Cooperativity and the switch-like response

Science · Biochemistry & Molecular Biology · ages 19-20

Name ______________________ Date ____________

Which description matches a sigmoid rate-versus-substrate curve?
- A straight rise with no plateau even at high substrate concentration
- A falling rate followed by a steep rise at every substrate concentration
- A shallow beginning, steep middle and plateau
In positive cooperativity, substrate binds at one site. How does this affect other sites?
- Further binding becomes more favourable
- Further binding becomes less favourable
- Further binding remains independent of the occupied site
In a model with curve shape and maximum rate fixed, an allosteric inhibitor lowers apparent substrate affinity. What response shift is expected?
- Left, so less substrate is needed for the same rate
- Right, so more substrate is needed for the same rate
- No shift, because regulatory-site binding cannot affect substrate response
A pathway uses feedback inhibition. Its end product accumulates. What response reduces further production?
- The end product activates an early enzyme to increase its own production
- The end product inhibits an early regulatory enzyme
- The end product removes every enzyme from the pathway permanently
A regulatory molecule changes an enzyme's activity without occupying its active site. Where does it bind in this allosteric model?
- At a distinct regulatory site
- At the active site instead of the substrate
- Within the product molecule after the enzyme has released it
What makes the steep middle of a sigmoid response useful for regulation?
- Every small substrate change produces exactly the same rate change
- The enzyme remains inactive until substrate reaches a perfect on-off threshold
- A small substrate change can produce a large rate change
In a model with curve shape and maximum rate fixed, an activator increases apparent substrate affinity. A student predicts a right shift. Which correction fits this model?
- Keep the right shift because every activator lowers apparent affinity
- Predict a left shift because less substrate gives the same rate
- Predict a higher plateau because every affinity change alters maximum rate
At fixed substrate concentration near the sigmoid middle, an inhibitor shifts the response right while preserving curve shape and maximum rate. What happens to this enzyme's rate?
- It decreases at the fixed substrate concentration
- It increases because the curve moved toward higher substrate
- It stays at its maximum because every sigmoid middle is saturated

Answer key

For grown-ups. Fold this page away before handing over the rest.

Cooperativity and the switch-like response W1-mt_u8f5QLHq-g-s1

A shallow beginning, steep middle and plateau · The sigmoid response has a shallow beginning, a steep middle and a plateau.
Further binding becomes more favourable · Positive cooperativity favours further binding at other sites.
Right, so more substrate is needed for the same rate · With shape and maximum rate fixed, lower apparent affinity requires more substrate for the same rate below the plateau, giving a right shift.
The end product inhibits an early regulatory enzyme · Feedback inhibition reduces further production by inhibiting an early regulatory enzyme.
At a distinct regulatory site · An allosteric effector binds at a regulatory site distinct from the active site.
A small substrate change can produce a large rate change · A steep region gives high sensitivity to changes in substrate concentration without requiring a literal on-off switch.
Predict a left shift because less substrate gives the same rate · In the stated model, increased apparent affinity means less substrate gives the same rate below the unchanged plateau, so the response shifts left.
It decreases at the fixed substrate concentration · In this shape-preserving right shift, more substrate is needed for the previous rate. At the unchanged concentration near the middle, rate decreases.