Enzymatic characteristics of two adenylate kinases, AdkA and AdkB, from Myxococcus xanthus.

Adenylate kinase (Adk) plays a critical role in energy metabolism and adaptation of bacteria to environmental stresses. We have previously shown that Myxococcus xanthus expresses polyphosphate kinase 1 (Ppk1) that also has Adk activity in the absence of polyphosphates. In this study, we investigated the Adk activity of the other two M. xanthus enzymes, AdkA and AdkB. The activity of AdkA was increased by dithiothreitol (DTT), which also enhanced enzyme stability. Site-directed mutagenesis of three cysteine residues (C130, C150, and C153) present in the LID domain of AdkA revealed that the Adk activity and stability of C150S and C153S mutants were not affected by DTT addition, suggesting formation of a disulfide bond between C150 and C153 in AdkA. The Km of AdkA for AMP was 8 and 17 times lower than that for ADP and ATP, respectively. AdkB is a polyphosphate kinase 2 (Ppk2) homolog lacking the Ppk2 middle region and, consequently, Ppk activity. According to our analysis, AdkB also had Adk activity and its affinity for substrates was higher than that of AdkA. Thus, M. xanthus expresses three enzymes, AdkA, AdkB, and Ppk1, with Adk activity, which may function to support energy metabolism of the bacteria in different environmental conditions.