Proteins

• How do enzymes catalyse specific reactions?

  • Lowers activation energy through the formation of enzyme substrate complexes
• Describe what is meant by primary structure?

  The sequence of amino acids in a polypeptide chain - this determines the specific shape of a protein.
• Describe non competitive inhibition?

  • Attaches to the enzyme at a site other that the active site
  • This changes the shape of the active site
  • Which means the active site is no longer complementary to substrate so less E-S complexes form
• Describe what is meant by Quaternary structure and give an example?

  • Highly complex proteins that consist of more than one polypeptide chain
  • these polypeptide chains are held together by ionic, hydrogen and sometimes disulfide bridges
  • Example: Haemoglobin
• Describe how the tertiary structure is formed and give an example?

  • formed by further folding and coiling of the secondary structure due to hydrogen, ionic and disulfide bonds
  • Example: Enzymes
• Describe the effect of substrate concentration on the rate of enzyme-controlled reaction?

  • Rate of reaction initially increases as the collisions between substrate and enzyme molecules are more likely
  • Rate of reaction then levels out as the active sites of all enzymes become fully saturated by the substrate molecules
  • At this point, the rate of reaction is limited by enzyme concentration
• Describe how amino acids join together to form dipeptides and polypeptides?

  • Amino acids are joined together by peptide bonds (-CONH-) during a condensation reaction
  • Two amino acids joined together = dipeptide
  • Many amino acids joined together = polypeptide

    
• Describe how you would test for the presence of proteins?

  • Add biuret reagent
  • Purple/lilac colour indicates that protein is present
  • If solution remains blue, then no protein is present
• Describe the effect of enzyme concentration on rate of enzyme-controlled reactions?

  • When substrate concentration is in excess, an increase in enzyme concentration will increase the rate of reaction
  • There are more enzymes molecules and therefore, more active sites available
  • This increases the number of collisions of enzymes and substrates to form more E-S complexes
• Describe competitive inhibition?

  • Inhibitor has a similar structure to the substrate and competes with it for attachment to the active site
  • Rate of reaction is reduced as the substrate cannot bind to an occupied active site
  • Competitive inhibition can be reduced by the addition of more substrate
• Describe the effect of pH on the rate of enzyme controlled reactions?

  • Very different pH from the optimum causes denaturation
  • Changes in pH effects the ionic charges of the acidic and basic groups
  • Hydrogen and ionic bonds are broken altering the tertiary structure and therefore the shape of the active site
• Describe the lock and key hypothesis?

  • Only substrate molecules that are complementary in shape to the active site are able to form an E-S complex
• Describe the induced fit model?

  • Substrate binds to active site of enzyme
  • Active site changes shape slightly so that is is complementary to substate
• Describe how the secondary structure is formed and give 2 examples?

  • Folding or coiling of a polypeptide chain as a result of hydrogen bonds between ‎‏‏‎amino acids ‎‏‏‎ ‎
  • Alpha helix and Beta pleated sheet
• Describe the effect of temperature on the rate of enzyme controlled reactions?

  • Increases temperature results in increased energy in the particles which results in more collisions between active sites and substrate molecules and therefore, forms more E-S complexes
  • This increases the rate of reaction up to the optimum temperature where the rate of reaction is at its maximum
• What happens to an enzyme if the temperature is too high?

  • Increasing the temperature above this causes the tertiary structure of the enzyme to denature as the hydrogen and ionic bonds are broken
  • Active site changes shape and is no longer complementary to the substrate so less E-S complexes forms
  • Rate of reaction decreases as substrate has difficulty binding to an altered active site