The mechanism of the elongation and branching reaction of Poly(ADP-ribose) polymerase as derived from crystal structures and mutagenesis

The binding site for the acceptor substrate poly(ADP-ribose) in the elongation reaction of the ADP-ribosyl transferase poly(ADP-ribose) polymerase (PARP) was detected by cocrystallizing the enzyme with an NAD + analogue. The site was confirmed by mutagenesis studies. In conjunction with the binding site of the donor NAD +, the bound acceptor reveals the geometry of the elongation reaction. It shows in particular that the strictly conserved glutamate residue of all ADP-ribosylating enzymes (Glu988 of PARP) facilitates the reaction by polarizing both, donor and acceptor. Moreover, the binding properties of the acceptor site suggest a mechanism for the branching reaction, that also explains the dual specificity of this transferase for elongation and branching, which is unique among polymer-forming enzymes.