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Proposed molecular mechanism in both directions of the reaction for hAPRT. A, in the forward reaction, SN2-type molecular mechanism including some SN1 characteristics. B, in the reverse reaction, a proposed magnesium-assisted SN2-type mechanism. For clarity, the nucleotide is shown in a syn configuration whereas it is always found in an anti configuration in the hAPRT active site. Sugar puckering configurations are indicated in blue next to the sugar moieties.
