At lower substrate concentrations, sigmoid kinetics typically produces slower rates than the corresponding hyperbolic form.
Our size exclusion chromatography experiments under conditions of pH 7.5 and 11.0 gave same molecular weight, thus excluding oligomerization as the potential basis for sigmoid kinetics. Furthermore, CD spectroscopy data indicates the absence of any major structural changes.
The possibility of a second substrate molecule binding to the acceptor amine binding site is ruled out as sigmoid kinetics was evident in the transpeptidation reactions also (data to be published).