3.1.4.2 Many proteins are enzymes

• Enzymes speed up chemical reactions by


  acting as biological catalysts
• Enzymes catalyse metabolic reactions at

  ● a cellular level — e.g respiration● for the organism as a whole — e.g digestion in mammals
• Intracellular


  within the cell
• Extracellular


  outside the cell
• Enzymes are

  proteins
• Enzymes have an active site, which has


  a specific shape
• Active site


  part of the enzyme where the substrate molecules bind to
• Substrate molecules

  the substance that the enzymes interact with
• Enzymes are highly specific due to

  their tertiary structure
• Activation energy


  the certain amount of energy needed to be supplied to the chemicals before the reaction will start— in a chemical reaction— it’s often provided as heat
• Enzymes can affect structures in an organism

  ● involved in the production of collagen ● as well as functions — like respiration
• Collagen


  an important protein in the connective tissues of animals
• Enzyme action can be

  ● intracellular — within cellsor ● extracellular — outside cells
• Enzymes lower


  the amount of activation energy that’s needed— often making reactions happen at a lower temperature than they could without an enzyme— speeds up the rate of reaction​

  
• When a substrate fits into the enzyme's active site it forms an enzyme-substrate complex this lowers the activation energy because


  ● If two substrate molecules need to be joined— being attached to the enzyme holds them close together— reducing any repulsion between the molecules so they can bond more easily● If the enzyme is catalysing a breakdown reaction— fitting into the active site puts a strain on bonds in the substrate— so the substrate molecule breaks up more easily
• Scientists soon realised that the lock and key model didn't give the full story

  ● The enzyme and substrate do have to fit together ● new evidence showed that the enzyme-substrate complex changed shape slightly to complete the fit— This locks the substrate even more tightly to the enzyme● Scientists modified the old lock and key model and came up with the ‘induced fit’ model
• Catalyst


  substance that speeds up a chemical reaction without being used up in the reaction itself
• The 'lock and key' model

  ● Enzymes only work with substrates that fit their active site● Early scientists studying the action of enzymes came up with the ‘lock and key’ model— where the substrate fits into the enzyme in the same way that a key fits into a lock — the active site and the substrate have a complementary shape

  
• The 'induced fit' model


  ● helps to explain why enzymes are so specific and only bond to one particular substrate ● The substrate doesn’t only have to be the right shape to fit the active site— it has to make the active site change shape in the right way ● example of how a widely accepted theory can change with new evidence

  
• Scientists now have a pretty good understanding of how enzymes work


  As with most scientific theories, this understanding has changed over time
• When describing enzyme action you need to say

  the active site and the substrate have a complementary shape
• enzymes break substrates down

  e.g one substrate molecule goes into the active site and two products come out— After the products are released, the active site returns to its original shape and can bind to the next substrate molecule
• Enzymes can also catalyse synthesis reactions


  e.g two substrate molecules go into the active site, bind together and one product comes out— After the products are released, the active site returns to its original shape and can bind to the next substrate molecule
• Enzyme properties are


  related to their tertiary structure
• Enzymes are very specific

  ● they usually only catalyse one reaction● e.g :— maltase only breaks down maltose— sucrase only breaks down sucrose

  
• Enzymes usually only catalyse one reaction because


  ● only one complementary substrate will fit into the active site— if the substrate shape doesn’t match the active site— an enzyme-substrate complex won’t be formed — the reaction won’t be catalysed

  
• The active site's shape is determined by


  the enzyme’s tertiary structure
• The enzymes tertiary structure is determined by


  the enzyme’s primary structure
• Each different enzyme has

  a different tertiary structure — so a different shaped active site
• If the tertiary structure of a protein is altered

  the shape of the active site will change— means the substrate won’t fit into the active site— an enzyme-substrate complex won’t be formed — enzyme will no longer be able to carry out its function
• The tertiary structure of an enzyme may be altered by changes in

  ● pH● temperature
• The primary structure

  amino acid sequence

  
• The primary structure protein is determined by

  a gene— If a mutation occurs in that gene — it could change the tertiary structure of the enzyme produced
• The rate of a reaction can be measured by

  ● How fast the product is made● How fast the substrate is broken down
• Rate of reaction can be measured by how fast the product is made because


  there are different molecules present at the end of a chemical reaction than there are at the beginning
• To measure how fast the product is made

  measure the amount of end product present at different times during the experiment— the reaction rate can be calculated
• To measure how fast the substrate is broken down

  measure the amount of substrate molecules left at different times during the experiment — the reaction rate can be calculated
• Rate of reaction can be measured by how fast the substrate is broken down because


  To produce the end products in a reaction, substrate molecules have to be used up
• The rate of an enzyme-controlled reaction increases when

  the temperature’s increased

  
• More heat means more kinetic energy


  ● molecules move faster● makes the substrate molecules more likely to collide with the enzymes’ active sites ● energy of these collisions also increases— means each collision is more likely to result in a reaction