Density functional theory calculations which include the enzyme environment show that NO can bind to the copper atom of reduced nitrite reductase in both side-on and end-on conformations with the end-on motif being somewhat preferred. The calculations do not predict the unusually long N-O distance found experimentally by Tocheva et al. [E.I. Tocheva, F.I. Rosell, A.G. Mauk, M.E.P. Murphy, Science 304 (2004) 867], but are in closer agreement with the structures of Antonyuk et al. [S.V. Antonyuk, R.W. Strange, G. Sawers, R.R. Eady, S.S. Hasnain, Proc. Natl. Acad. Sci. USA 102 (2005) 12041]. © 2005 Elsevier B.V. All rights reserved.