An NMR study of the 7Fe-8S ferredoxin from Rhodopseudomonas palustris and reinterpretation of data on similar systems

The oxidized 7Fe-8S ferredoxin from Rhodopseudomonas palustris is shown to possess a unique H-1 NMR spectrum displaying at least one hyperfine-shifted beta-CH2 signal for each cysteine bound to the [3Fe-4S] cluster. COSY and TOCSY spectra and 1- and 2-dimensional NOE experiments, in conjunction with a thorough reexamination of the H-1 NMR data on similar systems, permitted the sequential assignment of all of the cysteine beta-CH2 protons even in the absence of the amino acid sequence. The sequential assignment stems on the homology of the hyperfine shift pattern with those of other sequenced 7Fe-8S ferredoxins, which points to a substantial homology in tertiary structure. From the assignment, an analysis of the antiferromagnetic coupling in the [3Fe-4S] system was performed on the basis of a general model of exchange coupling. The NMR signal patterns of [3Fe-4S] clusters in both 3Fe-4S and 7Fe-8S ferredoxins have been discussed, and some correlations are proposed between signal patterns and the primary sequence.