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 recognition of the enzyme were investigated in relation to the manner in which polyphosphate is bound to the enzyme. Multiple polyphosphate binding sites were identified on distant portions of the enzyme and were determined to be responsible for the polymer length recognition of the enzyme. In addition, two independently folded domains were identified. The N-terminal domain contained a quasi-processive polyphosphatase active site belonging to the sugar kinase/actin/hsp70 superfamily. The C-terminal domain contained a single polyphosphate binding site and was responsible for nearly all of the PPX affinity for polyphosphate. This domain was also found to confer a highly processive mode of action to PPX. Collectively, these results were used to describe the interaction of polyphosphate with PPX.