hypothesis’
• Although the lock and key hypothesis assumes the active site of an enzyme is rigid, crystallographic studies indicate proteins are flexible
• Induced fit:

  -the lock and key model was modified and adapted to our current understanding of enzyme activity, permitted by advances in the molecular sciences

  -modified model was first proposed in 1959 and is known as the induced fit hypothesis
• Induced fit:

  -the enzyme and substrate interact with each other

  -the enzyme and it’s active site (and sometimes the substrate) can change shape slightly as the substrate molecule enters the enzyme

  -these shape changes are known as conformational changes

  -the conformational changes ensure an ideal binding arrangement is achieved

  -this maximise the ability of the enzyme to catalyse the reaction
• All chemical reactions are associated with energy changes. For a reaction to proceed there must be enough activation energy. This is the minimum amount of energy required for a chemical reaction to occur. The activation energy is the barrier that the particles must overcome in order to react with each other
• How enzymes lower the activation energy:

  -substrates held close together in the active site, reducing repulsion between the molecules and helping them to bond together

  -fitting the substrates into the active site puta a strain on the configuration, making it easier for specific bonds to break

  -enzyme substrate complex is lower in energy
• How enzymes lower the activation energy:

  -they reduce the stability of bonds in the reactants

  -the destabilisation of bonds in the substrate makes it more reactive

  -enzymes provide an alternative energy pathway with lower activation energy

  -without enzymes, extremely high temperatures and pressures would be needed to reach the activation energy

  -enzymes avoid the need for these extreme conditions (that would otherwise kill cells)