biochemistry Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...

, electron bifurcation (EB) refers to a system that enables an unfavorable (

endergonic In chemical thermodynamics, an endergonic reaction (; also called a heat absorbing nonspontaneous reaction or an unfavorable reaction) is a chemical reaction in which the standard change in free energy is positive, and an additional driving fo ...

) transformation by coupling to a favorable (

exergonic An exergonic process is one which there is a positive flow of energy from the system to the surroundings. This is in contrast with an endergonic process. Constant pressure, constant temperature reactions are exergonic if and only if the Gibbs fr ...

) transformation. Two electrons are involved: one flows to an acceptor with a "higher reduction potential and the other with a lower reduction potential" than the donor. The process is suspected of being common in bioenergetics. Two versions of EB are recognized. One involves redox of quinones and the other involves flavins. Quinones and flavins are cofactors that are capable of undergoing 2 – 2 proton redox. A pervasive example of electron bifurcation is the

Q cycle The Q cycle (named for ''quinol'') describes a series of reactions that describe how the sequential oxidation and reduction of the lipophilic electron carrier, Coenzyme Q (CoQ), between the ubiquinol and ubiquinone forms, can result in the net mov ...

, which is part of the machinery that results in oxidative phosphorylation. In that case one electron from

ubiquinol A ubiquinol is an electron-rich (reduced) form of coenzyme Q (ubiquinone). The term most often refers to ubiquinol-10, with a 10-unit tail most commonly found in humans. The natural ubiquinol form of coenzyme Q is 2,3-dimethoxy-5-methyl-6-poly ...

is directed to a Rieske cluster and the other electron is directed to a

cytochrome b Cytochrome b within both molecular and cell biology, is a protein found in the mitochondria of eukaryotic cells. It functions as part of the electron transport chain and is the main subunit of transmembrane cytochrome bc1 and b6f complexes. F ...

.{{Lehninger4th

References

Thermodynamic processes Chemical thermodynamics Iron–sulfur proteins