Chapter 4

• What are enzymes?


  • biological catalysts that facilitate chemical reactions
  • globular proteins that interact w substrate molecules causing them to react at faster rate
• Anabolic reactions 


  • join smaller molecules
  • e.g. protein synthesis
• Catabolic reactions


  • break down larger molecules
  • e.g. digestion
• Metabolism 


  • sum of all diff reactions in cell/organism
  • affected by enzymes
• Specificity 


  specific tertiary structure determines shape of active site complementary to specific substrate
• Lock & key hypothesis

  • only a specific substrate will fit the active site of an enzyme
  • when substrate is bound to active site - enzyme-substrate complex is formed
  • substrate reacts, products formed in an enzyme-product complex
  • products released - enzyme unchanged & reusable
• Induced-fit hypothesis 


  • initial interaction between enzyme & substrate is weak
  • weak interactions induce changes in enzyme's tertiary structure
  • strengthens bonds between substrate & enzyme & puts strain on bonds within substrate
  • which lowers activation energy
• Intracellular enzymes 


  • act within cell
  • usually used for synthesis of polymers from monomers
  • e.g. catalase - catalyses decomposition of hydrogen peroxide into water & oxygen
• Extracellular enzymes


  • work outside the cell
  • released from cells to break down large molecules into smaller molecules
  • used in digestion - digestive enzymes secreted from cells into the digestive system
  • e.g. amylase
• 5 factors affecting enzyme activity

  • temperature
  • pH
  • enzyme concentration
  • substrate concentration
  • concentration of inhibitors
• Effect of temperature on enzyme activity

  inc temp = inc KE of enzyme & substrate molecules = move faster = more frequent successful collisions between substrate & enzyme = inc rate of reaction
• Denaturation from temperature 


  • as temp increases, vibration of bonds inc, until bonds strain & then break
  • result in change of tertiary structure - enzyme changed shape - substrate can't fit - rate of reaction decreases
• What is the temperature coefficient?

  • measure of how much rate increases w a 10°C rise in temp
  • Q10= R2/R1
  • for enzyme-controlled reactions - rate doubles w a 10°C increase
• Effect of pH on enzyme activity

  • active site will only be right shape at a certain hydrogen ion concentration - optimum pH
  • when pH changes from optimum, the charges of amino acids in the active site are altered which can prevent substrate molecules from binding - rate of reaction decreases
  • if pH changes significantly, hydrogen & ionic bonds in active site are broken which causes a permanent change in enzymes tertiary structure - enzyme is denatured

  
• How does substrate concentration affect enzyme concentration?

  • substrate conc increases = higher collision rate w the active sites of enzymes & the formation of more enzyme-substrate complexes = rate of reaction increases
  • rate inc up to Vmax - all active sites occupied by substrate particles - no more enzyme-substrate complexes can be formed until products released
  • substrate conc is the limiting factor - only way to inc rate would be to add more enzyme or inc temp

  
• How does enzyme concentration affect enzyme activity?

  • enzyme conc inc = inc number of available active sites in particular area/volume, leading to formation of enzyme-substrate complexes at a faster rate
  • rate inc up to Vmax - enzyme concentration becomes limiting factor
• Inhibitors 


  molecules that prevent enzymes from carrying out their normal function of catalysis
• Competitive inhibition 


  • a molecule or part of molecule that has similar shape to substrate of an enzyme can fit into active site of enzyme
  • blocks substrate from entering active site - preventing enzyme from catalysing reaction - is inhibited
  • substrate & inhibitor molecule compete w each other to bind to the active sites - reduce no. of substrate molecules binding to active sites in given time & slows rate of reaction

  
• Is competitive inhibition reversible or irreversible?

  most competitive inhibitors only bind to active site temporarily so effect is reversible (exceptions include aspirin)
• Competitive inhibition effect on rates of reaction

  • reduces rate for a given concentration of substrate but doesn't change Vmax of enzyme
  • if substrate concentration is increased enough, there will be so much more substrate than inhibitor that the original Vmax (max rate) can still be reached
• Examples of competitive inhibitions

  • statins - reduce blood cholesterol concentration
  • aspirin - inhibits active site of COX enzymes, preventing synthesis of prostaglandins & thromboxane, chemicals responsible for producing pain & fever
• Non-competitive inhibition 


  • inhibitor binds to enzyme at alternative site called an allosteric site
  • binding causes tertiary structure of enzyme to change shape - means active site changes shape
  • active site no longer complementary to substrate so substrate unable to bind to enzyme
  • enzyme can't carry out function - is inhibited

  
• Is non-competitive inhibition reversible or irreversible?

  both - sometimes reversible, sometimes not
• Examples of non-competitive inhibitors

  • cyanide ions
  • organophosphates
• End-product inhibition 


  • occurs when product of a reaction acts as an inhibitor to the enzyme that produces it
  • serves as negative feedback control mechanism for reactions
• Cofactor 


  • non-protein substance required for enzymes to function
  • can be organic or inorganic
  • obtained via diet as minerals
• Coenzyme 


  • organic cofactor
  • derived from vitamins
• Examples of inorganic cofactors

  • Cl- ions (enzyme = amylase
  • Mg2+ (enzyme = DNA polymerase)
• Examples of coenzymes 


  • coenzyme A (break down fatty acids & carbohydrates)
  • NAD+ (enzyme = lactate dehydrogenase)
• How do cofactors interact with enzymes?

  form temporary bonds w the enzyme but leave following the reaction
• Prosthetic groups 


  • cofactors
  • inorganic or organic
  • tightly bound to enzyme to form permanent feature of the protein
• Examples of prosthetic groups

  • Zn2+ (enzyme = carbonic anhydrase)
• Precursor activation 


  • enzymes produced in inactive form = inactive precursor enzymes
  • activated by a change in its tertiary structure (shape)
  • change achieved by:
  • cofactor binding
  • change in conditions such as pH or temp