chapter 4 - enzymes

• Enzymes

  Biological catalysts that are globular proteins
• Enzymes

  • They interact with substrate molecules causing them to react at much larger rates without the need for harsh environmental conditions
  • They are necessary for many of the processes required for life
• Role of enzymes in reactions

  1. Catalyse the synthesis of large polymer-based components like cellulose and protein molecules

  2. Catalyse catabolic reactions that release energy from large organic molecules

  3. Catalyse digestion of food
• Reactions rarely happen in isolation but as part of multi-step pathways
• Metabolism is the sum of all the different reactions and reaction pathways happening in a cell or organism, and it can only happen as a result of the control and order imposed by enzymes
• Vmax


  Maximum initial velocity or rate of the enzyme-catalysed reaction
• Enzymes

  • They can only increase the rates of reaction up to a certain point called the Vmax
  • The speed at which different cellular reactions proceed varies considerably and is usually dependent on environmental conditions like temperature, pressure, and pH
• The enzyme-substrate complex then undergoes catalysis to produce the product, which is then released, leaving the unchanged enzyme to catalyse more reactions
• Enzymes can be induced to change shape to better accommodate the substrate, a process called induced fit
• More complex interactions between the enzyme and substrate can also modify the active site and the way the enzyme binds the substrate
• Enzymes can lower the activation energy required for a reaction to occur
• Increase in collisions


  Increases the overall rate of reaction
• Many different enzymes are produced by living organisms, as each enzyme catalyses one biochemical reaction, of which there are thousands in any given cell
• Specialty of the enzyme

  Each enzyme catalyses one biochemical reaction
• Activation energy


  Energy needed for a reaction to start
• Sometimes, the amount of energy needed is so large it prevents the reaction from happening under normal conditions
• How enzymes work

  Enzymes help the molecules collide successfully, and therefore reduce the activation energy required
• Hypotheses for how enzymes work


  • Lock and key hypothesis
  • Induced-fit hypothesis
• Lock and key hypothesis


  An area within the tertiary structure of the enzyme has a shape that is complementary to the shape of a specific substrate molecule (the active site)
• Substrate binds to active site


  Enzyme-substrate complex is formed
• Enzyme-catalysed reaction

  1. Substrate binds to active site

  2. Enzyme-substrate complex forms

  3. Substrate reacts

  4. Products are released

  5. Enzyme unchanged
• Induced-fit hypothesis


  The active site of the enzyme actually changes shape slightly as the substrate enters, inducing changes in the enzyme's tertiary structure that strengthen binding and put strain on the substrate molecule
• Intracellular enzymes


  Enzymes that act within cells
• The synthesis of polymers from monomers, for example making polysaccharides from glucose, requires enzymes
• Hydrogen peroxide is a toxic product of many metabolic pathways. The enzyme catalase ensures hydrogen peroxide is broken down to oxygen and water quickly
• extracellular enzymes


  enzymes which are released from cells to break down these large nutrient molecules into smaller molecules
• single-celled organisms release enzymes into their immediate environment and multicellular organisms release enzymes into the bloodstream
• digestion of starch

  1. starch polymers are partially broken down into maltose, a disaccharide, by the enzyme amylase - which is produced by the salivary glands and pancreas
  2. maltose is then broken down into glucose, which is a monosaccharide, by the enzyme maltase - which is present in the small intestine
• digestion of proteins

  trypsin is a protease, a type of enzyme which catalyses the digestion of proteins into smaller peptides, which are broken down further into amino acids

  trypsin is produced in the pancreas and released into the small intestine
• Increasing the temperature of a reaction environment


  Increases kinetic energy of the particles
• As temperature increases


  The particles move faster and collide more frequently
• In an enzyme-controlled reaction, an increase in temperature


  Will result in more frequent successful collisions between substrate and enzyme
• An increase in temperature


  Leads to an increase in the rate of reaction
• Temperature coefficient, Q10


  Measure of how much the rate of a reaction increases with a 10% increase in temperature
• For enzyme-controlled reactions, the temperature coefficient is usually 2, which means that the rate of reaction doubles with a 10% temperature increase
• Denaturation


  Changes to the structure of enzymes caused by temperature
• As enzymes are proteins, their structure is affected by temperature
• At higher temperatures


  The bonds holding the protein together vibrate more, until they strain and break, resulting in a change in the precise tertiary structure of the protein
• When an enzyme is denatured, the active site changes shape and is no longer complementary to the substrate, so the enzyme will no longer function as a catalyst
• Optimum temperature


  The temperature at which an enzyme has the highest rate of activity