Biochemical requirements for the maturation of mitochondrial c-type cytochromes

Cytochrome c biogenesis: the Ccm system

Cytochromes of c-type contain covalently attached hemes that are formed via thioether bonds between the vinyls of heme b and cysteines within C(1)XXC(2)H motifs of apocytochromes. In diverse organisms this post-translational modification relies on membrane-associated specific biogenesis proteins, referred to as cytochrome c maturation (Ccm) systems. A highly complex version of these systems, Ccm or System I, is found in Gram-negative bacteria, archaea and plant mitochondria. We describe emerging functional interactions between the Ccm components categorized into three conserved modules, and present a mechanistic view of the molecular basis of ubiquitous vinyl-2 approximately Cys(1) and vinyl-4 approximately Cys(2) heme b-apocytochrome thioether bonds in c-type cytochromes.

Cytochrome c biogenesis: mechanisms for covalent modifications and trafficking of heme and for heme-iron redox control

Heme is the prosthetic group for cytochromes, which are directly involved in oxidation/reduction reactions inside and outside the cell. Many cytochromes contain heme with covalent additions at one or both vinyl groups. These include farnesylation at one vinyl in hemes o and a and thioether linkages to each vinyl in cytochrome c (at CXXCH of the protein). Here we review the mechanisms for these covalent attachments, with emphasis on the three unique cytochrome c assembly pathways called systems I, II, and III. All proteins in system I (called Ccm proteins) and system II (Ccs proteins) are integral membrane proteins. Recent biochemical analyses suggest mechanisms for heme channeling to the outside, heme-iron redox control, and attachment to the CXXCH. For system II, the CcsB and CcsA proteins form a cytochrome c synthetase complex which specifically channels heme to an external heme binding domain; in this conserved tryptophan-rich "WWD domain" (in CcsA), the heme is maintained in the reduced state by two external histidines and then ligated to the CXXCH motif. In system I, a two-step process is described. Step 1 is the CcmABCD-mediated synthesis and release of oxidized holoCcmE (heme in the Fe(+3) state). We describe how external histidines in CcmC are involved in heme attachment to CcmE, and the chemical mechanism to form oxidized holoCcmE is discussed. Step 2 includes the CcmFH-mediated reduction (to Fe(+2)) of holoCcmE and ligation of the heme to CXXCH. The evolutionary and ecological advantages for each system are discussed with respect to iron limitation and oxidizing environments.

Substrate specificity of three cytochrome c haem lyase isoenzymes from Wolinella succinogenes: unconventional haem c binding motifs are not sufficient for haem c attachment by NrfI and CcsA1

Bacterial c-type cytochrome maturation is dependent on a complex enzymic machinery. The key reaction is catalysed by cytochrome c haem lyase (CCHL) that usually forms two thioether bonds to attach haem b to the cysteine residues of a haem c binding motif (HBM) which is, in most cases, a CX(2)CH sequence. Here, the HBM specificity of three distinct CCHL isoenzymes (NrfI, CcsA1 and CcsA2) from the Epsilonproteobacterium Wolinella succinogenes was investigated using either W. succinogenes or Escherichia coli as host organism. Several reporter c-type cytochromes were employed including cytochrome c nitrite reductases (NrfA) from E. coli and Campylobacter jejuni that differ in their active-site HBMs (CX(2)CK or CX(2)CH). W. succinogenes CcsA2 was found to attach haem to standard CX(2)CH motifs in various cytochromes whereas other HBMs were not recognized. NrfI was able to attach haem c to the active-site CX(2)CK motif of both W. succinogenes and E. coli NrfA, but not to NrfA from C. jejuni. Different apo-cytochrome variants carrying the CX(15)CH motif, assumed to be recognized by CcsA1 during maturation of the octahaem cytochrome MccA, were not processed by CcsA1 in either W. succinogenes or E. coli. It is concluded that the dedicated CCHLs NrfI and CcsA1 attach haem to non-standard HBMs only in the presence of further, as yet uncharacterized structural features. Interestingly, it proved impossible to delete the ccsA2 gene from the W. succinogenes genome, a finding that is discussed in the light of the available genomic, proteomic and functional data on W. succinogenes c-type cytochromes.

A conserved haem redox and trafficking pathway for cofactor attachment

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 Fe(3+)), yet the final attachment requires reduced haem (Fe(2+)). 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.

Biogenesis of cytochrome b6 in photosynthetic membranes

In chloroplasts, binding of a c′-heme to cytochrome b₆ on the stromal side of the thylakoid membranes requires a specific mechanism distinct from the one at work for c-heme binding to cytochromes f and c₆ on the lumenal side of membranes. Here, we show that the major protein components of this pathway, the CCBs, are bona fide transmembrane proteins. We demonstrate their association in a series of hetero-oligomeric complexes, some of which interact transiently with cytochrome b₆ in the process of heme delivery to the apoprotein. In addition, we provide preliminary evidence for functional assembly of cytochrome b₆f complexes even in the absence of c′-heme binding to cytochrome b₆. Finally, we present a sequential model for apo- to holo-cytochrome b₆ maturation integrated within the assembly pathway of b₆f complexes in the thylakoid membranes.

CcsBA is a cytochrome c synthetase that also functions in heme transport

Little is known about trafficking of heme from its sites of synthesis to sites of heme-protein assembly. We describe an integral membrane protein that allows trapping of endogenous heme to elucidate trafficking mechanisms. We show that CcsBA, a representative of a superfamily of integral membrane proteins involved in cytochrome c biosynthesis, exports and protects heme from oxidation. CcsBA has 10 transmembrane domains (TMDs) and reconstitutes cytochrome c synthesis in the Escherichia coli periplasm; thus, CcsBA is a cytochrome c synthetase. Purified CcsBA contains heme in an "external heme binding domain" for which two external histidines are shown to serve as axial ligands that protect the heme iron from oxidation. This is likely the active site of the synthetase. Furthermore, two conserved histidines in TMDs are required for heme to travel to the external heme binding domain. Remarkably, the function of CcsBA with mutations in these TMD histidines is corrected by exogenous imidazole, a result analogous to correction of heme binding by myoglobin when its proximal histidine is mutated. These data suggest that CcsBA has a heme binding site within the bilayer and that CcsBA is a heme channel.