Krebs cycle anaerobic functions

Most anaerobically functioning mitochondria use endogenously produced fumarate as a terminal electron-acceptor (see before) and thus contain a FRD as the final respiratory chain complex (Behm 1991). The reduction of fumarate is the reversal of succinate oxidation, a Krebs cycle reaction catalysed by succinate dehydrogenase (SDH), also known as complex II of the electron-transport chain (Fig. 5.3). The interconversion of succinate and fumarate is readily reversible by FRD and SDH complexes in vitro. However, under standard conditions in the cell, oxidation and reduction reactions preferentially occur when electrons are transferred to an acceptor with a higher standard redox potential therefore, electrons derived from the oxidation of succinate to fumarate (E° = + 30 mV) are transferred by SDH to ubiquinone,... 

As we have seen elsewhere, the cell is divided into various compartments, nucleus, mitochondria, cytosol and so on, each one separated from the rest by membrane barriers. If everything could move freely across these membranes there would be no point in having them. They would be bfce national borders with no one checking passports, visas, smuggling, etc. The organelles keep certain processes inside their boundaries, e.g. the Krebs cycle, which takes place exclusively inside the mitochondria, or anaerobic glycolysis exclusively in the cytosol, etc. On the other hand, if there were no communication and traffic between compartments, there would in effect be no cell and no organism - the whole has to be more than the mere sum of its parts, and these parts are not set up to be autonomous but to carry out specialised functions as part of an overall team effort. 

Although not completely functional, certain enzymes Krebs cycle are, active under anaerobic conditions (Fig. 1.11). These additional pathways are very important to Saccharomyces during fermentation because NADH can be reoxidized and other precursors important for cellular functions synthesized (e.g., a-keto-glutarate involved in NH4 assimilation). 

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