The 40s Omega-loop plays a critical role in the stability and the alkaline conformational transition of cytochrome c

The structural and redox properties of a non-covalent complex reconstituted upon mixing two non-contiguous fragments of horse cytochrome c, the residues 1 - 38 heme-containing N-fragment with the residues 57 - 104 C-fragment, have been investigated. With respect to native cyt c, the complex lacks a segment of 18 residues, corresponding, in the native protein, to an omega ( W)loop region. The fragment complex shows compact structure, native-like alpha-helix content but a less rigid atomic packing and reduced stability with respect to the native protein. Structural heterogeneity is observed at pH 7.0, involving formation of an axially misligated low-spin species and consequent partial displacement of Met80 from the sixth coordination position of the heme-iron. Spectroscopic data suggest that a lysine ( located in the Met80-containing loop, namely Lys72, Lys73, or Lys79) replaces the methionine residue. The residues 1 - 38/57 - 104 fragment complex shows an unusual biphasic alkaline titration characterized by a low (pK(a1)= 6.72) and a high pK(a)-associated state transition (pK(a2)= 8.56); this behavior differs from that of native cyt c, which shows a monophasic alkaline transition ( pK(a)= 8.9). The data indicate that the 40s Omega-loop plays an important role in the stability of cyt c and in ensuring a correct alkaline conformational transition of the protein.