Polyphosphate binding and chain length recognition of Escherichia coli exopolyphosphatase

Exopolyphosphatase of Escherichia coli (PPX) is a highly processive enzyme demonstrating the ability to recognize polyphosphates of specific lengths. The mechanisms responsible for the processivity and polymer length recognit ion of the enzyme were investigated in relation to the manner in which poly phosphate is bound to the enzyme. Multiple polyphosphate binding sites were identified on distant portions of the enzyme and were determined to be res ponsible for the polymer length recognition of the enzyme. In addition, two independently folded domains were identified. The N-terminal domain contai ned a quasi-processive polyphosphatase active site belonging to the sugar k inase/actin/hsp70 superfamily. The C-terminal domain contained a single pol yphosphate binding site and was responsible for nearly all of the PPX affin ity for polyphosphate. This domain was also found to confer a highly proces sive mode of action to PPX. Collectively; these results cc-ere used to desc ribe the interaction of polyphosphate with PPX.