Monoclinic crystal structure of C-terminal desundecapeptide nitrite reductase (NiRc-11) from Achromobacter cycloclastes was determined at 2.6 A. NiRc-11 exists as a loose trimer in the crystal. Deletion of 11 residues eliminates all intersubunit hydrogen bonds mediated by the C-terminal tail. The rigid irregular coil 105-112, which constitutes part of the sidewall of the active site pocket, undergoes conformational changes and becomes highly flexible in NiRc-11. Correspondingly, the linker segments between the two copper sites 95-100 and 135-136 are partly relaxed in conformation, which leads to disrupted active site microenvironments responsible for the activity loss and spectral change of NiRc-11. Comparison with the native structure revealed a bulky residue Met331 fastened by hydrogen bonding, which may play a direct role in keeping the right copper site geometry by protruding its side chain against the irregular coil 105-112. Sequence alignment showed that the bulky residue is conserved at position 331, indicating an equal importance of C-terminal segment in other copper-containing nitrite reductases. (c) 2005 Elsevier Inc. All rights reserved.