Cytochromes

Cytochromes are a family of heme-containing proteins that play a central role in biological electron transfer. The iron atom in the heme group cycles between ferrous (Fe2+) and ferric (Fe3+) states, enabling the controlled flow of electrons between redox partners. Cytochromes are essential components of cellular respiration and photosynthesis and are found across bacteria, archaea, and eukaryotes.

Cytochromes are often classified by the type of heme they contain. C-type cytochromes have covalently attached heme groups bound to the apoprotein through thioether bonds formed with cysteine residues in the CXXCH motif. B-type cytochromes contain two hemes of type b and are typically bound non-covalently. A-type cytochromes carry heme a and are characteristic of several steps in mitochondrial respiration, notably in Complex IV. Other members include various d- and e-type cytochromes that occur in different organisms and cellular contexts.

In mitochondria, cytochromes participate in the electron transport chain, with cytochrome c acting as a small, soluble carrier that transfers electrons between Complex III (cytochrome bc1) and Complex IV (cytochrome c oxidase). In chloroplasts and photosynthetic bacteria, cytochromes such as cytochrome b6f components and cytochrome f mediate electron transfer between photosynthetic complexes, contributing to the formation of a proton gradient used to synthesize ATP.

Structural and functional properties of cytochromes are tuned by the nature of the heme group, its coordination, and the surrounding protein environment. Their redox potentials span a range that supports sequential electron transfer in biological energy conversion processes. The term cytochrome derives from the historical observation of their colors, and these proteins remain important tools in research and biotechnology as redox mediators and spectroscopic probes.