Structure of free fumarase C from Escherichia coli

Previous crystal structures of fumarase C from Escherichia coli have noted two occupied dicarboxylate-binding sites termed the active site and the B site. Here, the first known fumarase C structure is reported in which both sites are unoccupied by bound ligand. This so-called `free' crystal form shows conservation of the active-site water in a similar orientation to that reported in other fumarase C crystal structures. More importantly, a shift of His129 has been observed at the B site. This new crystallographic information suggests the use of water as a permanent member of the active site and the use of an imidazole-imidazolium conversion to control access at the allosteric B site.