Studies on AsK are rare. The first described enzyme was PAE2307, a protein from the hyperthermophilic archaeon Pyrobaculum aerophilum . Structural studies in combination with fluorescence spectroscopic analysis indicated a binding of adenosine and AMP to the adenosine-specific kinase like protein. Furthermore, a phosphorylation of the conserved histidine residue in the putative substrate-binding site was observed. Hence, it was suggested that the enzyme is a new class of AK. Therefore, we evaluated if TmAsK can phosphorylate adenosine. However, no AMPforming activity was observed (data not shown). As biochemical and structural studies on a homologous enzyme of Thermus thermophilus indicated that the enzymatic function of the "adenosine-specific kinase" family is rather a adenosine diphosphate (ADP) cleavage to AMP , we evaluated if the TmAsK shows ADP phosphatase activity. However, no activity was observed using ADP as substrate for TmAsK (data not shown). Additionally, no phosphatase activity was detected with AMP and ATP. Since MgCl 2 was described to inhibit phosphatase activity of AsK , reactions were also performed without the addition of MgCl 2 and DTT. However, no activity was detected as well (data not shown). Thus, neither a phosphotransferase nor a phosphatase activity was shown for TmAsK under the conditions tested.