ON THE 2 IRON CENTERS OF DESULFOFERRODOXIN

Desulfoferrodoxin from Desulfovibrio vulgaris, strain Hildenborough, i s 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 iro n centers, Fe-A, has previously been described as a 'strained rubredox in-like' site, EPR of the ferric form proves very similar to that of t he pentagonal bipyramidally coordinated iron in ferric complexes of DT PA, diethylenetriaminepentaacetic acid: both systems have spin S=5/2 a nd 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 a n unusually sharp S=2 resonance in parallel-mode EPR. The second iron, Fe-B, has S=5/2 and E/D=0.33; upon reduction (E(m,7.5)=+90 mV) FeB tu rns EPR-silent.