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The conformation of eukaryotic cytochrome c around residues 39, 57, 59 and 74
Insights
Nuclear magnetic resonance studies reveal conserved residue clusters in cytochromes c that shield the heme. These clusters exhibit temperature and redox-dependent conformational changes, impacting protein flexibility.
Area of Science:
- Biochemistry
- Structural Biology
- Spectroscopy
Background:
- Cytochromes c are essential electron transport proteins.
- Understanding their structure-function relationship is crucial for cellular respiration.
- Previous studies indicated conserved structural features in cytochromes c.
Purpose of the Study:
- To investigate the structural dynamics of cytochromes c from different species.
- To elucidate the role of specific residues in heme shielding and protein stability.
- To correlate structural findings with spectroscopic data.
Main Methods:
- Proton nuclear magnetic resonance (1H-n.m.r.) spectroscopy was employed.
- Comparative analysis of horse, tuna, Candida krusei, and Saccharomyces cerevisiae cytochromes c.
- Spectroscopic measurements of pKa values.
Main Results:
- A conserved cluster of residues at the heme's base was identified in all studied cytochromes c.
- The positions of these residues dynamically change with temperature and protein redox state.
- pKa values for key lysine and histidine residues align with structural predictions.
Conclusions:
- Cytochrome c proteins possess a flexible heme-shielding mechanism.
- Conformational heterogeneity is inherent to cytochrome c structure.
- The findings support existing models of cytochrome c structure and function.
Abstract:
1H-n.m.r. studies of horse, tuna, Candida krusei and Saccharomyces cerevisiae cytochromes c showed that each of the proteins contains a similar cluster of residues at the bottom of the protein that assists in shielding the haem from the solvent. The relative positions of the residues forming these clusters vary continuously with temperature, and they change with the change in protein redox state. This conformational heterogeneity is discussed with reference to the conformational flexibility of cytochrome c around residues 57, 59 and 74. Spectroscopic measurements of pKa values for Lys-55 (horse and tuna cytochromes c) and His-33 and His-39 (C. krusei and S. cerevisiae cytochromes c) are in excellent agreement with expectations based on chemical-modification studies of horse cytochrome c. [Bosshard & Zürrer (1980) J. Biol. Chem. 255, 6694-6699] and on the X-ray-crystallographic structure of tuna cytochrome c [Takano & Dickerson (1981) J. Mol. Biol. 153, 79-94, 95-115].