The first crystal structures of the 11-haem cytochrome from Shewanella sp. strain HRCR-6 have been solved , both ligand-free  and in complex with iron chelates Fe(III)-citrate  and Fe(III)-nitrilotriacetate . The authors propose that
the region around heme 7 could be a rudimentary active site for association of soluble organic redox partners, which would be consistent with the UndA functioning as an enzyme with broad, but differential, specificity to a variety of substrates, in contrast to a nonspecific cathode on the cell surface <such as decaheme cytochrome MtrF>.
- Edwards, M.J., Hall, A., Shi, L., Fredrickson, J.K., Zachara, J.M., Butt, J.N., Richardson, D.J. and Clarke, T.A. (2012) The crystal structure of the extracellular 11-heme cytochrome UndA reveals a conserved 10-heme motif and defined binding site for soluble iron chelates. Structure 20, 1275—1284.