A Biochemistry Question

Does anyone know a real-world example of a cycle like this:

or in other words, this:

\begin{array}{ccc}  \mathrm{A} + \mathrm{C}_1 \longrightarrow \mathrm{C}_2 \\   \mathrm{X} + \mathrm{C}_2 \longrightarrow \mathrm{C}_3  \\    \mathrm{C}_3 \longrightarrow \mathrm{B} + \mathrm{C}_4   \\    \mathrm{C}_4 \longrightarrow \mathrm{Y} + \mathrm{C}_1   \end{array}

where the reaction

\mathrm{A} \to \mathrm{B}

is exergonic (i.e., involves a decrease in free energy) while

\mathrm{X} \to \mathrm{Y}

is endergonic (i.e., involves a free energy increase)?

The idea is that the above cycle, presumably catalyzed so that all the reactions go fairly fast under normal conditions, ‘couples’ the exergonic reaction, which ‘wants to happen’, to the endergonic reaction, which doesn’t… thus driving the endergonic one.

I would love an example from biochemistry. This is like a baby version of much more elaborate cycles such as the citric acid cycle, shown here:

in a picture from Stryer’s Biochemistry. I’m writing a paper on this stuff with Jonathan Lorand, Blake Pollard and Maru Sarazola, and we have—presumably obvious—reasons to want to discuss a simpler cycle!

11 Responses to A Biochemistry Question

  1. Perez-Mercader, Juan says:

    Hi John,

    the first figure does not show in my Safari.

    Could you resend?



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