Redox loop model

However, some of the properties of electron carriers (such as their observed redox potentials) do not fit in such a simple loop model. This has led Mitchell [11] to propose a modified mechanism, the so-called proton-motive Q cycle (Fig. 4B). In this model quinones function in two separate reactions in the QH2/QH- and the Q/QH- couple. These couples have different midpoint redox potentials and would operate at the reducing and the oxidizing site of cytochrome b, respectively. During these reactions proton translocation is supposed to occur by diffusion of the quinones in the fully oxidized (Q) and fully reduced (QH2) forms through the hydrophobic environment between their successive reaction sites at both sides of the membrane. Recently some experimental support for such a role of quinones has been obtained. Alternative models which will not be discussed here, have been postulated by Papa [12] and Williams [13]. Currently there is no conclusive support for a specific model. 

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