Hydrogen bonds in rubredoxins from mesophilic and hyperthermophilic organisms

The extent and strength of the hydrogen bond networks in rubredoxins from the hyperthermophile Pyrococcus furiosus (PfRd), and its mesophilic analogue Clostridium pasteurianum (CpRd), are examined and compared using NMR spectroscopy. NMR parameters examined in this study include through-hydrogen bond (h3)J(NC)(') scalar couplings and (1)H, (13)C, and (15)N chemical shifts, as well as covalent (1)J(NH) and (1)J(NC)(') scalar couplings. These parameters have allowed the characterization in solution of 12 hydrogen bonds in each protein. Despite a 83% sequence homology and a low RMSD for the backbone heavy atoms (0.648 A) in the crystalline state, subtle, but definite, changes have been identified in the detailed hydrogen-bonding patterns. CpRd shows an increased number of hydrogen bonds in the triple-stranded beta-sheet and an additional hydrogen bond in the multiple-turn segment including residues 14-32. On the other hand, PfRd exhibits an overall strengthening of N-H...O=C hydrogen bonds in the loops involved at the metal binding site as well as evidence for an additional NH...S(Cys) hydrogen bond involving the alanine residue 44. These data, as well as temperature dependence of the NMR parameters, suggest that the particular NMR hydrogen bond pattern found in the hyperthermophile rubredoxin leads to an increased stabilization at the metal binding pocket. It seems to result from a subtle redistribution of hydrogen-bonding interactions between the triple-stranded beta-sheet and the actual metal binding site.