Removal of Pyroglutamic Acid Residues from the N-Terminus of Peptides and Proteins

Abstract

In both peptides and proteins, N-terminal glutamine residues can readily cyclize to the pyroglutamyl derivative (Fig. 1). This can occur during peptide and protein purification (it is uncertain whether the N-terminal pyroglutamyl residues of a number of naturally occurring peptides and proteins are genuine posttranslational modifications or were introduced by cyclization of N-terminal glutamine during purification). This cyclized derivative does not have a free amino group, and therefore, the peptide or protein is not amenable to sequence determination, unless the pyroglutamyl derivative is removed. This can be achieved by using the enzyme pyroglutamate aminopeptidase, a thiol exoprotease that cleaves N-terminal pyroglutamyl residues (pyrrolidone carboxylic acid) from peptides and proteins (1–5). The enzyme was first purified from Pseudomonas fluoresens (6), but nowadays, the calf liver enzyme is used, and it is this enzyme that we describe here.