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Enzymes are usually proteins of high molecular weight (15,000 < MW < several million Daltons) that act as catalysts. Recently, it has been shown that some RNA molecules are also catalytic, but the vast majority of cellular reactions are mediated by protein catalysts. RNA molecules that have catalytic properties are sometimes called ribozymes or RNA enzymes. Enzymes are specific, versatile, and very effective biological catalysts, resulting in much higher reaction rates as compared to chemically catalyzed reactions under ambient conditions. More than 2000 enzymes are known. Enzymes are named by adding the suffix-ase to the end of the substrate, such as urease, or the reaction catalyzed, such as alcohol dehydrogenase. Some enzymes have a simple structure, such as a folded polypeptide chain (typical of most hydrolytic enzymes). Many enzymes have more than one subunit. Some protein enzymes requirenonprotein group for their activity. This group is either a cofactor, such as metal ions Mg, Zn, Mn, Fe, or a coenzyme such as a complex organic molecule, NAD, FAD, CoA, and some vitamins. An enzyme containing a non-protein group is called a holoenzyme. The protein part of this enzyme is the apoenzyme (holoenzyme = apoenzyme + co-factor). Enzymes that occur in several different molecular forms, but catalyze the same reaction, are called isozymes. Some enzymes are grouped together to form enzyme complexes. Enzymes are substrate specific and are classified according to the reaction they catalyze. Major classes of enzymes and their functions are listed in Table-1.