Normal Mode Analysis of Pyrococcus furiosus Rubredoxin via Nuclear Resonance Vibrational Spectroscopy (NRVS) and Resonance Raman Spectroscopy

We have used 57Fe nuclear resonance vibrational spectroscopy (NRVS) to study the Fe(Scys)4 site in reduced and oxidized rubredoxin (Rd) from Pyrococcus furiosus (Pf). The oxidized form has also been investigated by resonance Raman spectroscopy. In the oxidized Rd NRVS, strong asymmetric Fe−S stretching modes are observed between 355 and 375 cm-1; upon reduction these modes shift to 300−320 cm-1. This is the first observation of Fe−S stretching modes in a reduced Rd. The peak in S−Fe−S bend mode intensity is at ∼150 cm-1 for the oxidized protein and only slightly lower in the reduced case. A third band occurs near 70 cm-1 for both samples; this is assigned primarily as a collective motion of entire cysteine residues with respect to the central Fe. The 57Fe partial vibrational density of states (PVDOS) were interpreted by normal mode analysis with optimization of Urey−Bradley force fields. The three main bands were qualitatively reproduced using a D 2 d Fe(SC)4 model. A C1 Fe(SCC)4 model based on crystallographic coordinates was then used to simulate the splitting of the asymmetric stretching band into at least 3 components. Finally, a model employing complete cysteines and 2 additional neighboring atoms was used to reproduce the detailed structure of the PVDOS in the Fe−S stretch region. These results confirm the delocalization of the dynamic properties of the redox-active Fe site. Depending on the molecular model employed, the force constant K Fe - S for Fe−S stretching modes ranged from 1.24 to 1.32 mdyn/Å. K Fe - S is clearly diminished in reduced Rd; values from ∼0.89 to 1.00 mdyn/Å were derived from different models. In contrast, in the final models the force constants for S−Fe−S bending motion, H S - Fe - S, were 0.18 mdyn/Å for oxidized Rd and 0.15 mdyn/Å for reduced Rd. The NRVS technique demonstrates great promise for the observation and quantitative interpretation of the dynamical properties of Fe−S proteins.