MOLECULAR MODEL OF THE SOLUTION STRUCTURE FOR THE PARAMAGNETIC FOUR-IRON FERREDOXIN FROM THE HYPERTHERMOPHILIC ARCHAEON THERMOCOCCUS LITORALIS

: A molecular model for the three-dimensional solution structure of the paramagnetic, four-iron ferredoxin (Fd) from the hyperthermophilic archaeon Thermococcus litoralis (Tl) has been constructed on the basis of the reported 1H NMR spectral parameters [Donaire, A. (1996) J. Biomol. NMR 7, 35-47]. The conventional use of long mixing time NOESY cross-peak intensity, backbone angles, and hydrogenbonding constraints for building the structure was augmented by short mixing time NOESY, steady-state NOE, paramagnetic relaxation constraints, and the angular dependence of the ligated Cys H beta contact shifts. Distance geometry was used to generate various initial structures, and these structures were refined with the simulated annealing protocol. The family of structures with inconsequential violations exhibited low RMS deviations for the backbone except for a few residues in the immediate cluster vicinity and traces out a secondary structure very similar to those of the structurally characterized single cubane cluster Fds. The ability to describe the cluster environment depended on the use of numerous paramagnetic relaxation constraints which resulted in even the cluster loop residues exhibiting well-defined orientations, with the exception of one residue (Ilel1) whose 1H signals have not been located. Comparison of the structure of Tl Fd to those of mesophilic ferredoxins reveals that Tl Fd possesses the same secondary structural elements, two beta-sheets, two helices, and four turns, with the exception that the beta-sheet involving the termini incorporates a third strand in Tl Fd. Several minor structural adjustments in Tl Fd relative to other Fds, in addition to the third strand for beta-sheet, include the incorporation of the termini into the beta-sheet, a likely salt bridge from the side chain of the third beta-strand to the N-terminus, and a more hydrophobic and compact interaction between the large beta-sheet and the long helix. It is likely that each of these modifications, among others not yet well-defined (i.e., surface salt bridges), contributes to the extraordinary thermostability of Tl Fd.