1.4.2 Many proteins are enzymes

How do enzymes act as biological catalysts?

• each enzyme lowers activation energy of reaction it catalyses by providing an alternative reaction pathway

• 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.

Described 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 its 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• initially lock and key model (now outdated)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 concentration increases, rate of reaction increases

• → enzyme concentration = 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 = limiting factor (all substrates in use)

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

• as substrate concentration increases, rate of reaction increases 

• → substrate concentration = limiting factor (too few substrate molecules to occupy all active sites)

• → more enzyme-substrate complexes form 

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

• → enzyme concentration = limiting factor 

• → as all active sites saturated / occupied (at given time)

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

• as temperature increases to optimum, rate of reaction increases

• → more kinetic energy

• → so more enzyme-substrate complexes form

• as temperature exceeds optimum, rate of reaction decreases

• → enzymes denature - tertiary structure and active site changes shape

• → as hydrogen bonds / ionic bonds break

• → so active site no longer complementary

• → so fewer enzyme-substrate 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 bonds / ionic bonds break

• → so active sites no longer complementary

• → so fewer enzyme-substrate complexes form

Describe and explain the effect of concentration of competitive inhibitors on 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 fewer enzyme-substrate complexes form

• increasing substrate concentration 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 inhibitors 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 fewer enzyme-substrate complexes form 

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