1.4.2 many proteins are enzymes

• how do enzymes act as biological catalysts

  each enzyme lowers activation energy of reaction is catalyses

  to speed up rate of reaction
• induced-fit model of enzyme action

  substrate binds to (not completely complementary) active site of enzyme

  causing active site to change shape (slightly) so it is complementary to substrate

  so enzyme-substrate complex forms

  causing bonds in substrate to bend/distort

  lowering activation energy
• describe how models of enzyme action have changed over time

  initially lock and key model (now outdated)

  where active site is a fixed shape and complementary to one substrate

  now induced fit model
• explain the specificity of enzymes

  specific tertiary structure determines shape of active site

  dependent on sequence of amino acids (primary structure)

  active site is complementary to a specific substrate

  only this substrate can bind to active site

  inducing fit and forming an enzyme-substrate complex
• effect of enzyme concentration on the rate of enzyme-controlled reactions

  as enzyme concentration increases rate of reaction increases

  enzyme concentration is limiting factor (excess substrate)

  more enzymes so more available active sites

  so more enzyme-substrate complexes form

  at a certain point rate of reaction stops increasing/levels off

  substrate concentration is limiting factor (all substrates in use)
• effect of substrate concentration on the rate of enzyme-controlled reactions

  as substrate concentration increases rate of reaction increases

  substrate concentration is the limiting factor (too few enzyme molecules to occupy all active sites)

  more enzyme substrate complexes form

  at a certain point rate of reaction stops increasing/levels off

  enzyme concentration is limiting factor

  as all active sites saturated/occupied (at a given time)
• effect of temperature on the rate of enzyme-controlled reactions

  as temperature increases up to optimum rate of reaction increases

  more kinetic energy

  so more enzyme-substrate complexes form

  as temperature increases above optimum rate of reactions decreases

  enzymes denature so tertiary structure and active site change shape

  as hydrogen/ionic bonds break

  so active site no longer complementary

  so fewer enzyme-substrate complexes form
• effect of pH on the rate of enzyme-controlled reactions

  as pH increases/decreases above/below an optimum rate of reaction decreases

  enzymes denature so tertiary structure and active site change shape

  as hydrogen/ionic bonds break

  so active site no longer complementary

  so fewer enzyme-substrate complexes form
• effect of concentration of competitive inhibitors on the rate of enzyme-controlled reactions

  as concentration of competitive inhibitor increases rate of reaction decreases

  similar shape to substrate

  competes for/binds to/blocks active site

  so substrates can't bind and fewer enzyme-substrate complexes form

  increasing substrate concentration reduces effect of inhibitors (dependent on relative concentrations of substrate and inhibitor)
• effect of concentration of non-competitive inhibitors on the rate of enzyme-contolled reactions

  as concentration of non-competitive increases rate of reaction decreases

  binds to site other than the active site (allosteric site)

  changes enzyme tertiary structure/active site shape

  so active site no longer complementary to substrate

  so substrates can't bind so fewer enzyme-substrate complexes form

  increasing substrate concentration has no effect on rate of reaction as change to active site is permanent