Chapter summary
Enzymes and investigations, at a glance
Scan the key ideas, or hide the answers and try to recall them.
01
How an enzyme lowers the barrier
Why does an enzyme speed a reaction without changing its overall energy change?
Key idea and reminders
An enzyme provides a lower-activation-energy route; it does not change the reactants, products or overall energy difference.
- Lower activation energy, unchanged overall energy difference.
- Enzyme-substrate complex forms temporarily.
- Induced fit includes conformational change.
Keep in mind: It lowers the activation barrier through a different reaction pathway.
02
Temperature and pH change enzyme rate
Why is a temperature-rate curve not symmetrical?
Key idea and reminders
Below the optimum, more kinetic energy raises the frequency of effective collisions. Above it, the enzyme is denatured and the 3D shape of the active site changes. pH changes the charges that hold the shape and bind the substrate.
- Below the optimum: more kinetic energy, higher frequency of effective collisions.
- Above the optimum: bonds break, the 3D shape of the active site changes, the enzyme is denatured.
- pH can alter charge and binding.
- An optimum depends on assay conditions.
Keep in mind: Rate can fall because of lower temperature, limited substrate or altered ionisation without complete denaturation.
03
Measure a rate, explain a plateau
What becomes limiting when more substrate no longer increases rate?
Key idea and reminders
At high substrate concentration, occupied active sites limit rate; measure initial slopes or a justified reciprocal-time proxy.
- Substrate plateau: enzyme capacity limits rate.
- More enzyme helps only while substrate is sufficient.
- Initial slope and 1/time are different rate measures.
Keep in mind: Substrates still move; enzyme availability and turnover limit further increases in rate.
04
Competitive and non-competitive inhibition
Can adding substrate overcome this inhibitor?
Key idea and reminders
A competitive inhibitor competes at the active site; a non-competitive model reduces functional catalytic capacity through another binding site.
- Competitive: active-site competition.
- More substrate can overcome reversible competitive inhibition.
- Non-competitive capacity loss lowers the plateau in the simple model.
- Allosteric enzyme: several subunits, cooperative binding, sigmoid rate curve.
- End-product inhibition: the final product allosterically inhibits an early enzyme in its pathway (negative feedback), e.g. ATP and phosphofructokinase.
Keep in mind: Many inhibitors bind reversibly, and different binding mechanisms produce different kinetic effects.