Structural changes in cytochrome c upon hydrogen-deuterium exchange.
The resonance Raman spectra of yeast ferri- and ferro-iso-1-cytochrome c dissolved in H2O and D2O are reported. Hydrogen exchange in the protein leads to distinct spectral changes of heme vibrational bands, particularly in the region between 670 and 710 cm-1 and at approximately 443 and approximately 450 cm-1. The latter two bands, which have previously been assigned to porphyrin modes including bending vibrations of the propionate side chains [Hildebrandt, P. (1991) J. Mol. Struct. 242, 379-395], reveal frequency shifts by up to 4 cm-1. These shifts are attributed to structural changes of the propionate groups caused by the energetic differences of the hydrogen and deuterium bonds between these substituents and the adjacent amino acid residues. The frequency shifts of the bands between 670 and 710 cm-1 most likely reflect structural differences of the tetrapyrrole macrocycle itself. Time-dependent experiments revealed that the hydrogen exchange processes associated with the changes in the resonance Raman spectra are complete in less than 15 min. The protons which are involved are those in the interior of the heme pocket as concluded by comparison with the exchange rate constants previously determined by NMR spectroscopy [Mayne, L., Paterson, Y., Cerasoli, D., & Englander, S. W. (1992) Biochemistry 31, 10678-10685]. These protons are part of a hydrogen bonding network including the amide protons of Asn-52, Met-80, and Lys-79, the side chain protons of Asn-52, Tyr-67, Thr-78, Trp-59, and Thr-49, and the water molecules 121 and 166.(ABSTRACT TRUNCATED AT 250 WORDS)