Arginyl-tRNA Synthetase fromEscherichia coliK12: Specificity with Regard to ATP Analogs and their Magnesium Complexes

Fifteen analogs of ATP have been tested in the ATP/PPi pyrophosphate exchange and the aminoacylation of arginyl-tRNA synthetase from E. coli K12. Six compounds are substrates in both reactions, whereas seven of the triphosphates were inhibitors for both reactions. The Km, V and Ki values have been determined. The enzyme is less specific against base modifications of the ATP molecule than arginyl tRNA synthetase from baker's yeast in the aminoacylation and is inhibited by more base-modified compounds in the ATP/PPi exchange. The enzyme accepts 3'-deoxy-ATP as substrate and is inhibited by 2'-methoxy-ATP, whereas the reversed observation is made for the yeast arginyl-tRNA synthetase. The stoichiometry and association constants of complexes formed by six ATP analogs (four substrates and two inhibitors) and magnesium ions were investigated; five analogs form 1:1 complexes, one analog (3'-deoxy-ATP) binds two magnesium ions. The enzyme must accept different complexes formed with one or two magnesium ions as substrate, which must be different in structure from complexes proposed in literature.