Scientists have long investigated the importance of metals in biology. Its availability has evolutionarily driven its use in biological systems such as the active site of over fifty enzymes. One of the enzymes is xanthine dehydrogenase (XOR) and has many varied roles in purine catabolism, drug metabolism, and oxidative stress response. Moco deficiency causes a severe disease in humans that usually results in premature death in early childhood and is inherited as an autosomal recessive trait. Mutations in the genes encoding the proteins involved in Moco biosynthesis lead to loss of activity of the enzymes requiring this cofactor in humans. Determination of reaction mechanism for the hydroxylation of heterocyclic substrates (Xanthine, Hypoxanthine and Allopurinol) by Oxidoreductase Enzyme using computational software packages has a great importance for the Science societies and as the whole the world. Now a day theoretical chemistry is the growing science. The Transition structure, Mulliken atomic Charge, bond distance, energy of the enzyme bound to the heterocyclic substrates were determined using the computational software package (Gaussian 03 and Gaussian view etc).