Abstract
Dark-operative protochlorophyllide oxidoreductase (DPOR) is a nitrogenase-like enzyme catalyzing the reduction of rotochlorophyllide D-ring to form chlorophyllide. DPOR consists of two separable components, L-protein and NB-protein, which are structurally related to Fe-protein and MoFe-protein of nitrogenase, respectively. Here we show structural models for L-protein and NB-protein from Rhodobacter capsulatus based on analyses of molecular mass and Fe-S centers. The elution profiles of the two components in gel filtration chromatography indicated that L and NB-protein are a homodimer [(BchL)2] and a heterotetramer [(BchN)2(BchB)2], respectively. L-protein semi-purified with 6xHN affinity tag showed an EPR signal with g-values of 1.95 and 1.86, suggesting that L-protein carries a [4Fe-4S] cluster similar to Fe-protein. In contrast, semi-purified NB-protein showed complex EPR signals with g-values of 2.03, 1.94 and 1.92, which suggest the presence of [4Fe-4S] cluster(s) different from MoFe-protein. Structural similarity between DPOR and nitrogenase will be discussed.
