The historical background of cytochrome
Macmunn first in 1886, found in animal cells containing a heme pigment, known as tissue heme. But the main biochemist at the time thought it was an experimental error. Until 1925, Keilin confirmed the discovery of Macmunn, and renamed the cytochrome.
An overview of cytochrome
Cytochrome is an electron transfer protein with iron porphyrin (or hemoglobin) as the cofactor. It is widely involved in the oxidation-reduction reaction of plants and animals, aerobic bacteria, anaerobic photosynthetic bacteria and so on. Cytochrome as an electron carrier electron transfer way is through its heme-iron base in the reduced state (Fe2+) and oxidation state between the reversible changes. Any kind of cell protein (heme protein) plays a very important role in cell energy transfer. Cytochrome can be divided into three categories according to the wavelength of light they absorb, and at least 30 different cytochromes have been identified.
The definition of cytochrome
The porphyrin ring is linked to the iron atom by four valence bonds, forming a four-ligand chelate complex, commonly known as heme. According to the different structure of heme, the cytochromes can be divided into a, b, c and d categories. The cofactor of the class b cytochrome is protohemoglobin iron-protoporphyrin IX. The side chain substituents on the porphyrin ring are 4 methyl groups, the two vinyl groups and the two propionic groups are the same as those of hemoglobin and myoglobin.
A type of cytochrome prosthesis is the structure of hemoglobin A, which is different from the original heme is porphyrin ring on the eighth position to formyl group instead of methyl, the second place on the behalf of alkenylene instead of hydroxyl base.
D-type cytochrome only found in bacteria, its cofactor for the iron dihydroporphyrin, and other cytochrome different.
The cofactor of the C-type cytochrome is the covalent binding of heme to the thioether bond of the vinyl group on the porphyrin ring and the cysteine thiol group in the protein molecule. Other types of cytochrome prosthesis are non-covalent bond and protein combination. The cytochrome of the reduced state has a characteristic optical absorption band in the visible region: α band, β band, γ band (or absorption band). In general, the alpha absorption band of cytochrome A is located in the range of 598 to 605 nm. The largest α-absorption band of class b is 556-564 nm; the class c is 550-555 nm; and the class d is 600-620 nm.
The classification of cytochrome
All aerobic organisms, whether bacteria or higher animals and even people there are cytochrome, all the cytochrome in the visible region has three absorption peaks, respectively, called α, β, γ peak, see Table 1. Various reduced cytochromes of β-and γ-peak wavelength are relatively close, only the α-peak wavelengths have a greater difference. According to the different wavelengths of α peak, the cytochromes in animals can be divided into a, b, c family, a family including a, a3, b family including bK, bT, b5, P4 and so on, c group including c, c1.
In addition to cytochrome b and cytochrome P450 mainly present in the endoplasmic reticulum, the cytochrome of animal cells are present in the mitochondrial inner membrane protein and lipid complexes. Extracted with water can only get cytochrome c, while other cytochrome is still fixed in the mitochondrial insoluble particulate matter.
Cytochrome oxidase has been determined to have two components, a and a3, but not the two separate. Actually a and a3 combine to form a macromolecular oligomer, but the number and structure of subunits is unclear, usually known as cytochrome aa3 or cytochrome oxidase, molecular weight of about 200 000, containing two molecular heme A and two copper atoms. The reactivity of two hemoglobin A and some ligands is different, and the heme A of oxidized a3 is easily bound to CN-1 and can not be reduced again. Reduced a3 hemoglobin A and CO to form a stable complex, thereby interrupting the electron transport of the respiratory chain, and cytochrome a hemoglobin A and CO and CN can not be combined, can not be combined with O2.
Cytochrome c family
Cytochrome c has c and c1, are mitochondrial cytochrome, the spectral properties of the two are very similar, both the cofactor and the enzyme protein are connected in the same way. C1 monomer molecular weight of 38 000, c1 is the polymer exists, whether monomer or polymer, can not react with aa3. Cytochrome c is the smallest molecular weight of cytochrome, because of its molecular weight is small, soluble, easy extraction and purification, so more research, its structure is also the most clear, more than 50 kinds of organisms have been measured cytochrome c level Structure, its high-level structure has also been studied in detail. The cytochrome c of vertebrates consists of 104 residues. The cytochrome c of the plant consists of 112 residues. The cytochrome c of human and chimpanzee is exactly the same, although there are many variations in the cytochrome c residues of various organisms. But its three-dimensional structure is basically the same.
Cytochrome b family
BT, bK, b5, P450, etc., bT, bK exists in the mitochondrial inner membrane, is a component of mitochondrial respiratory chain, can not be combined with O2. B5 and P450 mainly exist in the endoplasmic reticulum, which is the component of the microsomal mixed functional oxidase system (see "Microsomal Oxidation"). After the combination of reduced P450 and CO, it has the maximum absorption peak at 450nm, So named cytochrome P450, which can combine with O2.