A Pivotal Heme-transfer Reaction Intermediate in Cytochrome c Biogenesis

Abstract

Background: Heme attachment to cytochrome c is a catalyzed post-translational modification.Results: We identify a ternary complex of the cytochrome c biogenesis protein CcmE, heme, and a cytochrome, and demonstrate its functional significance.Conclusion: The complex is a trapped catalytic intermediate at the point of heme transfer from the cytochrome biogenesis apparatus to the cytochrome.Significance: An insight into biosynthesis of heme proteins.

The Heme Chaperone ApoCcmE Forms a Ternary Complex with CcmI and Apocytochrome c

“…2C). A similar, highly stable holoCcmE dimer has been reported previously (18,24), and our data with the apoCcmE derivative indicated that the presence of heme is not required for this oligomerization.…”

“…The heme was covalently ligated to both CcmE and apocytochrome b 562 variant via the His and Cys residues, respectively. A similar complex was also obtained in vitro using the Hydrogenobacter thermophilus apocytochrome c 552 (18).…”

“…In E. coli cells a cytochrome b 562 derivative with a C 1 XXXH (R98C) binding motif forms a complex with heme and CcmE (18). The heme was covalently bound to the Cys-98 of this c-type cytochrome mimic and the His-130 of CcmE.…”

“…We thought that the specific regions of these proteins involved in these interactions might be important for transferring heme from CcmE to apocytochrome c. R. capsulatus cytochrome c 2 , like the H. thermophilus cytochrome c 552 , recently shown to form a ternary complex in vitro with CcmE and heme (18), belongs to the Class I of c-type cytochromes (26). These c-type cytochromes have a N-terminally located heme binding (C 1 XXC 2 H) motif, a C-terminally located Met residue acting as the sixth axial ligand of the heme-iron, and a general globular fold with interacting N-and C-terminal helices (27,28).…”

Cytochrome c assembly

“…Recent progress has been made in establishing the functional relevance of the covalent bond, which is between a histidine N and the b-carbon of one of the heme vinyl groups (45). A complex between the membrane-anchored CcmE and a variant cytochrome c was trapped in vivo and characterized (46). The effects on the formation of the complex caused by changes in the other Ccm proteins indicate that the covalent bond is on-pathway and physiologically relevant.…”