Phenylalanyl-tRNA synthetase from yeast and its discrimination of 19 amino acids in aminoacylation of tRNA(Phe)-C-C-A and tRNA(Phe)-C-C-A(3'NH2)

For discrimination between phenylalanine and 18 other naturally occurring non-cognate amino acids by the class II aminoacyl-tRNA synthetase specific for phenylalanine, discrimination factors, D, of 190-6300 have been determined from k(cat) and K-m values. Generally, phenylalanyl-tRNA synthetase is more specific than the class II enzymes specific for Lys and Thr, but works with lower accuracy than the class I enzymes specific for Ile, Tyr, and Arg. In aminoacylation of tRNA(Phe)-C-C-A(3'NH2) discrimination factors D-1 vary between 80-1610. Pre-transfer proof-reading factors II1 are in the range 2.3-74, post-transfer proof-reading factors II2 in the range 1.0-4.6, showing that pre-transfer proof-reading is the main correction step, post-transfer proofreading is less effective or negligible. Initial discrimination factors (I-1 and I-2 caused by differences in Gibbs free energies of binding between phenylalanine and non-cognate amino acids have been calculated assuming a two-step binding process. Factors I-1 can be related to hydrophobic-interaction farces depending on accessible surface areas of the amino acids, factors I-2 scatter about a low mean value and do not show any relation to amino acid structures or surfaces, indicating less checking of amino acid side chains in the putative second binding step.