Q53._____ enzyme catalyzes conversion of fructose to fructose-6-phosphate requires _____ as cofactor.
(1) Phosphofructokinase-I, Mg²⁺
(2) Phosphofructokinase-II, Mg²⁺
(3) Phosphofructokinase-I, Mn²⁺
(4) Phosphofructokinase-II, Mn²⁺
Phosphofructokinase-1 (PFK-1) catalyzes fructose-6-phosphate → fructose-1,6-bisphosphate in glycolysis, requiring Mg²⁺ as cofactor. The correct answer for Q53 is (1) Phosphofructokinase-I, Mg²⁺.
Option Breakdown
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(1) Phosphofructokinase-I, Mg²⁺: Correct. PFK-1 (main glycolysis enzyme) transfers phosphate from ATP to F6P at C1 position. Mg²⁺ complexes with ATP substrate, essential for kinase catalysis.
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(2) Phosphofructokinase-II, Mg²⁺: Incorrect enzyme. PFK-2 makes fructose-2,6-bisphosphate (F2,6BP, PFK-1 activator), not F6P → F1,6BP.
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(3) Phosphofructokinase-I, Mn²⁺: Incorrect cofactor. Mn²⁺ substitutes in vitro but physiological = Mg²⁺ only.
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(4) Phosphofructokinase-II, Mn²⁺: Both wrong—wrong enzyme + cofactor.
Reaction Details
PFK-1: F6P + ATP → F1,6BP + ADP (irreversible, ΔG°’ = -14.2 kJ/mol)
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Role: Committed glycolysis step, major regulation point (AMP↑, ATP↓, citrate↓).
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Mg²⁺: Stabilizes negative ATP phosphates for nucleophilic attack.
Master phosphofructokinase enzyme Q53 for CSIR NET Life Sciences: PFK-I converts fructose-6-phosphate to F1,6BP (glycolysis committed step) requiring Mg²⁺ cofactor. Why PFK-II wrong, cofactor specificity, regulation—complete breakdown.
PFK-1 vs PFK-2: Critical Difference
Reaction Mechanism
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Mg-ATP binds active site
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F6P C1-OH attacks ATP γ-phosphate
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Mg²⁺ lowers energy barrier, stabilizes transition state
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F1,6BP + ADP released
Why Options Fail
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PFK-2: Regulatory, not flux enzyme (F2,6BP allosterically ↑PFK-1 Vmax 10-100x)
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Mn²⁺: Lab substitute; cells maintain ~0.5 mM free Mg²⁺
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CSIR trick: Tests PFK-1 vs PFK-2 confusion (common in glycolysis PYQs)
Regulation Context
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Activators: AMP, F2,6BP, F6P (feed-forward)
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Inhibitors: ATP, citrate (energy charge)
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Hormonal: Glucagon ↓F2,6BP → ↓PFK-1 activity
Exam Mnemonic
“1 makes 1,6BP for glycolysis; 2 makes 2,6BP regulator”
Nail phosphofructokinase enzyme fructose fructose-6-phosphate Mg²⁺ cofactor.
