Question

What proteins of the electron-transport chain contain iron and in what form? Select all that apply.

a. nonheme iron proteins; in the form of a nonheme prosthetic group

b. cytochromes; in the form of Fe–S clusters

c. iron–sulfur proteins; in the form of Fe–S clusters

d. iron–sulfur proteins; in the form of a heme prosthetic group

e. cytochromes; in the form of a heme prosthetic group

Answer 1

The correct options are c (iron- sulfur protein in the form of Fe-S clusters) and e (cytochromes; in the form of a heme prosthetic group).

Fe- S proteins are one of the major players in the electron transport chain. Iron- sulfur clusters are a major component of Complex I,II and III. They are non heme proteins. They contain 1-4 no of iron linked to sulfur of cysteine amino acid of the protein. Iron gets converted into Fe2+ and Fe3+ during oxidation reduction reactions. It transfer single electron during a cycle.

Iron present in cytochrome is in heme form. The iron here is tightly bound (covalently) and acts as prosthetic group to cytochrome. It undergoes oxidation reduction cycle easily. Heme group of cytochrome also transfers one electron at a time. Cytochrome containing Fe as heme prosthetic group plays a major role in Q cycle as well as in complex IV (cytochrome oxidase system). Reduced Fe of cytochrome is finally oxidized by cytochrome oxidase with simultaneous reduction of molecular oxygen to water.

Option A is not correct- non heme proteins in the form of nonheme prosthesic group is not found in electron transport chain.

Option b is not correct- Cytochromes contain Fe as in the form of heme prosthetic group not in the form of Fe-S clusters . Hence, this option is incorrect.

Option D is incorrect.- Iron sulfur proteins are found in form of non heme clusters bound to amino acid cysteine of protein, not in form of heme prosthetic groups. Hence this option is incorrect.