Fumarate respiration occurs in a wide range of bacteria growing anaerobically.48 This is prob-ably because fumarate itself is formed from carbohydrates and protein during growth.
We describe the electron transport pathway in W. succinogenes, a gram-negative anaerobe iso-lated from the rumen, by reference to Fig. 5.17.
W. succinogens can grow at the expense of H2 or formate, both produced in the rumen by other bacteria. The electron transport pathway is shown in Fig. 5.17A. The active sites for both the hydrogenase and the formate dehydroge-nase are periplasmic, whereas the active site for the fumarate reductase is cytoplasmic. An examination of the topology of the components of the respiratory chain reveals how a ∆p is generated.
Topology of the components of the electron transport pathway
The electron transport chain consists of a periplasmic enzyme that oxidizes the elec-tron donor, a membrane-bound menaquinone (MQ) that serves as an intermediate elec-tron carrier, and membrane-bound fumarate reductase, which accepts the electrons from the menaquinone and reduces fumarate on the cytoplasmic side of the membrane (Fig. 5.17B).
Both the hydrogenase and the formate dehydro-genase are made of three polypeptide subunits, two facing the periplasm and one an integral membrane protein (cytochrome b). Note that cytochrome b not only serves as a conduit for electrons, but also binds the dehydrogenases which is assimilated via the Calvin cycle: see
Chapter 14.) The electrons are transferred from the dehydrogenase to c-type cytochromes in the periplasm. The cytochromes c transfer the electrons to cytochrome aa3 oxidase in the membrane, which reduces oxygen to water on the cytoplasmic surface. A ∆p is established as a result of the inward fl ow of electrons and the outward pumping of protons by the cytochrome aa3 oxidase, as well as the release of protons in the periplasm during methanol oxidation and consumption in the cytoplasm during oxygen reduction. Since the oxidation of methanol bypasses the bc1 coupling site, the ATP yields are lower. In addition to methanol, the bacteria can grow on methylamine (CH3NH+3), which is oxidized by methylamine dehydrogenase to formaldehyde, NH+4 , and 2H+:
CH3NH+3 + H2O → HCHO + NH+4 + 2H+ Methylamine dehydrogenase is also located in the periplasm and donates electrons via cyto-chromes c to cytochrome aa3, bypassing the bc1 complex.
Periplasm cytoplasm
CH3OH
2 x 1e -cyt c551
cyt c
2H+ 2H+
1 2O2 + 2H+ H2O
cyt aa3 2 x 1e -cell membrane
HCHO + 2H+
methanol dehydrogenase
Fig. 5.16 Oxidation of methanol by P. denitrifi cans.
Methanol is oxidized to formaldehyde by a periplas-mic methanol dehydrogenase. The electrons are transferred to periplasmic cytochromes c and to a membrane-bound cytochrome aa3, which is also a proton pump. A ∆p is created as a result of the elec-trogenic infl ux of electrons and the elecelec-trogenic effl ux of protons, accompanied by the release of protons in the periplasm (methanol oxidation) and uptake in the cytoplasm (oxygen reduction).
into the membrane. The two periplasmic sub-units of the hydrogenase are a Ni-containing protein subunit and an iron–sulfur protein. The two periplasmic subunits of the formate dehy-drogenase are a Mo-containing protein subunit and an iron–sulfur protein.
The fumarate reductase is a complex contain-ing three subunits. One subunit of the fumar-ate reductase is a fl avoprotein with FAD as the prosthetic group (subunit A). A second subunit has several FeS centers (subunit B). And the third subunit has two hemes of the b type (sub-unit C), which binds the fumarate reductase to the membrane. Fumarate reductase is similar in structure to succinate dehydrogenase isolated from several different sources, which catalyzes the oxidation of succinate to fumarate in the cit-ric acid cycle.
Electron fl ow and the establishment of a Δp Electron fl ow is from the dehydrogenase or hydrogenase to cytochrome b to menaquinone to fumarate reductase. Two electrons are elec-trogenically transferred across the membrane to fumarate for every H2 or formate oxidized, leaving two protons on the outside, thus estab-lishing a ∆p. In a study of whole cells, a value of 1.1 was obtained for the ratio of protons to electrons during fumarate reduction, sug-gesting perhaps that a mechanism of proton translocation through the membrane may also exist.48 If one assumes that 1.1 is a correct num-ber, and also assumes a stoichiometry for the ATP synthase of 3H+/ATP, then the theoretical maximum number of ATPs that can be formed from the transfer of two electrons to fumarate is 2.2/3, or 0.73. The actual number measured by Fig. 5.17 A model for the electron transport system of Wolinella succinogenes. (A) Electrons fl ow from H2 and formate through menaquinone (MQ) to fumarate reductase. (B) Illustration that the catalytic portions of hydrogenase and formate dehydrogenase are periplasmic, whereas fumarate reductase reduces fumarate on the cytoplasmic side. Electrons fl ow electrogenically to fumarate. A ∆p is created because of electrogenic infl ux of electrons together with the release of protons in the periplasm and their consumption in the cyto-plasm. Source: Modifi ed from Kroger, A., V. Geisler, E. Lemma, F. Theis, and R. Lenger. 1992. Bacterial fumarate respiration. Microbiology 158:311–314.
used when oxygen levels are high, and to cyto-chrome d, which is used when oxygen becomes limiting. Other bacteria may have in addition, or instead, an electron transport pathway in which electrons travel from reduced quinone through a bc1 complex, to cytochrome aa3, which reduces oxygen. This is the same as the electron transport pathway found in mitochon-dria, although the carriers may not be identi-cal. The alternative branches may differ in the number of coupling sites, and this could have regulatory signifi cance regarding the rates of oxidation of reduced electron carriers, as well as ATP yields.
Another difference from mitochondria is that bacteria can have either aerobic or anaerobic electron transport chains; or, as is the case with facultative anaerobes such as E. coli, either can be present, depending upon the availability of oxgyen or alternative electron acceptors. A hierarchy of electron acceptors is used. For E.
coli, oxygen is the preferred acceptor, followed by nitrate, and fi nally fumarate.
With respect to cytochrome c oxidase, there are two classes. Cytochrome aa3 is the major class and has been reported in many bacteria, includ-ing Paracoccus denitrifi cans, Nitrosomonas europaea, Pseudomonas AM1, Bacillus subti-lis, and Rhodobacter sphaeroides. A different cytochrome c oxidase has been reported for Azotobacter vinelandii, Rhodobacter capsulata, R. sphaeroides, R. palustris, and Pseudomonas aeruginosa, as well as P. denitrifi cans.13,49 Apparently, both classes of cytochrome c oxi-dase coexist in the same organism and serve as alternate routes to oxygen.
The main energetic purpose of the respira-tory electron transport pathways is to convert a redox potential (∆Eh) into a proton potential (∆p). This is done at coupling sites. A mem-brane potential is created by electrogenic infl ux of electrons, leaving the positively charged proton on the outside, or during electrogenic effl ux of protons during proton pumping, leav-ing a negative charge on the inside. Infl ux of electrons occurs when oxidations take place on the periplasmic membrane surface or in the periplasm, and electrons move vectorially across the membrane to the cytoplasmic sur-face, where reductions take place. This occurs in two situations: (1) when the substrate (e.g., H2, methanol) is oxidized by dehydrogenases experimentation was 0.56. Note that the
qui-none functions as an electron carrier between the cytochromes b but does not take part in hydrogen translocation across the membrane as in a Q loop or Q cycle.
5.8 Summary
All electron transport schemes can be viewed as consisting of membrane-bound dehydrogenase complexes, such as NADH dehydrogenase or succinate dehydrogenase, that remove electrons from their substrates and transfer the electrons to quinones, which in turn transfer the electrons to oxidase or reductase complexes. The latter complexes reduce the terminal electron accep-tors. In contrast to mitochondria, which all have the same electron transport scheme, bac-teria differ in the details of their electron trans-port pathways, although the broad outlines of all such schemes are similar. In bacteria, the dehydrogenase, oxidase, and reductase com-plexes are sometimes referred to as modules because specifi c ones are synthesized under cer-tain growth conditions and “plugged into” the respiratory pathway. For example, in faculta-tive anaerobes such as E. coli, the oxidase mod-ules are synthesized in an aerobic atmosphere and the reductase modules under anaerobic conditions.
Other dehydrogenases besides NADH dehy-drogenase and succinate dehydehy-drogenase exist.
These oxidize various electron donors (e.g., methanol, hydrogen, formate, H2, glycerol) and are located in the periplasm or the cyto-plasm. The coenzyme or prosthetic groups for these soluble dehydrogenases vary (e.g., they may be NAD+ or fl avin). The electrons from the various dehydrogenases are transferred to one of the electron carriers (e.g., quinone, cyto-chrome) and from there to a terminal reductase or oxidase.
An important difference between elec-tron transport chains in bacteria and those in mitochondria is that the former are branched.
Branching can occur at the level of quinone or cytochrome. The branches lead to different oxi-dases or reductases, depending upon whether the bacterium is growing aerobically or anaer-obically. Many bacteria, including E. coli, transfer electrons from reduced quinone to cytochrome o, which is the major cytochrome
3. What are two features that distinguish