Hyperfine-Shifted ~(13)C and ~(15)N NMR Signals from Clostridium pasteurianum Rubredoxin: Extensive Assignments and Quantum Chemical Verification

摘要

Stable isotope-labeling methods, coupled with novel techniques for detecting fast-relaxing NMR signals, now permit detailed investigations of paramagnetic centers of metalloproteins. We have utilized these advances to carry out comprehensive assignments of the hyperfine-shifted ~(13)C and ~(15)N signals of the rubredoxin from Clostridium pasteurianum (CpRd) in both its oxidized and reduced states. We used residue-specific labeling (by chemical synthesis) and residue-type-selective labeling (by biosynthesis) to assign signals detected by one-dimensional ~(15)N NMR spectroscopy, to nitrogen atoms near the iron center. We refined and extended these ~(15)N assignments to the adjacent carbonyl carbons by means of one-dimensional ~(13)C['5N] decoupling difference experiments. We collected paramagnetic-optimized Su-perWEFT ~(13)C[~(13)C] constant time COSY (SW-CT-COSY) data to complete the assignment of ~(13)C signals of reduced CpRd. By following these ~(13)C signals as the protein was gradually oxidized, we transferred these assignments to carbons in the oxidized state. We have compared these assignments with hyperfine chemical shifts calculated from available X-ray structures of CpRd in its oxidized and reduced forms. The results allow the evaluation of the X-ray structural models as representative of the solution structure of the protein, and they provide a framework for future investigation of the active site of this protein. The methods developed here should be applicable to other proteins that contain a paramagnetic center with high spin and slow electron exchange.