The regulatory properties of yeast pyruvate kinase

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Abstract
The kinetics of pyruvate kinase from S. cerevisiae were studied in assays at pH 6.2 where the relationships between the initial velocities of the catalyzed reaction and the concentrations of the substrates ADP, phosphoenolpyruvate and Mg2+ are non-hyperbolic. The findings were represented empirically by the exponential model for a regulatory enzyme. ADP, phosphoenolpyruvate and Mg2+ display positive homotropic interaction in their binding behavior with (calculated) Hill slopes at half-saturation equal to 1.06, 2.35 and 3.11, respectively. The direct heterotropic interaction between ADP and phosphoenolpyruvate is small and negative, but the overall interaction between these substrates become positive when their positive interactions with Mg2+ are taken into account. The heterotropic interactions of the substrates, though smaller in magnitude, are comparable with those revealed by the rabbit muscle enzyme, and it is suggested that they have a common origin in charge interactions within the active site.