A ribosomal-boand aminopeptidase in Escherichia coli B: substrate specificity

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Abstract
The substrate specificity of the ribosomal-bound aminopeptidase from Escherichia coli B has been studied using di-, tri-, and tetrapeptides. The enzyme shows strong activity to leucyl, methionyl, threonyl, and lysyl peptides. Of the other dipeptides tested considerable hydrolysis was observed only if the C-terminal amino acid was leucine or methionine. In a given series of peptides the rate of hydrolysis of the N-terminal peptide bond increased as the size of the peptide increased. Although leucyi dipeptides were hydroiyzed more rapidly than the corresponding methionyl dipeptide the reverse was true with the tripeptides tested. No carboxypeptidase activity was observed and peptides containing D-amino acids were not hydroiyzed. The substrate specificity of the aminopeptidase was compared with the known N-terminal sequences of E. coli proteins to determine whether the enzyme may be involved in the removal of N-formylmethionyl from newly synthesized polypeptides.

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