Cytochromes C proteins are central proteins in energy transduction processes by virtue of their functions in electron transfer in respiration and photosynthesis. They have heme covalently attached to a characteristic CXXCH motif via protein-catalyzed post-translational modification reactions. The cytochrome c family is widespread and influences the balance between life and death, acting in energy provision for cellular functions or triggering programmed cell death.
Mitochondria possess two c-type cytochromes, the soluble redox-active heme protein CYTc that transfers electrons between complex III (cytochrome c reductase) and complex IV (cytochrome c oxidase, CcO) and the inner membrane anchored CYTc1 that constitutes an integral part of complex III. Structurally, they both contain covalently bound heme, which is attached by two thioether bonds established between two reduced cysteines present in the conserved apocytochrome c (apo-CYTc) heme-binding motif (C1XXC2H), and two vinyl groups present in heme b. Covalent ligation of the heme cofactor confers stability to the apo-CYTc, avoiding its rapid degradation.
CYTc biogenesis: synthesis and maturation of CYTc
Biogenesis of c-type cytochromes is an intricate process. First, apo-CYTc and heme should be translocated from their site of synthesis (cytosol and themitochondrial matrix or the chloroplast, respectively) to the assembly site in the IMS. Once in the IMS, apo-CYTc and heme must be maintained in a reduced redox state until the heme attachment reaction occurs. There are five CYTc maturation pathways that have been described in different organisms. System I, also referred as CCM (cytochrome C maturation) pathway, was first described in 𝛼- and 𝛾-proteobacteria, and is found in bacteria, archea and plant mitochondria . System II (Ccs, CYTc synthesis) is used by 𝛽-, 𝛿- and 𝜀-proteobacteria, Gram-positive bacteria, algae and is also is present in chloroplasts. System III is characteristic of yeast and mammalian mitochondria where enzymes named CYTc heme lyase (CCHL) or holo-CYTc synthase (HCCS) participate in apo-CYTc transport and heme attachment. Systems I to III were described on the positive (-p) side of bioenergetics membranes of bacterial periplasm, chloroplast lumen and mitochondrial IMS. Finally, two other systems (IV and V) were recently described for the biogenesis of unusual c-type cytochromes. System IV catalyzes heme attachment through a single thioether bond on the negative (-n) side of bioenergetic membranes presents in bacterial cytoplasm and plastid stroma. This system is found in all organisms with oxygenic photosynthesis but not in Firmicutes (e.g. Bacillus subtilis). System V is described in mitochondria of Euglenozoans, were the heme is hexacoordinated with a single tioether bond .
CYTc proteins are multifunctional proteins
CYTc family are soluble heme proteins of the IMS that transports electrons between cytochrome bc1 reductase (complex III) and CcO (complex IV). Under physiological conditions, this reaction constitutes the rate-limiting step of the mETC in mammalian cells [3,4]. CYTc proteins is essential for aerobic energy production and both knockout of CYTc genes or defects in the maturation of the holoprotein are lethal in plants due to defects in embryogenesis [5,6]. In addition to providing energy for cellular functions and growth, another, somewhat contradictory function has been demonstrated for CYTc: to promote cell death when necessary during normal development or after conditions of irreparable damage imposed to the cell. Also, CYTc has the final decision between life and death as, according to several parameters, it can exit from mitochondria and play an active role in triggering PCD .