A conserved haem redox and trafficking pathway for cofactor attachment

Abstract

A pathway for cytochrome c maturation (Ccm) in bacteria, archaea and eukaryotes (mitochondria) requires the genes encoding eight membrane proteins (CcmABCDEFGH). The CcmABCDE proteins are proposed to traffic haem to the cytochrome c synthetase (CcmF/H) for covalent attachment to cytochrome c by unknown mechanisms. For the first time, we purify pathway complexes with trapped haem to elucidate the molecular mechanisms of haem binding, trafficking and redox control. We discovered an early step in trafficking that involves oxidation of haem (to Fe3+), yet the final attachment requires reduced haem (Fe2+). Surprisingly, CcmF is a cytochrome b with a haem never before realized, and in vitro, CcmF functions as a quinol:haem oxidoreductase. Thus, this ancient pathway has conserved and orchestrated mechanisms for trafficking, storing and reducing haem, which assure its use for cytochrome c synthesis even in limiting haem (iron) environments and reducing haem in oxidizing environments.

Heme Utilization in the Caenorhabditis elegans Hypodermal Cells Is Facilitated by Heme-responsive Gene-2

“…its covalent attachment to apocytochrome c (51,52). In Gramnegative bacteria, the cytochrome c synthetase CcmF was proposed to function as a quinol:heme oxidoreductase (52).…”

“…In Gramnegative bacteria, the cytochrome c synthetase CcmF was proposed to function as a quinol:heme oxidoreductase (52). In addition, the lipocalin ␣1-microglobulin has the ability to reduce hemin in cytochrome c and methemoglobin (53).…”

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

“…This complex contained oxidized heme (Fe 3ϩ ) covalently bound to the His residue of CcmE, with CcmC providing two additional His residues as axial ligands of the heme iron (11).…”

“…CcmF is a large multispan membrane protein with a b-type heme (11) and belongs to the heme handling protein (HHP) family like CcmC (supplemental Table S1) (14). Recently, it has been suggested that CcmF reduces the heme of holoCcmE to facilitate its transfer to the apocytochrome c (11,15). In ␣-proteobacteria (e.g.…”

CcmI Subunit of CcmFHI Heme Ligation Complex Functions as an Apocytochrome c Chaperone during c-Type Cytochrome Maturation

“…1). CcmF contains heme b and belongs to the heme handling protein family (8,9). It has a conserved periplasmic WWD motif and two conserved His residues that are proposed to acquire heme from holoCcmE (9).…”