Lesson 3 of 4 / Enzymes and investigations
What becomes limiting?
Why does adding more substrate eventually stop helping?
I can explain substrate- and enzyme-concentration effects on reaction rate.
I can investigate and explain temperature, pH, enzyme and substrate concentration effects using rate measurements.Syllabus
Syllabus 8876, Core 1(o). Investigate and explain temperature, pH, enzyme and substrate concentration effects using rate measurements.
About 5 min
Make a guess. You are not marked.
Adding more substrate no longer raises the rate. What would raise it?
Show the answer
Adding more enzyme.
At saturation every active site is busy. More enzyme means more active sites.
The key idea
More substrate increases productive encounters until available active sites become limiting. More enzyme adds active sites if enough substrate is available.
Explore the idea
Find the limiting factor
At low substrate, additional substrate increases productive encounters. Near the plateau, available active sites and cycle speed limit rate.
Original qualitative model, not a fitted experimental curve. More enzyme changes rate; equal starting substrate can still give the same final product amount.
Explanation
With enzyme concentration fixed, increasing a low substrate concentration increases the chance of substrate reaching an available active site. More enzyme-substrate complexes form per unit time, so initial rate rises.
At high substrate concentration, most active sites are occupied much of the time. The enzyme must finish one catalytic cycle before starting another, so a maximum rate is approached. Adding still more substrate has little effect: enzyme availability is now limiting.
Increasing enzyme concentration provides more active sites. When substrate is in excess and other conditions remain suitable, initial rate can increase approximately in proportion to enzyme concentration. If substrate becomes limiting, the relationship no longer remains proportional.
Rate and final yield are different. Doubling enzyme can make a fixed amount of substrate react faster without doubling the total possible product. Use early measurements before substantial substrate depletion when comparing initial rates.
Step by step
- 1
Fix the other concentration
Interpret a substrate curve at a stated enzyme concentration.
- 2
Identify the limiting factor
At low substrate, encounters limit rate; at high substrate, available catalytic capacity limits it.
- 3
Separate rate from amount
An enzyme changes how quickly the substrate is converted, not the initial amount supplied.
Worked example
Twice as much enzyme
Two tubes contain equal substrate amounts. One receives twice the enzyme and initially reacts twice as fast. Must it make twice the final product?
One way to explain it
No. It can reach the same substrate-limited final amount sooner. The doubled initial rate reflects more active sites while substrate is plentiful, not a doubled supply of starting material.
Why this answer works
- Identify what was changed and what was held constant.
- Distinguish product per time from eventual product amount.
Watch out for this
A student says: "A substrate-rate plateau means the enzyme has stopped working." What is wrong with this?
Show the answer
At saturation the enzyme is working near its maximum rate; active sites are occupied much of the time.
Check your understanding
What would usually raise the high-substrate plateau while keeping conditions suitable?
Show the answer
Adding more active enzyme.
More active sites increase total catalytic capacity.
Exam-style questions
Check your understanding
At a high substrate concentration, the rate of an enzyme reaction stays constant. Which statements explain this? 1: All active sites are occupied. 2: The enzyme has been denatured. 3: Enzyme concentration is now limiting. 4: Substrate molecules have stopped moving.
Show the answer
1 and 3 only
At saturation every active site is busy, so the amount of enzyme limits the rate. The enzyme is not denatured and molecules keep moving.
Quick recall
Why does adding more substrate eventually stop helping?