A Tour of Cytochrome C


Please be patient while the structures in the left frame load. In order to display all of the structures in the tour properly, press 'View' buttons below in order (from 1 to the end).



 

This is the oxidized form of cytochrome C. The major function of cytochrome C is as an electron carrier in the process of respiration. In the electron transport chain, cytochrome C accepts an electron from the Fe2+ centre in Complex III and then donates an electron to a Cu2+ centre in cytochrome C oxidase in Complex IV.

The protein contains a Fe3+ ion in a heme C group (Fe-porphyrin) surrounded by the protein chain shown as a ribbon with helical regions coloured in blue.

Positively charged lysines form a ring around an exposed edge of the heme c group. This positive surface patch is used to align the electron transfer "hot spot" (the heme) containing the iron(III) near a negative surface patch on the electron donor protein (not shown). Since the Fe donor-acceptor orbitals can mix with the porphyrin π * system, the effective d-electron density is extended to the edge and thus to the surface of the protein. This allows fast electron transfer to occur.

This display highlights the amino acids that are coordinated to the heme group. The porphyrin ring is attached to the polypeptide chain at two cysteines through thioether bridges (Cys14 and Cys17). The Fe ion lies in the plane of the ring, and the two axial positions are occupied by sulfur from methionine (Met91) and nitrogen from histidine (His18).

This is the core of the oxidized form and the next display will show the core of the reduced form. There is very little strucural change on going from Fe3+ to Fe2+ and back again. Here the Fe3+ is octahedral with Fe-N and Fe-S bond lengths of 1.99 Å and 2.37 Å, respectively.

The bond distances in the reduced form Fe2+ are longer. The Fe-N and Fe-S bond lengths are 2.18 Å and 2.43 Å, respectively, compared with 1.99 and 2.37 Å for Fe3+. Nature minimizes such structural changes to make electron transfer fast.

This is the oxidized form of cytochrome C to explore.

The crystal structure of cytochrome C was reported by F.R.Salemme et al. in J.Biol.Chem. 1973, 248, 3910.


Copyright Robert H. Morris, Adrian Lee and Alen Hadzovic, 1998, 2009, 2011.