Subsystem: Biogenesis of c-type cytochromes

This subsystem's description is:

Cytochromes of the c type bind heme covalently via two thioether bonds to the two cysteines within the sequence motif Cys-Xaa-Xaa-Cys-His located in the periplasmic domain of the cytochrome. The mechanism of this covalent stereo-specific attachment of heme to the cysteine residues of apocytochrome is the key question of biogenesis of c-type cytochromes. At least three different pathways of variable complexity have been described so far:

System III of cytochrome c maturation, the simplest kind known, operates in mitochondia (mt) of fungi, vertebrates, invertebrates, and some green algae (i.e., Chlamydomonas reinhardtii). It has been studied in detail in yeast. A single enzyme, cytochrome c haem lyase (CCHL) is required for the formation of two thioether bonds. However, in fungal (and possibly animal) mt two separate CCHLs are used for the two c-type cytochromes c and c1. CCHL genes are nuclear encoded.

System II is found in Gram-positive bacteria, cyanobacteria and some proteobacteria, and also in plant and algal chloroplasts, and perhaps in archaea. While the main components of the system II - proteins CcdA and Ccs1 are homologous across different species, protein thiol:disulfide oxidoreductases that are associated with CcdA (ResA in B. subtilis Erlendsson et al., 2003) can be rather diverse in different organisms. Based on co-localization of these proteins with CcdA (and Ccs1) they have been included in this SS as possibly involved in cyt c biogenesis (Hyp3, Hyp4). Generally thiol:disulfide oxidoreductases (i) share an active site with two Cys residues arranged in a motif: C-X-X-C; (ii) despite very low primary sequence similarity often show the same tertiary structure thioredoxin-like fold.

System I cytochrome c maturation occurs in alpha- and gamma-proteobacteria, Deinococci, and mitochondria of plants and protozoa [13,13]. It requires eight or nine specific cytochrome c maturation (ccm) genes often located in a cluster, and three dsb genes (for disulfide bond formation). In addition, genes of the general secretion pathway and of cellular redox control contribute to efficient cytochrome C biogenesis. System I pathway has a higher affinity for haem than system II for cytochrome c biogenesis due to the presence of ATP-binding cassette (ABC) transporter that is required for release of the periplasmic haem chaperone CcmE to the last step of cytochrome c assembly (Feissner et al., 2006). This ABC transporter is composed of the ATP-binding subunit CcmA, and two membrane proteins, CcmB and CcmC. In the absence of CcmA or CcmB, holo(haem)CcmE binds to CcmC in a stable dead-end complex, indicating high affinity binding of haem to CcmC (Feissner et al., 2006).
CcmCDE are involved in heme delivery. CcmC is required for the covalent attachment of heme to the heme chaperone CcmE. CcmF has been proposed to form a bacterial cytochrome c heme lyase complex together with CcmH (and CcmL). Cysteine thiols of the heme-binding motif of apocytochromes c are oxidized by the periplasmic DsbA. CcmG, a thioredoxin-like protein, and CcmH, a putative thiol-disulfide oxidoreductase, reduce intramolecular disulfide bonds in apocytochromes before heme attachment by transferring electrons from the transmembrane disulfide oxidoreductase DsbD (DipZ)

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