Hemes of the Electron Transport Chain
There are 7 hemes in the mammalian mitochondrial ETC: heme a and heme a3
in cytochrome oxidase, cytochrome c, heme c1 which is part of the high potential chain in the
bc1
complex, hemes
bL
and
bH
which are part of the low potential chain in the
bc1
complex and heme
b560
in complex II.
The high absorption originates from the heme porphyrin ring and the spectrum changes when the iron is reduced (Fe2+) or oxidized (Fe3+). Typically, the reduced heme has a strong peak in the α-band, and the oxidized heme has a weaker broader spectrum. We use the reduced minus oxidized difference spectrum to fit the change in attenuation giving us concentration changes from baseline (Figure 1). Heme b560 in complex II has a very low midpoint potential and remains fully oxidized in the absence of an extrinsic strong reductant and so does not contribute to the change in attenuation spectrum under physiological conditions.