Phenylalanyl-tRNA synthetases from cytoplasm and mitochondria of yeast and hen liver: comparison of their structural and catalytic properties.

Amino acid compositions and tryptic maps of the cytoplasmic and mitochondrial phenylalanyl-tRNA synthetases from yeast and hen liver, respectively, demonstrate the similarity of these enzymes, although they are clearly not identical. Moreover, similarity is noted for catalytic properties like stoichiometries of complex formation with tRNAPhe and negative cooperativity of tRNAPhe binding, triggered by substrates. Analysis of the kinetics at saturating and subsaturating substrate concentrations indicates the contribution of the transfer of phenylalanine from the adenylate to tRNAPhe to the rate-determining step in aminoacylation and subunit interactions in the tetrameric enzymes. Furthermore, the locations of substrate-binding sites appear rather constant within species and in interspecies comparison. Subtle differences at certain sites, although homology exists, are exemplified by a special regulatory effect on activity by other amino acids only in the case of the cytoplasmic enzyme from hen liver. The results support the idea of a common ancestry by gene duplication for the cytoplasmic and mitochondrial phenylalanyl-tRNA synthetases in fungi and animals, respectively.