The two metals present in the active site serve specific purposes . The ions bound to the so-called A-and B-sites (Figure ) play crucial roles, including polarizing the 3'OH, resulting in lowering of its pKa, which facilitates its deprotonation and subsequent nucleophilic attack on the -phosphate of the incoming nucleotide. The metals are also responsible for neutralizing the developing negative charge brought about by the nucleotidyl transfer reaction. This B site metal is co-ordinated to the -, βand -nonbridging phosphate oxygens of the incoming nucleotide and helps in removal of PPi after the reaction has occurred.