Hemin a.k.a. ferriprotoporphyrin IX
accepts single electrons e^-
the H nucleus from ½“H₂” is liberated as a proton in solution

hemin is a prosthetic group, tightly -- or, in the case of cytochrome c, covalently -- bound to the cytochromes which utilize it;

the ability of CoQ/CoQH₂ to function as either a one- or two-electron donor/acceptor is critical in the changeover from the “H₂”-transporting portion [Complexes I and II] to the e^- -only portion [complexes III and IV] of the ETS

basic reaction: Fe³⁺ + e- à Fe²⁺

sample reactions: CoQH₂ + Fe³⁺ à CoQH• + Fe²⁺ + H⁺
CoQH• + Fe³⁺ à CoQ + Fe²⁺+ H⁺

Hemin [Fe³⁺] found in the oxidized form of cytochrome b
In hemoglobin, oxidation of the iron to the 3+ state renders the molecule incapable of transporting O₂, but for cytochromes both 3+ and 2+ states are critical for their function.

Heme [Fe²⁺] as found in hemoglobin, myoglobin, and the reduced forms of b-type cytochromes

Hemin c, found in cytochromes c and c₁, where it is covalently bound to two cysteine residues as shown [their thiol groups have added to the vinyl C=C of regular heme]

Heme a, found in cytochromes a & a₃ of Complex IV
note the long, hydrophobic side chain on ring A

See a chime structure of heme at http://www.wellesley.edu/Chemistry/Flick/molecules/heme.html
or http://www.molecularmodels.ca/molecule/modelfiles/jb09heme.html

See cytochrome c in Chime at http://c4.cabrillo.cc.ca.us/projects/viewers/index.html
>> Macro Viewer 800 x 600 >> cytochrome c