Is there a tool to find the action of an enzyme in a metabolic pathway?
I think the KEGG pathway database may be of some use to you. 

Link is here: http://www.genome.jp/kegg/pathway.html

This a database of manually drawn pathway maps, I have used the site myself and it is very useful in determining if your enzyme is in a pathway and where it lies in it. This is assuming though, your enzyme is in the database and has been mapped.