Mass spectrometric studies of a modified active-site tetrapeptide from delta 5-3-ketosteroid isomerase of Pseudomonas testosteroni.

Examination of the product of affinity labeling of delta 5-3-ketosteroid isomerase (EC 5.3.3.1) of Pseudomonas testosteroni by the suicide substrate [7-3H]5,10-secoestr-5-yne-3,10,17-trione has demonstrated that the steroid becomes bound by an acid- and base-labile linkage to an active-site peptide representing residues 55-58 (H2N-Tyr-Ala-Asn-Ser-CO2H) of the primary structure (Penning, T. M., and Talalay, P. (1981) J. Biol. Chem. 256, 6851-6858). Upon release of the steroid by mild acid hydrolysis, the peptide is converted into a more basic structure while retaining its amino acid composition (as determined by conventional means). These findings were rationalized by postulating that the steroid is bound as an imido ester via the amide group of asparagine 57 and that the polypeptide backbone participates (via attack by nitrogen or oxygen) in the hydrolysis of this ester with the formation of a cyclic amidine or basic oxazine. By comparing the properties of the isolated tetrapeptide, from which the steroid has been removed, with those of synthetic H2N-Tyr-Ala-Asn-Ser-CO2H and H2N-Tyr-Ala-Asp-Ser-CO2H by electron impact and fast atom bombardment mass spectrometry, we now have evidence for the presence of an oxazine (5,6-dihydro-6-iminio-4H-1,3-oxazine) in the modified peptide. Our results draw attention to the hitherto unsuspected degree of nucleophilicity of the amide group of the side chain of asparagine and the participation of this group in the formation of an imido ester.