Cytochromes and Related Peptides、細(xì)胞色素及相關(guān)肽的介紹

Definition

Cytochromes can be defined as electron­ or proton­transfer proteins. They are membrane-bound hemoproteins.

Discovery

In 1884 MacMunn initially described cytochromes as respiratory pigments (myohematin or histohematin). Later in 1920s, Keilin explained these respiratory pigments and named them as cytochromes and classified these heme proteins was done on the basis of the position of their lowest energy absorption band in the reduced state, as cytochromes a (605 nm), b (~565 nm), and c (550 nm 1.   Cytochromes can be classified according to their haem iron coordination, by haem type, and further by sequence similarity. Some haemoproteins, such as P450s and nitric oxide synthase (NOS), have been termed `cytochromes' as well. Although both P450s and NOS can be considered as b­type cytochromes, their main function is catalysis. The recommended name for the group of enzymes which includes P450s and NOS is `haem-thiolate proteins.

Structural Characteristics

The heme group present in a cytochrome is a highly-conjugated ring system which surrounds a metal ion. The metal ion present can interconverts between Fe2+ (reduced) and Fe3+ (oxidised) states (electron-transfer processes). Sulfate-reducing bacteria contain a variety of multi-heme c-type cytochromes. The cytochrome of highest molecular weight (Hmc) contains 16 heme groups and is part of a transmembrane complex involved in the sulfate respiration pathway.  Crystal structure of the Desulfovibrio vulgaris Hildenborough cytochrome Hmc and a structural model of the complex with its physiological electron transfer partner, cytochrome c3 have been obtained by NMR restrained soft-docking calculations. The Hmc is composed of three domains, which exist independently in different sulfate-reducing species, namely cytochrome c3, cytochrome c7, and Hcc. The complex involves the last heme at the C-terminal region of the V-shaped Hmc and heme 4 of cytochrome c3, and represents an example for specific cytochrome-cytochrome interaction 2. Itoh Y et al., demonstrated that residues 46 and 54 on a synthetic peptide, AEGFSYTVANKNKGIT (50V), work as an agretope (site contacts with major histocompatibility complex molecules) and residues 50 and 52 function as an epitope (site contacts with T cell receptor), when tri-molecular complexes are formed among 50V,I-Ab and the T cell receptor 3.

Mode of Action

Respiratory pigments found widely in aerobic organisms which transfer energy within cells. They are proteins containing a chelate complex of an iron prosthetic group and a porphyrin system, and so include hemoglobin. Cytochromes undergo a series of oxidation and reduction reactions, transferring electrons from a substrate to each other and finally to an electron-acceptor. In cellular respiration the substrates are the hydrogen-acceptors in the citric acid cycle, the electron acceptor is oxygen, and the energy produced is stored in ATP.

Synthetic peptide 50V was composed of residues 43 to 58 of pigeon cytochrome c (p43-58) except that the aspartic acid at residue 50 was substituted by valine. Substitution of agretopic residues on 50V changed this I-Ab-binding peptide to an I-Ak-binding peptide, suggesting that positions 46 and 54 work as an agretope in I-Ak-restricted T cell responses. Later residues 46 and 54 of 50V was examined to work as agretopes in T cell responses restricted to other I-A haplotypes. The 50V-related peptides with phenylalanine at position 46 and alanine at position 54 bound tightly to I-Ab, I-Ad, I-Aq and I-As molecules and stimulated T cells most potently in mice bearing these I-A haplotypes. In contrast, 50V-related peptides carrying D at position 46 and A at position 54 bound most potently to I-Ak molecules, and the peptides with arginine at position 46 and A at position 54 bound most efficiently to I-Av molecules. Their findings demonstrate that the agretopic positions on the p43-58 related peptides are preserved in T cell responses restricted to each I-A haplotype studied, and that the specific amino acids on the agretopic positions exist a priori for each I-A allele-specific structure .

Functions

Oxidation of reduced coenzymes, some cytochromes react with oxygen directly; others are intermediates in the oxidation of reduced coenzymes. The iron in the haem of cytochromes undergoes oxidation and reduction.

Redox reactions, Cytochromes are of performing oxidation and reduction. Because the cytochromes are held within membranes in an organized way, the redox reactions are carried out in the proper sequence for maximum efficiency.

Respiratory chain, a spectroscopic examination of the cytochrome content of these mitochondria shows no detectable cytochrome c1, a, or a3 and does reveal cytochromes of types c and b. Subunits in the same size range are found in cytochrome c deficient mitochondria of the emergent bee, while the frequency of their occurrence along the cristae is decreased relative to the adult bee. Apparently, the cytochrome content of the respiratory chain is not related to the size of the subunits, but may be related to the frequency of occurrence of the subunits 4.

References

1. Reedy CJ, Gibney BR (2004).Heme protein assemblies. Chem Rev.,104(2):617–649.

2. Czjzek M, ElAntak L, Zamboni V, Morelli X, Dolla A, Guerlesquin F, Bruschi M (2002). The crystal structure of the hexadeca-heme cytochrome hmc and a structural model of its complex with cytochrome c3. Structure, (10)12:1677-1686.

3. Itoh Y, Ogasawara K, Takami K, Gotohda T, Naruse H, Good RA, Onoé K (1994). Determination of amino acids on agretopes of pigeon cytochrome c-related peptides specifically bound to I-A allelic products. Eur J Immunol., 24(1):76-83.

4. Chance B, Parsons DF (1963). Cytochrome function in relation to inner membrane structure of mitochondria. Science, 142(3596):1176-1180.