Question

Question:The ultimate electron acceptor in the respiration process is molecular oxygen. Electron transfer through the respiratory chain takes place through a complex series of oxidation– reduction reactions. Some of the electron transport steps use iron-containing proteins called cytochromes. All cytochromes transport electrons by converting the iron in the cytochromes from the 13 to the 12-oxidation state. Consider the following reduction potentials for three different cytochromes used in the transfer process of electrons to oxygen (the potentials have been corrected for pH and temperature):

$$\begin{gathered} \mathbf{cytochrome}\mathbf{}\mathbf{a}\left({\mathrm{Fe}}^{+3}\right)\mathbf{+}{\mathbf{e}}^{\mathbf{-}}\mathbf{\to }\mathbf{cytochrome}\mathbf{}\mathbf{a}\left({\mathrm{Fe}}^{+2}\right)\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{E}\mathbf{=}\mathbf{0}\mathbf{.}\mathbf{385}\mathbf{V} \\ \mathbf{cytochrome}\mathbf{}\mathbf{b}\mathbf{\left(}{\mathbf{Fe}}^{\mathbf{+}\mathbf{3}}\mathbf{\right)}\mathbf{+}{\mathbf{e}}^{\mathbf{-}}\mathbf{\to }\mathbf{cytochrome}\mathbf{}\mathbf{b}\mathbf{\left(}{\mathbf{Fe}}^{\mathbf{+}\mathbf{2}}\mathbf{\right)}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{E}\mathbf{=}\mathbf{0}\mathbf{.}\mathbf{030}\mathbf{V} \\ \mathbf{cytochrome}\mathbf{}\mathbf{c}\mathbf{\left(}{\mathbf{Fe}}^{\mathbf{+}\mathbf{3}}\mathbf{\right)}\mathbf{+}{\mathbf{e}}^{\mathbf{-}}\mathbf{\to }\mathbf{cytochrome}\mathbf{}\mathbf{c}\mathbf{\left(}{\mathbf{Fe}}^{\mathbf{+}\mathbf{2}}\mathbf{\right)}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{E}\mathbf{=}\mathbf{0}\mathbf{.}\mathbf{254}\mathbf{V} \end{gathered}$$

In the electron transfer series, electrons are transferred from one cytochrome to another. Using this information, determine the cytochrome order necessary for spontaneous transport of electrons from one cytochrome to another, which eventually will lead to electron transfer to O₂.

Short answer

Cytochrome b < cytochrome c < cytochrome a

Step-by-step solution

Analyzing the given data

$$\begin{gathered} \mathrm{cytochrome}\mathrm{a}\left({\mathrm{Fe}}^{+3}\right)+{\mathrm{e}}^{-}\to \mathrm{cytochrome}\mathrm{a}\left({\mathrm{Fe}}^{+2}\right)\mathrm{E}=0.385\mathrm{V} \\ \mathrm{cytochrome}\mathrm{b}\left({\mathrm{Fe}}^{+3}\right)+{\mathrm{e}}^{-}\to \mathrm{cytochrome}\mathrm{b}\left({\mathrm{Fe}}^{+2}\right)\mathrm{E}=0.030\mathrm{V} \\ \mathrm{cytochrome}\mathrm{c}\left({\mathrm{Fe}}^{+3}\right)+{\mathrm{e}}^{-}\to \mathrm{cytochrome}\mathrm{c}\left({\mathrm{Fe}}^{+2}\right)\mathrm{E}=0.254\mathrm{V} \end{gathered}$$

Arranging the cytochromes in the order necessary for spontaneous transport of electrons

We know that the transport of e^- happens due to the increase in reduction potential.Arranging the cytochromes in the increasing order of their reduction potential, we get:

0.03 V < 0.254 V < 0.385 V

Cytochrome b < cytochrome c < cytochrome a