Understanding a chemical reaction from kegg I came across a reaction that looks as follows: 
  
In this reaction the polyphosphate is common in both the sides of the reaction. Does this act as enzyme? But from the website it seems that the enzyme is ADP-specific glucokinase(in the enzyme section click 2.7.1.63). Then what is the function of polyphosphate here?
The $n$ of the polyphosphate should be decreased by 1 on the reactant side relative to the product side:

$\ce{(P)_{n} + Glc -&gt; G 6P + (P)_{n-1} }$

Polyphosphate, like ATP, contains several phosphoanhydride bonds.  These are "high energy" bonds that can be used for biosynthesis.  But to do so one of these bonds needs to be broken, and that is exactly what happens when one of the end phosphates on the phosphoanhydride chain is peeled off and trasferred to glucose.