1.4.2 enzymes

How do enzymes act as biological catalysts

  • each enzyme lowers activation energy of the reaction it catalyses

  • This speeds up the rate of reaction

The induced fit model

  1. Substrate binds to (not completely complementary) active site of enzyme

  2. Causing active site to change shape, so it is complementary to its substrate

  3. So enzyme-substrate complex form

  4. Causing bonds in substrate to bend/ distort, lowering activation energy

How have enzyme action models changed over time

  • initially lock and key model, which is outdated. Suggested the active site was a fixed shape, complementary to one substrate

  • Changed to the induced fit model

Explain the specificity of enzymes

  • specific tertiary structure determines shape of active site, dependent on the 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

The effect of enzyme concentration on the rate of enzyme controlled reactions

— As enzyme concentration increases, rate of reaction increases —

  • This is because the enzyme concentration was the limiting factor, so there’s excess substrate

  • So more enzymes means there more available active sites

  • So more enzyme- substrate complexes form

— At a certain point, rate of reaction stops increasing/ levels off —

This is because substrate concentration is the limiting factor (all substrates in use)

The effect of substrate concentration on the rate of enzyme controlled reactions

— As substrate concentration increases, rate of reaction increases —

  • This is because the substrate concentration was the limiting factor, there was too few substrate molecules to occupy all the active sites

  • So more enzymes-substrate complexes form

— At a certain point, rate of reaction stops increasing/ levels off —

  • This is because enzyme concentration is the limiting factor, as all active sites are saturated/ occupied

The effect of temperature on the rate of enzyme controlled reactions

— As temperature increases to optimum, rate of reaction increases —

  • This is because there is more kinetic energy

  • So more enzyme- substrate complexes form

— As temperature exceeds optimum, rate of reaction decreases —

  • Enzymes denature, tertiary structure and active site change shape

  • As hydrogen/ ionic bonds break

  • So active site no longer complementary

  • So fewer enzyme- substrate complexes form

The effect of pH on the rate of enzyme controlled reactions

— As pH increases or decreases, above or below optimum, rate of reaction decreases —

  • enzymes denature, the tertiary structure and active site change shape

  • As hydrogen/ ionic bonds break

  • So active site is no longer complementary

  • So fewer enzyme-substrate complexes form

The effect of competitive inhibitors on the rate of enzyme controlled reactions

— As concentration of competitive inhibitors increases, rate of reaction decreases —

  • The inhibitor is a similar shape to substrate

  • So competes for/ binds to/ blocks active site

  • So substrates can’t bind

  • So fewer enzyme-substrate complexes form

— Increasing substrate concentration reduced the effect of inhibitors —

The effect of non-competitive inhibitors on the rate of enzyme controlled reactions

— As concentration of non-competitive inhibitors increases, rate of reaction decreases —

  • The inhibitor binds to any site other than the active site (allosteric site)

  • Changes the enzyme tertiary structure/ active site shape

  • So active site is no longer complementary to substrate

  • So substrates can’t bind

  • So fewer enzyme-substrate complexes form

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