ATP‐mediated amino acid recognition by GlnRS

Abstract

Glutaminyl-tRNA synthetase has been a leading model for the study of structure-function correlations in class I tRNA synthetases. Recognition of the correct amino acid and tRNA by these enzymes is critical for the fidelity of the genetic code. The active site assembly of the quaternary and ternary complexes of GlnRS bound to tRNAGln, ATP and glutamine, or the aminoacyl-adenylate analog, QSI, revealed a unique interaction between ATP and glutamine distal to the reactive α-phosphate and carboxylate moieties: the amide oxygen of glutamine accepts a direct hydrogen bond from the 3′OH group of the ribosyl moiety. Interestingly, the quaternary structure of GlnRS bound to noncognate glutamate shows that the analogous hydrogen bond is not formed. Indeed, examination of all tRNA synthetase crystal structures shows that such a distal contact between ATP and the amino acid R-group is unique to GlnRS. To elucidate the importance of this interaction two approaches were taken: kinetic analysis using an ATP analog, 3′dATP, and crystallization of GlnRS bound to the noncognate aminoacyl-adenylate analog, glutamol-AMP. The kinetic findings demonstrate the importance of the unique hydrogen bond: while the Km of 3′-dATP for GlnRS is unaltered, kcat/Km is decreased by 103fold, and the affinity for glutamine is weakened by 20-fold. The crystal structure of the noncognate ternary complex confirms that the distal interaction is unique to glutamine binding. Funded by NIH.