Thioesterases are enzymes which belong to the esterase family. Esterases, in turn, are one type of the several hydrolases known. Thioesterases are enzymes which belong to the esterase family. Esterases, in turn, are one type of the several hydrolases known. Thioesterases exhibit Esterase activity (splitting of an ester into acid and alcohol, in the presence of water) specifically at a thiol group.