The electronic structure of a [4Fe-4S] cluster, so-called NB cluster [Fe4S4(SCH3)(3)(CH3COO)], found in dark-operative protochlorophyllide oxidoreductase (DPOR) has been studied using the density functional theory. The results show that this NB cluster can function as a better electron-donor than the conventional [4Fe-4S] cluster, [Fe4S4(SCH3)(4)], in the 2-/3- reduction in an environment with the dielectric constant higher than 10. This redox character of the NB cluster accounts for why Asp36 is essential in DPOR. It is also suggested that the unknown coordinating ligand of the [4Fe-4S] cluster in the D36A mutant of DPOR is a chloride ion. (C) 2011 Elsevier B.V. All rights reserved.