Anaerobic metabolism, catabolic reactions

A summary of the major catabolic and amphibolic reactions for the anaerobic metabolism of glucose is presented in Fig. 17.11. 

ANAEROBIC CARBOHYDRATE METABOLISM Yeasts growing in media containing high concentrations of fermentable carbohydrate invariably metabolize it fermentatively to produce ethanol and CO2. If air is present, and when the sugar concentration has been lowered, the ethanol is respired using the metabolic routes described above. Under the anaerobic conditions of a brewery fermentation the hexoses derived from wort fermentable carbohydrates are catabolized by the EMP pathway (Fig. 17.2) to pyruvic acid. The pyruvate produced is decarboxylated by the enzyme pyruvate decarboxylase, with the formation of acetaldehyde and CO2. The enzyme requires the cofactor thiamine pyrophosphate (TPP) for activity and the reaction is shown in Fig. 17.10. The acetaldehyde formed acts (in the absence of the respiratory chain) as an electron acceptor and is used to oxidize NADH with the formation of ethanol ... 

It has been established (Delwiche and Carson, 1953) that propionic acid bacteria are able to oxidize the intermediate products of the TCA cycle. Under anaerobic conditions the TCA cycle is also functional, and its role may not be limited to anabolic processes. In these conditions nitrate and fumarate can act as terminal electron acceptors in propionic acid bacteria. It is well known that the TCA cycle provides microorganisms with precursors for biosynthetic reactions, and plays an essential role in both the catabolic and anabolic metabolism. 

It is a common statement that life is always connected to the production of heat. That is true in most cases, perhaps with one (known) exception, i.e. the anaerobic production of CH and CO2 from acetate. In this process the entropy term outweighs the heat term and, at least in theory, life could be connected to net heat absorption [23]. However, if we ignore this exception and instead concentrate on what part of metabolism generates the heat. Is it due to catabolic or anabolic reactions The answer is in fact both catabolism and anabolism is involved. 

Figure 53. Half cycles in dissipative maintenance metabolism with steady state ATP turnover, decoupled by futile cycling. The fhictose 6-phosphate/fructose 1,6-bisphosphate cycle is shown as an example. The net enthalpy change is calculated from the net biochemical change which, at steady state levels of ATP and all anabolic intermediates, is exclusively due to the catabolic half cycle reaction, equivalent to uncoupled catabolism (oxycaloric equivalent), Enthalpy is intermittently conserved in endothermic half cycles (p, phosphorylation a, anabolic), but an equivalent amount of enthalpy is exothermic in the reversed exergonic half cycles (-p, dephosphorylation d, dissipative). Therefore, ATP turnover and futile cycling raise the heat flux strictly proportional to the catabolic flux which, however, can be augmented by anaerobic catabolism with a corresponding anaerobic contribution to total heat flux (Reproduced from Reference [25] with permission).

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