Reduction of the four Fe centers is not required to initiate the reaction of the Halomonas halodenitrificans nitric oxide reductase (NOR) based on the facts that NOR in the form that ferric heme b(3) and non-heme iron (Fell) are not bridged and/or the interaction between them is weakened and reversibly binds NO molecules, and that NOR in the form that only heme b(3) is oxidized reacts with NO molecules. (c) 2005 Elsevier Inc. All rights reserved.