Cytochrome is an electron transfer protein with iron porphyrin (or heme) as a prosthetic group, and is widely involved in redox reactions of animals, plants, yeasts, aerobic bacteria, anaerobic photosynthetic bacteria, and so on. The way in which cytochrome acts as an electron carrier to transfer electrons is through a reversible change between the reduced state (Fe2+) and the oxidized state (Fe3+) of the iron atom in its heme prosthetic group. Any type of cellular protein (heme protein) plays an extremely important role in cellular energy transfer.
The porphyrin ring is bonded to the iron atom by four coordinate bonds to form a tetraligand chelated complex, generally referred to as heme. Cytochromes can be classified into categories a, b, c and d depending on the structure of the heme prosthetic group.
The structure of a type of cytochrome prosthetic group is heme A, which differs from the original heme in that the form of the porphyrin ring is replaced by a formyl group instead of a methyl group, and the second place is replaced by a hydroxy-farnesyl group
The prosthetic group of the b-type cytochrome is the protohemoglobin, iron-protoporphyrin IX. The side chain substituent on the porphyrin ring is 4 methyl groups, and the two vinyl groups and the two propionate groups have the same structure as the hemoglobin and myoglobin.
The prosthetic group of the c-type cytochrome is that heme is covalently bound by a thioether bond of a vinyl group on the porphyrin ring to a cysteine thiol group in the protein molecule. The prosthetic groups of other types of cytochromes are combined with proteins by non-covalent bonds. The cytochrome in the reduced state has characteristic light absorption bands in the visible light region: an α band, a β band, and a γ band (or an absorption band). Usually, the α-absorption band of a type of cytochrome is located at 598-605 nm; the maximum α-absorption band of b is 556-564 nm; the c-class is 550-555 nm; and the d-class is between 600-620 nm.
Class d cytochromes are only found in bacteria, and their prosthetic group is iron dihydroporphyrin, which is different from other cytochromes.
The cytochrome complex or cyt c is a small hemoglobin that is loosely associated with the mitochondrial inner membrane. It belongs to the cytochrome c family of proteins. Unlike other cytochromes, cytochrome c is highly water soluble and is an important component of the electron transport chain, carrying an electron in it. When its iron atom is converted between the ferrous and iron forms, it is capable of oxidation and reduction, but does not combine with oxygen. It transfers electrons between complex III (Coenzyme Q - Cyt C reductase) and IV (Cyt C oxidase). In humans, cytochrome c is encoded by the CYCS gene
Figure 1. Three-dimensional structure of cytochrome c.
Cytochrome c is a component of the electron transport chain in mitochondria. The heme group of cytochrome c accepts electrons from the bcl complex and transfers electrons to complex IV. Cytochrome c is also involved in the initiation of apoptosis. When cytochrome c is released into the cytoplasm, the protein binds to apoptotic protease activator-1 (Apaf-1). Cytochrome c can also catalyze some redox reactions, such as hydroxylation and aromatic oxidation, and through various electron donors (eg, 2,2-azidobis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), oxidation of 2-) shows peroxidase activity. Keto-4-thiomethylbutyric acid and 4-aminoantipyrine.
Figure 2. Structure of heme c.
Role in apoptosis
Cytochrome c binds to cardiolipin on the inner mitochondrial membrane, anchoring its presence and preventing its release from mitochondria and triggering apoptosis. Although the initial attraction between cardiolipin and cytochrome c is electrostatic due to the strong positive charge on cytochrome c, the final interaction is hydrophobic, in which the hydrophobic tail from cardiolipin inserts itself into the cytochrome the hydrophobic portion of c. In the early stages of apoptosis, the production of mitochondrial ROS is stimulated, and cardiolipin is oxidized by the peroxidase function of the cardiolipin-cytochrome c complex. The blood protein is then separated from the mitochondrial inner membrane and can be extruded into the soluble cytoplasm through the pores in the outer membrane. Cyt c is released from the mitochondria before the calcium level continues to rise. The release of a small amount of Cyt c results in interaction with the IP3 receptor (IP3R) on the endoplasmic reticulum (ER), resulting in ER calcium release. An overall increase in calcium triggers a large release of cyt c and then acts in a positive feedback loop to maintain ER calcium release via IP3R. This explains how ER calcium release reaches cytotoxic levels. This release of cytochrome c in turn activates the cysteine protease caspase 9. Caspase 9 can then continue to activate caspase 3 and caspase 7, which are responsible for destroying cells from within.
As an antioxidative enzyme
Cytochrome c is known to play a role in the electron transport chain and apoptosis. However, a recent study has shown that it can also act as an antioxidant enzyme in mitochondria. It is achieved by removing superoxide (O2–) and hydrogen peroxide (H2O2) from the mitochondria. Therefore, mitochondria require not only cytochrome c for cellular respiration, but also cytochrome c in mitochondria to limit O2– and H2O2 production.
Figure 3. Removal of O2− and H2O2 by cytochrome c.
Cytochrome c has been used to detect the production of peroxides in biological systems. When superoxide is produced, the amount of oxidized cytochrome c3+ increases, while the amount of reduced cytochrome c2+ increases. However, nitric oxide is often produced with nitric oxide. The presence of nitric oxide inhibits the reduction of cytochrome c3+. This results in the oxidation of cytochrome c2+ to cytochrome c3+ by peroxynitrite, which is an intermediate formed by the reaction of nitric oxide and superoxide. Peroxynitrite or H2O2 and nitrogen dioxide NO2 in mitochondria may be fatal because they nitrate the tyrosine residue of cytochrome c, thereby destroying the function of cytochrome c as an electron carrier in the electron transport chain.
Figure 4. Peroxynitrous acid.
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