Figure 3.

(A). Generalized, traditional model of non-transferrin bound iron uptake. A reversible redox couple provides electrons to the ferri-reductase, through its oxidation from a reduced to an oxidized form. Facilitated by the reductase, these electrons are used to reduce two molecules of ferric ion to form ferrous ion in the extracellular compartment. This ferrous ion is co-transported across the cell membrane with H⁺ ions, into the intracellular space; this is facilitated by the DMT1 transporter. The DMT1 transporter also acts as a uniporter, and “leaks” H⁺ ions to the intracellular compartment from the extracellular compartment. (B) Ubiquinone shuttle of NADH-ferri-reductases. It has been shown that coenzyme Q can facilitate electron transport through the membrane of cells for the NADH-ferri-reductases (Oakhill et al., 2008). This schematic proposes a method of how this can occur, based upon the mechanisms of coenzyme Q electron shuttling within complex I and II of the electron transport chain. (C) Schematic of the favored electron transfer pathway in a general transplasma membrane electron transport system according to the (in some cases approximated) standard redox potential at pH 7.