| Abstract: | Inorganic pyrophosphate inhibits the aminoacylation of tRNALeu by the leucyl-tRNA synthetase from Neurospora crassa giving very low Kapp.i, PPi values of 3-20 microM. The inhibition by pyrophosphate, together with earlier kinetic data, suggest a reaction mechanism where leucine, ATP and tRNA are bound to the enzyme in almost random order, and pyrophosphate is dissociated before the rate-limiting step. A kinetic analysis of this mechanism shows that the measured Kapp.i values do not give the real dissociation constant but it is about 0.4 mM. Other dissociation constants are 90 microM for leucine, 2.2 mM for ATP and 1 microM for tRNALeu. At the approximate conditions of the living cell (2 mM ATP, 100 microM leucine and 150 microM PPi) the leucyl-tRNA synthetase is about 85% inhibited by pyrophosphate. |
