Characterization of Metal and Nucleotide Liganded Forms of Adenylate Kinase by Electrospray Ionization Mass Spectrometry

Complexes of adenylate kinase fromEscherichia coli, Bacillus subtilis,andBacillus stearothermophiluswith the bisubstrate nucleotide analogP1,P5-di(adenosine 5′)-pentaphosphate and with metal ions (Zn2+and/or Mg2+) were analyzed by electrospray ionization mass spectrometry.P1,P5-di(adenosine 5′)-pentaphosphate·adenylate kinase complex was detected in the positive mode at pH as low as 3.8. Binding of nucleotide to adenylate kinase stabilizes the overall structure of the protein and preserves the Zn2+chelated form of the enzyme from the gram-positive organisms. In this way, it is possible in a single mass spectrometry experiment to screen metal-chelating adenylate kinases, without use of radioactively labeled compounds. Binding of Mg2+to enzyme viaP1,P5-di(adenosine 5′)-pentaphosphate was also demonstrated by mass spectrometry. Although no amino acid side chain in adenylate kinase is supposed to interact with Mg2+, Asp93in porcine muscle cytosolic enzyme, equivalent to Asp84in theE. coliadenylate kinase, was proposed to stabilize the nucleotide·Mg2+complex via water molecules.