The stimulation of Escherichia coli aspartate transcarbamylase activity by adenosine triphosphate : Relation with the other regulatory conformational changes; a model

Abstract

The process of stimulation of Escherichia coli aspartate transcarbamylase activity by ATP was investigated. The efficiency of the phenomenon increases with the number of phosphate groups bound to adenosine. The pH dependence of the stimulation by ATP and adenylyl methylenediphosphonate indicates that the binding of these nucleotides requires the ionization of their last phosphate acidic group. The aspartate trans-carbamylase activity does not appear to be under the influence of the “energy charge ratio” but rather to depend directly on the ATP concentration. The stimulation decreases when the aspartate concentration increases. Saturating amounts of ATP do not provoke the shift in optimum pH for the catalytic activity which is shown to be associated with the homotropic co-operative interactions between the catalytic sites. This result provides additional evidence that homotropic and heterotropic interactions correspond to different molecular mechanisms. ATP reverses the effect of the feedback inhibitor CTP. A model is proposed to account for the relationship between the process of stimulation by ATP and the other regulatory conformational changes.