Inhibition of O-acetylserine sulfhydrylase by fluoroalanine derivatives

O-acetylserine sulfhydrylase (OASS) is the pyridoxal 50-phosphate dependent enzyme that catalyses the
formation of L-cysteine in bacteria and plants. Its inactivation is pursued as a strategy for the identification
of novel antibiotics that, targeting dispensable proteins, holds a great promise for circumventing resistance
development. In the present study, we have investigated the reactivity of Salmonella enterica serovar
Typhimurium OASS-A and OASS-B isozymes with fluoroalanine derivatives. Monofluoroalanine reacts with
OASS-A and OASS-B forming either a stable or a metastable a-aminoacrylate Schiff’s base, respectively, as
proved by spectral changes. This finding indicates that monofluoroalanine is a substrate analogue, as previously
found for other beta-halogenalanine derivatives. Trifluoroalanine caused different and timedependent
absorbance and fluorescence spectral changes for the two isozymes and is associated with
irreversible inhibition. The time course of enzyme inactivation was found to be characterised by a biphasic
behaviour. Partially distinct inactivation mechanisms for OASS-A and OASS-B are proposed.