Figure 1.

Catalysis by PNP. Schematic diagrams showing the chemical reactions catalysed by PNP: (a) phosphorolysis, and (b) arsenolysis. These diagrams are based on arsenolytic/hydrolysis transition-state structures of PNPs [17,3,4]. Note protonation at N7, leading to formation of a positive charge, and glycosidic-bond cleavage resulting in formation of a ribo-oxocarbenium ion and negative charge in the purine ring at the transition state. This process occurs prior to nucleophilic attack as the catalytic reaction of PNP follows a SN1 type mechanism. (c) shows the transition state mimc inhibitor, Immucillin H (ImmH).