General Microbiology - Schlegel H. 1987

Electron transport under anaerobic conditions
Succinate formation via fumarate reduction

We have repeatedly discussed succinate (succinic acid) as a Fermentation product, but we have not yet mentioned that succinate formation can also be linked to Oxidative Phosphorylation. Succinate is produced via the reduction of fumarate:

Class="center">2 [Н] + Фумарат → Сукцинат

The function of fumarate is not limited to acting as a simple electron acceptor that interacts with NADH₂ generated during hexose oxidation. The redox potential for the fumarate/succinate couple is E'₀ = - 30 mV. Fumarate can accept electrons supplied by hydrogen-carrying Coenzymes that have already traversed part of the Respiratory Chain; thus, it enables oxidative phosphorylation. This type of phosphorylation, with fumarate serving as the terminal electron acceptor, can be classified as an "anaerobic respiratory process" and is referred to as fumarate Respiration (Fig. 9.8).

Fig. 9.8. Formation of succinate from fumarate by membrane-bound fumarate reductase. This process can be coupled with the generation of a proton motive force across the cytoplasmic membrane, thereby enabling oxidative phosphorylation.

Fumarate respiration is widespread among chemoorganotrophic anaerobic Bacteria. Supplementing the growth medium with fumarate stimulates rapid growth in many bacteria and increases biomass yield. This indicates that fumarate enables efficient ATP regeneration. Corresponding results have been obtained for enterobacteria (Escherichia, Proteus, Salmonella, Klebsiella), as well as Bacteroides, Propionibacterium, and Vibrio succinogenes (see Fig. 8.8). Under anaerobic conditions, exogenous fumarate added to the medium can serve as a terminal electron acceptor for bacteria. However, it can also be produced endogenously from CARBOHYDRATES and many other substrates via oxaloacetate and malate (see Fig. 7.13).

The cellular localization of fumarate reductase is consistent with its proposed role in electron transport: the enzyme is membrane-bound. The reduction of fumarate is coupled with the consumption of protons. Based on the hypothesis that fumarate reduction leads to the generation of a proton motive force (with a positive charge on the outside), it is predicted that fumarate reductase must be located on the inner surface of the membrane. This appears to be the case. Measurements have shown that the reduction of 1 mole of fumarate yields 1 mole of ATP.

It is likely that fumarate reductase participates in most fermentation processes involving succinate formation, resulting in additional ATP synthesis. Examples of such processes include succinate production in Escherichia coli and propionate production in Propionibacterium. However, "fumarate respiration" is not restricted to prokaryotes. Several species of facultatively anaerobic worms (Ascaris lumbricoides, Fasciola hepatica, Trichuris vulpis, Arenicola marina) are capable of surviving under anaerobic conditions. These organisms excrete succinate and propionate, with propionate formation proceeding via the methylmalonyl-CoA pathway (see Fig. 8.3). The Role of succinate and propionate production in lower animals is apparently analogous to that of Lactic acid fermentation in mammalian Muscle. Various Types of fermentation, as well as fumarate respiration, can provide a certain amount of energy even to strictly aerobic organisms, thereby enabling them to survive brief periods of anoxia.