(DEC 2019 ASSAM)
55. An allosteric enzyme has two heterotropic effectors, X and Y. The allosteric constant, L for the enzyme in the absence of effector molecules is 180. For the X- saturated form, the value of L increases from 180 to 1200, while for Y-saturated form it decreases to 60.
What kind of effector molecules are X and Y?
(1) X and Y both are positive regulators.
(2) X is a negative regulator while Y is a positive regulator.
(3) X is a positive regulator while Y is a negative regulator.
(4) X and Y are not allosteric regulators.
The correct answer is (2) X is a negative regulator while Y is a positive regulator.
Introduction
Allosteric enzymes undergo conformational changes upon effector binding, which modulates their activity. Effectors binding at regulatory sites can be positive regulators that enhance activity or negative regulators that inhibit activity. The allosteric constant L=[T]/[R] — the ratio of the tense (inactive) to relaxed (active) enzyme states — quantifies this regulation. Values of L reflect the equilibrium between inactive and active conformations, shifting in response to effectors.
Role of Allosteric Constant L
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When L increases, the equilibrium favors the T (inactive) state, reducing enzyme activity.
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When L decreases, the equilibrium favors the R (active) state, enhancing enzyme activity.
Interpreting the Given Data
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Without effectors, L=180, indicating baseline inactive/active state distribution.
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With effector X saturation, L increases to 1200 → equilibrium shifts toward inactive form → X is a negative regulator.
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With effector Y saturation, L decreases to 60 → equilibrium shifts toward active form → Y is a positive regulator.
Why Other Options Are Incorrect
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(1) Both positive regulators would both decrease L, not increase/decrease oppositely as shown.
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(3) Reverse roles contradict observed L shifts.
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(4) Clear changes in L confirm X and Y act as allosteric regulators.
Biological Significance
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Positive and negative effectors allow precise control of enzyme activity in response to cellular needs.
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This dynamic regulation maintains metabolic balance and responsiveness.
Summary Table
| Effector | Change in L | Regulatory Role |
|---|---|---|
| X | 180→1200 (increase) | Negative regulator |
| Y | 180→60 (decrease) | Positive regulator |
Conclusion
Effector X increases the allosteric constant L, promoting enzyme inactivity, marking it as a negative regulator. Effector Y decreases L, promoting enzyme activity, marking it as a positive regulator. Hence, option (2) is the correct answer.
This explanation clarifies the biochemical principles of allosteric regulation, aiding students and researchers in interpreting enzyme modulation mechanisms accurately.
28 Comments
Khushi Vaishnav
September 12, 2025X is a negative regulator while Y is a positive regulator.
Kajal
September 13, 2025Option 2 is correct as x is negative regulator while y is positive regulator ..
Kanica Sunwalka
September 13, 2025When L increases – > enzyme activity decreases
L decreases -> enzyme activity increases
so here , effector X increases the value of L to 1200 => -ve regulator
effector Y decreases the value of L to 60 => + ve regulator
option 2 is correct
Aakansha sharma Sharma
September 13, 2025When L increases – > enzyme activity decreases
L decreases -> enzyme activity increases
so here , effector X increases the value of L to 1200 => -ve regulator
effector Y decreases the value of L to 60 => + ve regulator
Pratibha Jain
September 14, 2025correct answer is option (2)
X is a negative regulator while Y is a positive regulator.
Mohd juber Ali
September 14, 2025X is negative regulators bcs Allosteric constant increase 180to 1200)favour t state(inactive
Y is positive regulators bcz allosteric decrease (1200 to 60 )R state active
Santosh Saini
September 14, 2025X is nagetive regulator while Y is positive regulator
Anju
September 14, 2025L=180
L inc-1200 towards in active (x-ve) regulator
L decrease -60 shift towards active form(y+ve) regulator
Dharmpal Swami
September 14, 2025X is a negative regulator and y is a positive regulator
yashika
September 14, 2025C increase allosteric constant and become negative regulator and vice versa
Pooja
September 14, 2025Option b is correct
X is a negative regulator while Y is a positive regulator
Soniya Shekhawat
September 14, 2025Allosteric constant is LLis increase then enzyme activity is decrease so X is processed 180 to 1200 means negative regulator and Y form is decrease LL that means increase enzymatic reaction that is positive regulator.
Aafreen Khan
September 14, 2025X is a negative regulator while Y is a positive regulator because if allosteric constant LL increases then enzyme activity decreases
In X the LL is increasing from 180-1200 then X is negative regulator and Y decreasing LL then it is positive regulator
Pallavi Ghangas
September 14, 2025When Allosteric constant is more equilibrium favours in active enzymes reducing enzyme activity and when the constant value is less active enzymes are more more enzyme activity
Kirti Agarwal
September 14, 2025X is negative Y is positive allosteric
Aartii sharma
September 14, 2025X is positive y is positive allosteric
Palak Sharma
September 14, 2025When L increases = enzyme activity decreases
L decreases = enzyme activity increases
so here , effector X increases the value of L to 1200 = -ve regulator
effector Y decreases the value of L to 60 =+ ve regulator
option 2
Deepika sheoran
September 15, 2025Option 2nd correct
X is negative regulator and y is positive regulator.
Roopal Sharma
September 15, 2025When L increases enzyme activity decreases .
When L decreases enzyme activity increases . X is negative regulator and Y is positive.
Parul
September 15, 2025Effector X increases the Allosteric constant L means increasing the enzyme inactivity – so it is a Inhibitor.
Effector Y decreases the allosteric constant L means increasing the enzymatic activity – so it is a activator.
Nilofar Khan
September 16, 2025Correct answer is 2
X is nagative regulator becouse L increase anzym activity decrease
Y is positive regulator becouse L decreases anzym activity increases
Khushi Agarwal
September 16, 2025The correct answer is (2) X is a negative regulator while Y is a positive regulator
Tanvi Panwar
September 16, 2025X is a negative regulator bcz it inactivates the enzyme while Y is a positive regulator as it activates the enzyme.
Avni
September 17, 2025The correct answer is (2) X is a negative regulator while Y is a positive regulator
Priti khandal
September 17, 20252is right
Minal Sethi
September 19, 2025X is a negative regulator and Y is a positive regulator
Muskan Yadav
September 19, 2025X is a negative regulator while Y is a positive regulator.
Kajal
September 25, 2025The correct answer is (2) X is a negative regulator while Y is a positive regulator