PRINCIPLES OF MODERN MICROBIOLOGY
Mark Wheelis
ANSWERS TO STUDYQUESTIONS
Chapter 7
Microbial Metabolism: Fermentation and Respiration
2. Hexoses are commonly fermented via glycolysis, which produces two moles of acetyl-CoA per mole of hexose, with a net yield of two moles of ATP by substrate-level phosphorylation. It also produces two moles of NADH, which has to be reoxidized by reducing some of the acetyl-CoA. In the ethanol/acetate fermentation, ethanol production reoxidizes two moles of NADH per mole of ethanol produced, so to reoxidize the two moles of NADH we need to use one mole of acetyl-CoA for this purpose. The other mole of acetyl-CoA can be converted to one mole of acetate, with the production of one mole of ATP. Thus this fermentation yields two moles of formate, one mole of acetate, and one mole of ethanol per mole of hexose fermented. The ATP yield is three moles per mole of hexose.
4.
Under high oxygen tensions, E. coli would pump eight protons per NADH oxidized (4 by NADH dehydrogenase, 4 by quinol oxidase), equivalent to 2.67 ATP per NADH. Under very low oxygen tension, it would pump only 2 protons per NADH oxidized (none by NADH dehydrogenase, 2 by quinol oxidase), for a total of ).67 ATP per NADH.
6.
Under high-oxygen condition, Paracoccus pumps 10 protons per NADH oxidized (4 by NADH dehydrogenase, 4 by the cytochome b/c complex, and 2 by the cytochrome oxidase). This is equivalent to 3.33 ATP per NADH oxidized. This is the same number of protons pumped as in the eucaryotic mitochondrion, which is not surprising considering that the mitochondrion and
Paracoccus share a common ancestor. However, in the mitochondrion, ATP yields are slightly lower due to the requirement to actively transport phosphate into the mitochondrion from the cytoplasm.
8. The Calvin-Benson cycle requires two NADPH and three ATP to fix one molecule of CO
2
. Two molecules of H
2
S have to be oxidized to provide the electrons for the two NADPH. These electrons are driven through NADH DH (requiring 4 protons per pair of electrons, or 8 protons total), and then transhydrogenase (requiring 2 protons per pair of electrons, or 4 total). These proton needs are partially offset by the 4 protons released into the periplasm by H
2
S oxidation, and consumed in the cytoplasm by CO
2
fixation. Thus we will need to pump out 8 protons (12-4) by respiration to provide the motive force for reverse electron transport. In addition, respiration will have to pump out 9 protons to make the 3 ATP we need. Thus we need to pump a total of 17 protons, and these have to be pumped by forward electron transport driven by H
2
S oxidation. Each H
2
S oxidized pumps 6 protons (4 by the cytochrome complex and 2 by cytochrome oxidase), as well as contributing 2 by
H
2
S oxidation in the periplasm and oxygen reduction in the cytoplasm, for a total of 8 protons per
H
2
S oxidized. Thus we need to oxidize 2.1 (two and one eighth) H
2
S to provide the protonic potential to drive ATP synthesis and reverse electron transport, for a grand total of 4.1 H
2
S per CO
2 fixed.