title: Spatial proximity of two divalent metal ions at the active site of S-adenosylmethionine synthetase.
abstract: S-Adenosylmethionine synthetase from Escherichia coli is shown to require 2 divalent metal ions/enzyme subunit for maximal enzymatic activity. In the absence of substrate, the tetrameric enzyme binds 1 Mn(II) ion/subunit, whereas in the presence of a nucleotide substrate, adenylylimidodiphosphate, or the product pyrophosphate, there are two Mn(II)-binding sites/subunit. Electron paramagnetic resonance spectra of Mn(II) bound to the enzyme reveal a spin exchange interaction between 2 Mn(II) ions in complexes of enzyme and Mn(II) which also contain adenosylmethionine, K+, and either pyrophosphate or imidotriphosphate. Since a spin exchange interaction requires orbital overlap between the 2 ions, the metal ions must be bound close to one another, and they may share a common ligand.
mesh_terms: D000266:Adenylyl Imidodiphosphate; D001665:Binding Sites; D004578:Electron Spin Resonance Spectroscopy; D004926:Escherichia coli / Q000201:enzymology*; D007700:Kinetics; D008345:Manganese / Q000494:pharmacology; D008716:Methionine Adenosyltransferase / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D014166:Transferases / Q000378:metabolism*
chemical_list: D000266:Adenylyl Imidodiphosphate; D008345:Manganese; D014166:Transferases; D008716:Methionine Adenosyltransferase