DNA polymerases facilitate faithful insertion of nucleotides, a central reaction occurring during DNA replication and repair. DNA synthesis (forward reaction) is "balanced," as dictated by the chemical equilibrium by the reverse reaction of pyrophosphorolysis. Two closely spaced divalent metal ions (catalytic and nucleotide-binding metals) provide the scaffold for these reactions. The catalytic metal lowers the pK a of O3′ of the growing primer terminus, and the nucleotide-binding metal facilitates substrate binding. Recent time-lapse crystallographic studies of DNA polymerases have identified an additional metal ion (product metal) associated with pyrophosphate formation, leading to the suggestion of its possible involvement in the reverse reaction. Here, we establish a rationale for a role of the product metal using quantum mechanical/molecular mechanical calculations of the reverse reaction in the confines of the DNA polymerase β active site. Additionally, site-directed mutagenesis identifies essential residues and metal-binding sites necessary for pyrophosphorolysis. The results indicate that the catalytic metal site must be occupied by a magnesium ion for pyrophosphorolysis to occur. Critically, the product metal site is occupied by a magnesium ion early in the pyrophosphorolysis reaction path but must be removed later. The proposed dynamic nature of the active site metal ions is consistent with crystallographic structures. The transition barrier for pyrophosphorolysis was estimated to be significantly higher than that for the forward reaction, consistent with kinetic activity measurements of the respective reactions. These observations provide a framework to understand how ions and active site changes could modulate the internal chemical equilibrium of a reaction that is central to genome stability.NA polymerases are responsible for high-fidelity DNA synthesis during replication and repair of the genome (1). Although there are at least 17 human DNA polymerases, they all use a general nucleotidyl transferase DNA synthesis reaction. This reaction requires deoxynucleoside triphosphates (dNTPs), divalent metal ions, and DNA substrate with a primer 3′-OH annealed to a coding template strand. An inline nucleophilic attack of the primer 3′-oxyanion on P α of the incoming dNTP results in products with DNA extended by one nucleotide [i.e., deoxynucleoside monophosphate (dNMP)] and pyrophosphate (PP i ). If the enzyme does not release PP i , pyrophosphorolysis (reverse reaction) can generate dNTP and a DNA strand that is one nucleotide shorter (Fig. 1) (2).Although the forward DNA synthesis reaction is preferred, the pyrophosphorolysis reaction can be biologically significant. Because DNA polymerases are an attractive chemotherapeutic target, chainterminating nucleoside drugs are often used in a strategy of blocking DNA synthesis (3-5). However, drug resistance to chain-terminating agents is influenced by the ability of a stalled DNA polymerase to remove chain-terminating nucleotides through pyrophosphorolysis (6-8)...