Fig. 3.

The dependence of enzyme activity on the magnesium ion concentration. The activity in the presence of 1 mM Mg²⁺ was taken as 100%. Reactions were conducted at a temperature of 75°C, at a MgCl₂ concentration that varied from 0 to 5 mM, in 0.5 mL of 20 mM Tris-HCl buffer, pH 8.0, containing: 1) 0.4 mM ATP, 1 mM ribose, 1 mM KH₂PO₄, 50 mM KCl, 0.15 μg of TspRK, 2) 1 mM ATP, 1 mM D-ribose 5-phosphate, 10 mM KH₂PO₄, 0.75 μg of TthPRPPS1 or TthPRPPS2, 3) 1 mM adenine, 1 mM PRPP, 0.125 μg of TthAPRT.