We have proposed negative cooperativity for\ud human liver L-type pyruvate kinase, a phenomenon\ud also observed in: Lineweaver-Burk plots V⁻¹\ud versus [phosphoenolpyruvatel]⁻¹ were straight lines\ud at low phosphoenolpyruvate concentrations, and\ud bent downward near they axis (high phosphoenolpyruvate\ud concentrations). Our conclusions were\ud drawn from experiments performed at Mg²⁺total =\ud 12 mM or 20 mM. However, phosphoenolpyruvate\ud has been proposed to bind Mg²⁺free with a Keq =\ud 5.5 X 1O⁻³ M⁻¹ at 25°C. So, at high phosphoenolpyruvate\ud concentrations Mg²⁺free would be removed\ud from the reaction mixture. We investigated the\ud hypothesis that the phenomenon interpreted as negative\ud cooperativity is in fact due to removing\ud inhibitory Mg²⁺free from the reaction mixture at high\ud phosphoenolpyruvate concentrations. Our results are\ud in favour of this hypothesis
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