Overall mechanism and rate equation for O-acetylserine sulfhydrylase

Saturation curves for O-acetylserine sulfhydrylase using the substrate analogues O-propionyl-L-serine, O-butyryl-L-serine, and beta-chloro-L-alanine all exhibit substrate inhibition and yield Km values comparable to O-acetyl-L-serine, except the O-butyryl derivative which has a Km 5-fold higher. Since all analogues are used as substrates and yield similar kinetic parameters in most cases, it is possible that they share a common intermediate. This evidence also suggests that specificity of O-acetylserine sulfhydrylase resides in the fact that the beta-substituted moiety on L-serine is a good leaving group. The overall rate equation for O-acetylserine sulfhydrylase was derived. A comparison of the numerical integration of the rate equation and an experimental time course is given.