Role of lysine-195 in the KMSKS sequence of E. coli tryptophanyl-tRNA synthetase.

Lysine 195 in the K195 MSKS sequence of E. coli tryptophanyl-tRNA synthetase (TrpRS) was replaced with alanine. The resulting K195A mutant TrpRS had essentially unchanged Km values for ATP and Trp, but a 1500-fold decreased kcat in a pyrophosphate-ATP exchange reaction. This large decrease in kcat reduces the rate of aminoacyladenylate formation (step 1) to a rate comparable to the rate of aminoacylation of tRNA(Trp) (step 2) by the K195A mutant enzyme. Both the TIGN and KMSKS sequences are important for step 1 of class I aminoacyl-tRNA synthetase reactions.