Use of Β3-Methionine As an Amino Acid Substrate of Escherichia Coli Methionyl-Trna Synthetase.

Polypeptides containing beta-amino acids are attractive tools for the design of novel proteins having unique properties of medical or industrial interest. Incorporation of beta-amino acids in vivo requires the development of efficient aminoacyl-tRNA synthetases specific of these non-canonical amino acids. Here, we have performed a detailed structural and biochemical study of the recognition and use of beta(3)-Met by Escherichia coli methionyl-tRNA synthetase (MetRS). We show that MetRS binds beta(3)-Met with a 24-fold lower affinity but catalyzes the esterification of the non-canonical amino acid onto tRNA with a rate lowered by three orders of magnitude. Accurate measurements of the catalytic parameters required careful consideration of the presence of contaminating alpha-Met in beta(3)-Met commercial samples. The 1.45 angstrom crystal structure of the MetRS: beta(3)-Met complex shows that beta(3)-Met binds the enzyme essentially like alpha-Met, but the carboxylate moiety is mobile and not adequately positioned to react with ATP for aminoacyl adenylate formation. This study provides structural and biochemical bases for engineering MetRS with improved beta(3)-Met aminoacylation capabilities.