Sulfur (S), as the second element in the main group 6A just below oxygen (O), has been often used as an isosteric
replacement for O in enzymatic mechanistic studies. In addition, S has also been used as an isosteric replacement for
CH2. These S-based mechanistic probes have been used in the studies with protein enzyme systems such as the sirtuin
family of the protein Nε-acyl-lysine deacylases, phosphotransferases, and fatty acid desaturase, as well as various RNA
enzymes (ribozymes). These probes are basically the O→S mutants of the corresponding O(or CH2)-containing substrates
for the enzymatic reactions under study. This article will review the significant contributions that the S-based probes have
been able to make toward an enhanced mechanistic understanding of different types of the enzyme-catalyzed reactions.