Copper-containing nitrite reductase (CuNIR) catalyzes the reduction of nitrite (NO(-)2)to nitric oxide (NO) during denitrification. We determined the crystal structures of CuNIR from thermophilic gram-positive bacterium, Geobacillus thermodenitrificans (GtNIR) in chloride- and formate-bound forms of wild type at 1.15 A resolution and the nitrite-bound form of the C135A mutant at 1.90 A resolution. The structure of C135A with nitrite displays a unique eta(1)-O coordination mode of nitrite at the catalytic copper site (T2Cu), which has never been observed at the T2Cu site in known wild-type CuNIRs, because the mobility of two residues essential to catalytic activity, Asp98 and His244, are sterically restricted in GtNIR by Phe109 on a characteristic loop structure that is found above Asp98 and by an unusually short CH-O hydrogen bond observed between His244 and water, respectively. A detailed comparison of the WT structure with the nitrite-bound C135A structure implies the replacement of hydrogen-bond networks around His244 and predicts the flow path of protons consumed by nitrite reduction. On the basis of these observations, the reaction mechanism of GtNIR through the eta(1)-O coordination manner is proposed.