Rate of enzyme activity

• Rates of reactions are very slow at low temperatures as enzyme and substrate molecules have little Kinetic Energy (K.E.) and few enzyme – substrate collisions taking place.  
• As temperature increases, the Kinetic Energy of substrate and enzyme molecules increases, there are more successful collisions between the enzyme and substrate and more Enzyme- Substrate complexes form. The rate of reaction will increase initially. 
• The rate of reaction reaches its maximum (v max) at the optimum temperature.
• Increasing the temperature further than the optimum will mean that molecules will also vibrate more which will cause weaker bonds such as hydrogen and ionic bonds holding the enzyme 3D shape together to break. 
• • The tertiary shape of the enzyme will then be altered when the ionic and hydrogen bonds break. This may alter the shape of the active site. 
  • Fewer enzyme substrate complexes form and the rate of reaction decreases.
  • The enzyme is described as denatured.
• Different enzymes have different optimum temperatures. The optimum temperature is likely to be related to the organism’s environmental temperature. 
• • pH is a measure of hydrogen ion (H+  ) concentration. The greater the concentration of H+ ions the lower the pH.   
  • The optimum pH is the pH value at which the rate of an enzyme-controlled reaction is at its maximum.  
• • Above and below the optimum pH the Hydrogen ions can interfere with R- group interactions between amino acids as H+ will cluster around negatively charged R groups and disrupt the Hydrogen bonds and ionic bonds which maintain the tertiary structure  of a protein and the shape of the  active site.  
  • This reduces the rate of the enzyme controlled reaction as it reduces the number of successful collisions and fewer enzyme substrate complexes form.
• • In extremes of pH changes, the enzyme denatures and H+ concentration alters the charges around the active site which may affect the binding of the substrate to the active site in the induced fit hypothesis. 
  • The hydrogen/ ionic bonds break.
  • The tertiary structure is altered- changing the shape of the active site.
• When the shape of the active site is changed, the substrate can no longer bind to the active site, so no Enzyme Substrate Complex can form. The rate of reaction will fall to zero. 
• Different enzymes have a different optimum pH condition e.g. pepsin is a protease found in the stomach which has an optimum of pH=2. 
• When measuring the effect of pH on an enzyme controlled reaction pH buffer solutions of different pH’s can be used. 
• Initial Rate - The rate of reaction is at its highest at the first few seconds/ beginning of the reaction when the concentration of substrate molecules is highest and therefore more enzyme - substrate collisions and so more Enzyme substrate complexes are being formed. 
• V Max – the reaction is fastest as all the enzyme active sites are occupied no more Enzyme substrate complexes can be formed.  
• As more substrate molecules are converted into product there are fewer substrate molecules to bind with the enzymes active sites and the reaction will get slower until it finally stops.  
• There are 2 main ways of measuring the rate of reaction -Start the reaction, then measure either the concentration of product formed or substrate used up after a fixed time intervals  
• Limiting factor - A variable that prevents V max from being achieved. If it is increased, then the rate of the process will increase.  
• The effect of increasing substrate concentration:

  • If the concentration of enzyme remains constant and the concentration of substrate increases - the number of collisions between enzyme and substrate molecules will increase.  
  • More enzyme substrate complexes form
  • The rate of reaction increases
  • A point is reached where all enzyme active sites are occupied at all times
  • Rate of reaction remains at the maximum (v max)
  • The concentration of enzymes is the limiting factor.
• The effect of increasing enzyme concentration:

  • Enzymes being catalysts are not used up in reactions
  • Therefore one enzyme molecule can catalyse millions of molecules per minute.
  • If the concentration of substrate is kept constant and the concentration of enzyme is increased each time the experiment is carried out then the rate of the reaction will increase and then remain constant at a maximum rate. 
• The effect of increasing enzyme concentration:

  • As the enzyme concentration increases there are more active sites available. 
  • Greater chance that substrates will collide with the active site. and more enzyme-substrate complexes can be formed.  
  • All the substrate molecules are occupying active sites. 
  • The initial rate of reaction will increase  
  • A further increase in enzyme concentration will not increase the rate of reaction further. 
  • The concentration of substrate limits any further increase in the rate of reaction. V max is achieved.