Enzyme Classifications Traditionally, enzymes were simply assigned names by the investigator who discovered the enzyme. As knowledge expanded, systems of enzyme classification became more comprehensive and complex. Currently enzymes are grouped into six functional classes by the International Union of Biochemists (I.U.B.). Number Classification Biochemical Properties 1 Oxidoreductases Act on many chemical groupings to add or remove hydrogen atoms. 2 Transferases Transfer functional groups between donor and acceptor molecules. Kinases are specialized transferases that regulate metabolism by transferring phosphate from ATP to other molecules. 3 Hydrolases Add water across a bond, hydrolyzing it. 4 Lyases Add water, ammonia or carbon dioxide across double bonds, or remove these elements to produce double bonds. 5 Isomerases Carry out many kinds of isomerization: L to D isomerizations, mutase reactions (shifts of chemical groups) and others. 6 Ligases Catalyze reactions in which two chemical groups are joined (or ligated) with the use of energy from ATP.