Which of the following statements is true?
(a) Ubiquinone is a small, hydrophobic protein containing a metal group that acts as an electron carrier.
(b) A 2Fe2S iron–sulfur center carries one electron, whereas a 4Fe4S center carries two.
(c) Iron–sulfur centers generally have a higher redox potential than do cytochromes.
(d) Mitochondrial electron carriers with the highest redox potential generally contain copper ions and/or heme groups.
Choice (d) is the correct answer. Cytochrome c oxidase, which is the last carrier in the mitochondrial electron-transport chain and therefore has the highest redox potential, contains copper ions and a heme group. Ubiquinone is not a protein and does not contain a metal group [choice (a)]. Both 2Fe2S and 4Fe4S centers carry one electron [choice (b)]. Iron–sulfur centers generally have a lower redox potential than do cytochromes [choice (c)]. The heme group in cytochrome c contains a charged iron ion. The interiors of proteins are often hydrophobic, favoring a relatively high redox potential, because reduction of the iron ion decreases its charge, and charges are energetically unfavorable in a hydrophobic environment.
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