Cytochromes are proteins containing heme as a cofactor. They are classified according to its mode of binding. Four varieties are recognized by the IUBMB, cytochromes a, cytochromes b, cytochromes c and cytochrome d. Cytochrome function is linked to the reversible redox change from ferrous to the ferric oxidation state of the iron found in the heme core. In addition to the classification by the IUBMB into four cytochrome classes, several additional classifications such as cytochrome o and cytochrome P450 can be found in biochemical literature. Cytochromes were described in 1884 by MacMunn as respiratory pigments. In the 1920s, Keilin rediscovered these respiratory pigments and named them the cytochromes, or “cellular pigments”, he classified these heme proteins on the basis of the position of their lowest energy absorption band in their reduced state, as cytochromes a, b, c. The ultra-violet to visible spectroscopic signatures of hemes are still used to identify heme type from the reduced bis-pyridine-ligated state, i.e. the pyridine hemochrome method.

Within each class, cytochrome a, b, or c, early cytochromes are numbered consecutively, e.g. cyt c, cyt c1, cyt c2, with more recent examples designated by their reduced state R-band maximum, e.g. cyt c559. The heme group is a conjugated ring system surrounding an iron ion; the iron in cytochromes exists in a ferrous and a ferric state with a ferroxo state found in catalytic intermediates. Cytochromes are, capable of performing electron transfer reactions and catalysis by reduction or oxidation of their heme iron; the cellular location of cytochromes depends on their function. They can be found as membrane proteins. In the process of dioxidative phosphorylation, a globular cytochrome cc protein is involved in the electron transfer from the membrane-bound complex III to complex IV. Complex II itself is composed of several subunits, one of, a b-type cytochrome while another one is a c-type cytochrome. Both domains are involved in electron transfer within the complex. Complex IV contains a cytochrome a/a3-domain that transfers electrons and catalyzes the reaction of oxygen to water.

Photosystem II, the first protein complex in the light-dependent reactions of oxygenic photosynthesis, contains a cytochrome b subunit. Cyclooxygenase 2, an enzyme involved in inflammation, is a cytochrome b protein. In the early 1960s, a linear evolution of cytochromes was suggested by Emanuel Margoliash that led to the molecular clock hypothesis; the constant evolution rate of cytochromes can be a helpful tool in trying to determine when various organisms may have diverged from a common ancestor. Several kinds of cytochrome exist and can be distinguished by spectroscopy, exact structure of the heme group, inhibitor sensitivity, reduction potential. Four types of cytochromes are distinguished by their prosthetic groups: There is no "cytochrome e," but cytochrome f, found in the cytochrome b6f complex of plants is a c-type cytochrome. In mitochondria and chloroplasts, these cytochromes are combined in electron transport and related metabolic pathways: A distinct family of cytochromes is the cytochrome P450 family, so named for the characteristic Soret peak formed by absorbance of light at wavelengths near 450 nm when the heme iron is reduced and complexed to carbon monoxide.

These enzymes are involved in steroidogenesis and detoxification. Scripps Database of Metalloproteins Cytochromes at the US National Library of Medicine Medical Subject Headings

Sierra de Gata is a comarca at the northern end of province of Cáceres in Extremadura, one of Spain's seventeen Autonomous Communities. The mountain range of Sierra de Gata has given its name to the comarca; the main town is Moraleja This comarca borders with Portugal in the west in the area of the Portuguese Serra da Malcata Natural Reserve. There are linguistic affinities between neighboring Las Hurdes; the traditional names of the towns and aldeas are in brackets. Acebo Cadalso Cilleros Parrera Descargamaría Eljas El Soto Gata Moheda de Gata Hernán Pérez Hoyos Moraleja Cañadas Malladas La Mata Pedrizas Porciones La Quinta Rozacorderos Perales del Puerto Robledillo de Gata San Martín de Trevejo Santibáñez el Alto Torre de Don Miguel Torrecilla de los Ángeles Valverde del Fresno Vegaviana Villamiel Trevejo Villanueva de la Sierra Villasbuenas de Gata Página oficial de la comarca de Sierra de Gata

The National Negro Network was a black-oriented radio programming service in the United States founded on January 20, 1954 by Chicago advertiser W. Leonard Evans, Jr, it was the first black-owned radio network in the country, its programming was broadcast on up to 45 affiliates. An article in the trade publication Broadcasting said that the network was expected "to reach 12 million of the 15 million Negroes in America."Evans was the network's president. Reggie Schuebel was vice president-treasurer, John M. Wyatt was executive vice president; the network featured a variety of different programming, including a popular soap opera The Story of Ruby Valentine, based on CBS's We Love and Learn and As the Twig is Bent, starred Juanita Hall, Ruby Dee and Terry Carter. The serial was sponsored among others, Philip Morris and Pet Milk. Other short-lived series included The Life of Anna Lewis with Hilda Simms, It's A Mystery Man with Cab Calloway; some shows were produced by Ethel Waters. Other fare included broadcasts of symphony concerts from black colleges, programs hosted by black DJs at affiliate stations.

The network drew up plans for several more series, but—with the TV era exploding—fell apart within a year due to inadequate capital. Jason Chambers wrote in his book, Madison Avenue and the Color Line: African Americans in the Advertising Industry that Evans felt that advertising agencies were hesitant to recommend NNN to clients. "Agencies are aware of our existence and watch our growth closely," Evans said, "but... are still reluctant to come right out and make a recommendation Negro radio, preferring to keep campaigns at a'test' level while watching to see what others do."