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Figure 2. How electron transfer down a membrane-embedded respiratory
chain might pump protons. In order to account for this process,
Mitchell postulated the existence of “loops” in the respiratory chain.
Translated into English, this means alternation of electron/proton carriers
with electron carriers across the membrane. More specifically, a
flavoprotein hydrogenase on the inner surface of the membrane [(Fp); upon
reduction, FAD or FMN accepts electrons and protons] transfers its
electron to a non-heme iron sulfur protein (NHFeS) disposed towards the
outer surface of the membrane (iron accepts electrons only), and the
proton is released into the external medium. The NHFeS then transfers
electrons to ubiquinone (Q) on the inner surface of the membrane which picks
up protons from the internal solvent. QH2 then transfers
electrons to cytochrome b (CYT b) towards the outer surface which accepts
only electrons, and 2 protons are released into the external medium.
Finally, CYT. B transfers electrons to the terminal oxidase (CYT ox) on the
inner surface which reduces oxygen on the inside utilizing another 2 protons
to make water. |