Enzymes

• Enzymes


  Biological catalysts that speed up the rate of chemical reactions without being used up or changed
• Enzymes


  • They function in living systems
  • They are globular proteins
  • They have an active site where specific substrates bind forming an enzyme-substrate complex

  
• Intracellular enzymes


  Enzymes produced and functioning inside the cell
• Extracellular enzymes


  Enzymes secreted by cells and catalysing reactions outside cells (e.g. digestive enzymes in the gut)
• Enzyme specificity


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

  
• Enzyme-substrate complex formation


  1. Substrate collides with enzyme's active site

  2. Binding occurs at correct orientation and speed

  3. Enzyme-substrate complex forms temporarily

  4. Enzyme catalyses reaction

  5. Products are released

  
• Catabolic reactions


  Breakdown of complex molecules into simpler products
• Anabolic reactions


  Building of more complex molecules from simpler ones
• Enzymes work by lowering the activation energy of a reaction
• Lock-and-key hypothesis


  Enzymes and substrates are rigid structures that lock into each other precisely
• Induced-fit hypothesis


  Enzyme and substrate interact, causing conformational changes to achieve ideal binding arrangement

  
• Investigating catalase activity


  1. Combine hydrogen peroxide and catalase

  2. Measure volume of oxygen generated in set time

  3. Calculate rate of reaction

  
• Investigating amylase activity using iodine

  1. Combine amylase and starch

  2. Test reaction mixture for starch at regular time intervals using iodine solution

  3. Measure time taken for starch to be broken down
• Investigating effect of starch concentration on amylase activity using colourimetry

  1. Calibrate colourimeter

  2. Prepare range of starch concentrations using serial dilutions

  3. Measure percentage absorbance or transmission

  4. Plot calibration graph of starch concentration vs absorbance/transmission
• Enzyme rate experiments


  Experiments to determine the effect of changing a factor (e.g. temperature, pH, enzyme concentration, substrate concentration) on the rate of a reaction catalysed by an enzyme

  
• Line graphs should be used to present the results of enzyme rate experiments
• The independent variable should be plotted on the x-axis and the dependent variable on the y-axis
• A line of best fit (straight or curved) should be added to the graph if a trend can be identified
• Interpolation and extrapolation can be used with the line of best fit to read off values
• Ways in which the reaction rate can be measured

  1. Measuring how much of a product is made in a given time period

  2. Measuring how much a substrate is broken down in a given time period
• The data should be plotted with the independent variable on the x-axis and the dependent variable on the y-axis
• If a trend can be identified, a line of best fit (straight or curved) should be added to the graph
• Interpolation


  Reading y values in between existing data points
• Extrapolation


  Going beyond the range of existing data points to read y values
• For some enzyme rate experiment graphs, it may be necessary to plot more than one set of data on the same graph
• When drawing a graph like this, make sure to clearly label each line
• The results of an enzyme experiment are plotted with the volume of product produced and not the rate of reaction on the y axis
• The initial rate of reaction is represented by the initial gradient of the lines on the graph
• As the temperature is the factor being manipulated it is the independent variable
• Tips for plotting line graphs


  • Plot data points accurately
  • Use appropriate linear scales on axes
  • Choose scales that enable all data points to be plotted within the graph area
  • Label axes, with units included
  • Make graphs that fill the space the exam paper gives you
  • Draw a line (or curve) of best-fit to identify trends. The line must be smooth and have a balance of data points above and below the line
  • In some cases, the line of best fit should be drawn through the origin, for example for rate–concentration graphs (the reaction cannot occur if the concentration of enzyme or substrate is 0). The line of best fit should only go through the origin if the data and trend allow it
• When drawing graphs for enzyme rate experiments, students often make mistakes when choosing the scales for their graphs
• If possible, you should try to avoid scales that involve using parts of grid squares on the graph paper and instead try and use whole grid squares, as this makes it much easier to plot data points accurately
• Tangent


  A straight line that is drawn so it just touches the curve at a single point
• The slope of the tangent matches the slope of the curve at just that point
• Initial rate of reaction

  The rate of reaction at the start of the reaction (i.e. where time = 0)
• Using a tangent to find initial rate of reaction

  1. Estimate the extrapolated curve of the graph

  2. Find the tangent to the curve at 0 seconds (the start of the reaction)

  3. Calculate the gradient of the tangent (this will give you the initial rate of reaction)
• When drawing tangents: always use a ruler and a pencil; make sure the line you draw is perfectly straight; choose the point where the tangent is to be taken and slowly line the ruler up to that point; try to place your ruler so that none of the line of the curve is covered by the ruler (it is much easier if the curve is entirely visible whilst the tangent is drawn)
• Rise over run


  Any increase/decrease vertically should be divided by any increase/decrease horizontally
• Eyect of temperature on the rate of an enzyme-catalysed reaction

  • Lower temperatures either prevent reactions from proceeding or slow them down
  • Higher temperatures speed up reactions
  • As temperatures continue to increase, the rate at which an enzyme catalyses a reaction drops sharply, as the enzyme begins to denature
• Very few human enzymes can function at temperatures above 50°C