On the two iron centers of desulfoferrodoxin

Abstract

Desulfoferrodoxin from Desulfovibrio vulgaris, strain Hildenborough, is a homodimer of 28 kDa; it contains two Fe atoms per 14.0 kDa subunit. The N-terminal amino-acid sequence is homogeneous and corresponds to the previously described Rbo gene, which encodes a highly charged 14 kDa polypeptide without a leader sequence. Although one of the two iron centers, Fe_A, has previously been described as a ‘strained rubredoxin-like’ site, EPR of the ferric form proves very similar to that of the pentagonal bipyramidally coordinated iron in ferric complexes of DTPA, diethylenetriaminepentaacetic acid: both systems have spin S = and rhombicity E/D = 0.08. Unlike the Fe site in rubredoxin the Fe_A site in desulfoferrodoxin has a pH dependent midpoint potential with pK _ox= 9.2 and pK _red = 5.3. Upon reduction (E _m,7.5 = +2 mV) Fe_A exhibits an unusually sharp S = 2 resonance in parallel-mode EPR. The second iron, Fe_B, has S = and E/D = 0.33; upon reduction (E _m,7.5 = +90 mV) Fe_B turns EPR-silent.