Structure at 1.5 Å resolution of cytochrome c₅₅₂ with its flexible linker segment, a membrane-anchored protein from Paracoccus denitrificans

Electron transfer (ET) between the large membrane-integral redox complexes in the terminal part of the respiratory chain is mediated either by a soluble c-­type cytochrome, as in mitochondria, or by a membrane-anchored cytochrome c, as described for the ET chain of the bacterium Paracoccus denitrificans. Here, the structure of cytochrome c₅₅₂ from P. denitrificans with the linker segment that attaches the globular domain to the membrane anchor is presented. Cytochrome c₅₅₂ including the linker segment was crystallized and its structure was determined by molecular replacement. The structural features provide functionally important information. The prediction of the flexibility of the linker region [Berry & Trumpower (1985), J. Biol. Chem. 260, 2458-2467] was confirmed by our crystal structure. The N-terminal region from residues 13 to 31 is characterized by poor electron density, which is compatible with high mobility of this region. This result indicates that this region is highly flexible, which is functionally important for this protein to shuttle electrons between complexes III and IV in the respiratory chain. Zinc present in the crystallization buffer played a key role in the successful crystallization of this protein. It provided rigidity to the long negatively charged flexible loop by coordinating negatively charged residues from two different molecules and by enhancing the crystal contacts.