Serine proteases are enzymes that cleave peptide bonds in proteins, in which serine serves as the nucleophilic amino acidat the active site. They are found ubiquitously in both eukaryotes and prokaryotes. Serine proteases fall into two broad categories based on their structure: chymotrypsin-like or subtilisin-like. In humans, serine proteases are responsible for co-ordinating various physiological functions, including digestion, immune response, blood coagulation and reproduction. Threnonine proteases are a family of proteolytic enzymes harbouring a threonine (Thr) residue within the active site. The prototype members of this class of enzymes are the catalytic subunits of the proteasome, however the acyltransferases convergently evolved the same active site geometry andmechanism.