Dark-operative protochlorophyllide oxidoreductase, a nitrogenase-like enzyme, contains two [4Fe-4S] clusters, one in the L-protein ((BchL) 2) and the other in the NB-protein ((BchN-BchB)2). The reduced NB-cluster in the NB-protein, which is ligated by 1Asp/3Cys residues, showed a broad S = 3/2 electron paramagnetic resonance signal that is rather rare in [4Fe-4S] clusters. A 4Cys-ligated NB-cluster in the mutated variant BchB-D36C protein, in which the Asp36 was replaced by a Cys, gave a rhombic normal S = 1/2 signal and lost the catalytic activity. The results suggest that Asp36 contributes to the low redox potential necessary to reduce protochlorophyllide.
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