Figure 9. Mechanism for the nucleotide addition cycle.

A kinetic scheme that is applicable to nucleic acids polymerases is shown. The first step involves binding of the substrate NTP to the enzyme in complex with RNA at the n^th stage of elongation i.e. the daughter chain is n nucleotides long. The second step involves a conformation change (shown by the red arrows) of the ERn•NTP complex into a “closed” state that is capable of catalysis. The third step is the chemistry step (shown by the blue arrows) extending the daughter chain by one nucleotide (n+1). The fourth step (shown by the red arrows) is a second conformational change that enables the release of the pyrophosphate product. The fifth and the final step involves release of pyrophosphate that is likely coupled to movement of the template forward in register to prime it for the next nucleotidyl-transfer reaction.