Bacillus subtilis (Bs) grows as shown in a fluid thioglycollate or shake tube: It can grow aerobically using glucose or succinate as a carbon and energy source. There are some interesting features about its electron transport chain, though. It only produces the NDH-2 type of electron transport complex I, and so complex I cannot pump H+ . Furthermore, it only produces menaquinone (MQ), not ubiquinone (UQ, or just Q). The energy levels of these various quinones are included in the table shown here.
A. How does mitochondrial complex I pump protons? What would be different about Bs complex I that would not allow it to pump protons?
B. When growing with glucose as carbon and energy source, enough NADH is available, and the NDH-2 complex is used. When growing with only succinate as carbon source, however, there is relatively little NADH produced (Why?), and MQ is reduced by succinate rather than by NADH. Under this condition, if a proton channel is used to cause membrane leaks that decrease the PMF, Bs fails to grow. (It does grow if the same proton channel is used when it is growing on glucose, though.) What is the problem reducing MQ using electrons from succinate? Why doesn’t this problem occur in E. coli, which produces UQ? How does Bs do this? What other example of this phenomenon did we talk about in class?
C. When growing with glucose as carbon and energy source, Bs has 12 rotor subunits in its ATP Synthase. But when growing with succinate as sole carbon and energy source, its ATP Synthase has only 9 rotor subunits. Explain why this difference is necessary to allow growth on succinate.
D. Although Bs grows as shown above in thioglycollate medium, it CAN grow anaerobically if it is provided with fumarate as a terminal electron acceptor. (Thus, in the diagram of the electron transport chain, oxygen is not available, but fumarate is.) What do we call this type of growth? It can only grow this way if glucose is the carbon source, though, not succinate. Explain how Bs growing with glucose as carbon source and fumarate as terminal electron acceptor produces a PMF.
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