Gramicidin S Synthetase. Stability of Reactive Thioester Intermediates and Formation of 3-Amino-2-piperidone

The reactive thioester complexes of gramicidin S synthetase with substrate amino acids and intermediate peptides are slowly hydrolyzed in neutral buffer solutions under mild conditions. Fully active enzyme is recovered. These processes are strongly accelerated by certain thiol protective agents. In the presence of 1 mM dithioerythritol the half-life times of these hydrolysis reactions are in the range of 1-90 h at 3 degrees C. The thioester complex of gramicidin S synthetase 2 (GS2, the heavy enzyme) with the tripeptide DPhe-Pro-Val is distinguished by the highest stability of all these intermediates. A different decomposition pattern is observed for the thioester complex of GS2 with LOrn. Here 3-amino-2-piperidone (cyclo-LOrn) is formed in a rapid cyclization reaction. This product specifically blocks the activation center of GS2 for LOrn at the thioester binding site. All other activation reactions of gramicidin S synthetase are unaffected. A procedure for a specific labelling of the reaction centers of the multienzyme is outlined.