1.4.2 Enzymes

• How do enzymes act as biological catalysts?

  ● Each enzyme lowers activation energy of reaction it catalyses ● To speed up rate of reaction Enzymes catalyse a wide range of intracellular and extracellular reactions that determine structures and functions from cellular to whole-organism level

  
• Describe the induced-fit model of enzyme action ?

  1. Substrate binds to (not completely complementary) active site of enzyme 2. Causing active site to change shape (slightly) so it is complementary to substrate 3. So enzyme-substrate complex forms 4. 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  ● Initially lock and key model (now outdated) ○ Active site a fixed shape, complementary to one substrate ● Now induced-fit model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
• Describe and explain the effect of enzyme concentration on the rate of enzyme-controlled reactions

  ● As enzyme conc. increases, rate of reaction increases ○ Enzyme conc. = limiting factor (excess substrate) ○ More enzymes so more available active sites ○ So more enzyme-substrate (E-S) complexes form ● At a certain point, rate of reaction stops increasing / levels off ○ Substrate conc. = limiting factor (all substrates in use)

  
• Describe and explain the effect of substrate concentration on the rate of enzyme-controlled reactions?

  ● As substrate conc. increases, rate of reaction increases ○ Substrate conc. = limiting factor (too few enzyme molecules to occupy all active sites) ○ More E-S complexes form ● At a certain point, rate of reaction stops increasing / levels off ○ Enzyme conc. = limiting factor ○ As all active sites saturated / occupied (at a given time)

  
• Describe and explain the effect of temperature on the rate of enzyme-controlled reactions

  ● As temp. increases up to optimum, rate of reaction increases ○ More kinetic energy ○ So more E-S complexes form ● As temp. increases above 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 E-S complexes form

  
• Describe and explain the effect of pH on the rate of enzyme-controlled reactions ;

  ● As pH increases / decreases above / below an 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 E-S complexes form

  
• Describe and explain the 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 E-S complexes form ● Increasing substrate conc. reduces effect of inhibitors (dependent on relative concentrations of substrate and inhibitor)

  
• Describe and explain the effect of concentration of non-competitive inhibitors on the rate of enzyme-controlled reactions;

  ● As concentration of non-competitive inhibitor 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 E-S complexes form ● Increasing substrate conc. has no effect on rate of reaction as change to active site is permanent

  
• Exam insight: common mistakes ❌-(1/6)

  ❌-“E-S complexes form.”

  ✅This abbreviation isn’t listed in the spec, so is often not accepted unless the full name is given elsewhere in the answer
• Exam insight: common mistakes ❌-(2/6)

  ❌“In the induced fit model, the active site is complementary to the substrate.”

  ✅The active site is not fully complementary to start with, but changes shape slightly when the substrate binds.
• Exam insight: common mistakes ❌-(3/6)

  ❌“The substrate changes shape in the induced fit model of enzyme action.”

  ✅It is the enzyme’s active site that changes shape slightly to become complementary to the substrate.
• Exam insight: common mistakes ❌-(4/6)

  ❌“High / low pH or temperature change the shape of the enzyme.”

  ✅This is too vague. They cause the tertiary structure to change, which causes the active site to change shape.
• Exam insight: common mistakes ❌-(5/6)

  ❌“Bonds break when an enzyme denature.”

  ✅Be specific - hydrogen and ionic bonds.
• Exam insight: common mistakes ❌-(6/6)

  ❌A competitive inhibitor has the same shape as the substrate.”

  ✅A competitive inhibitor has a similar shape to the substrate, but is not identical.