CytochromerC₅₅₂, Formed during Expression of the Truncated,Thermus thermophilusCytochromec₅₅₂Gene in the Cytoplasm ofEscherichia coli, Reacts Spontaneously To Form Protein-Bound 2-Formyl-4-vinyl (Spirographis) Heme^†,‡

Expression of the truncated (lacking an N-terminal signal sequence) structural gene of Thermus thermophilus cytochrome c(552) in the cytoplasm of Escherichia coli yields both dimeric (rC(557)) and monomeric (rC(552)) cytochrome c-like proteins [Keightley, J. A., et al. (1998) J. Biol. Chem. 273, 1200612016], which form spontaneously without the involvement of cytochrome c maturation factors. Cytochrome rC557 is comprised of a dimer and has been structurally characterized [McRee, D., et al. (2001) J. Biol. Chem. 276, 6537-6544]. Unexpectedly, the monomeric rC552 transforms spontaneously to a cytochrome-like chromophore having, in its reduced state, the Q(oo) transition (a-band) at 572 nm (therefore called p572). The X-ray crystallographic structure of rC(552), at 1.41 Angstrom resolution, shows that the 2-vinyl group of heme ring I is converted to a [heme-CO-CH2-S-CH2-C-alpha] conjugate with cysteine 11. Electron density maps obtained from isomorphous crystals of p572 at 1.61 Angstrom resolution reveal that the 2-vinyl group has been oxidized to a formyl group. This explains the lower energy of the Qoo transition, the presence of a new, high-frequency band in the resonance Raman spectra at 1666 cm(-1) for oxidized and at 1646 cm-1 for reduced samples, and the greatly altered, paramagnetically shifted H-1 NMR spectrum observed for this species. The overall process defines a novel mechanism for oxidation of the 2-vinyl group to a 2-formyl group and adds to the surprising array of chemical reactions that occur in the interaction of heme with the CXXCH sequence motif in apocytochromes c.