Adenylate kinases of thermophiles Aquifex aeolicus and Geobacillus stearothermophilus: biochemical and kinetic studies

Adenylate kinases (AK) play a pivotal role in the regulation of cellular energy. The aim of our work was to achieve the overproduction and purification of AKs from two groups of bacteria and to determine, for the first time, the comprehensive biochemical and kinetic properties of adenylate kinase from Gram-negative Aquifex aeolicus (AK_aq) and Gram-positive Geobacillus stearothermophilus (AK_st). Therefore we determined K_M and V_max values, and the effects of temperature, pH, metal ions, donors of the phosphate groups and inhibitor Ap₅A for both thermophilic AKs. The kinetic studies indicate that both AKs exhibit significantly higher affinity for substrates with the pyrophosphate group than for adenosine monophosphate. AK activation by Mg²⁺ and Mn²⁺ revealed that both ions are efficient in the synthesis of adenosine diphosphate and adenosine triphosphate; however, Mn²⁺ ions at 0.2-2.0 mmol/L concentration were more efficient in the activation of the ATP synthesis than Mg²⁺ ions. Our research demonstrates that zinc ions inhibit the activity of enzymes in both directions, while Ap₅A at a concentration of 10 µmol/L and 50 µmol/L inhibited both enzymes with a different efficiency. Sigmoid-like kinetics were detected at high ATP concentrations not balanced by Mg²⁺, suggesting the allosteric effect of ATP for both bacterial AKs.