Microbes and Metabolism - Environmental Biotechnology: Theory and Application - page 35

by association with electron transport. For 1mol of glucose passing through
glycolysis by the Embden-Meyerhof pathway to produce 2mol of pyruvate, there
is net production of 2mol of ATP. For most fermentation pathways, no further
ATP is synthesised. There are exceptions, of course, such as the conversion of an
acyl CoA derivative such as acetyl CoA or butyryl CoA to the free acid which in
these cases are acetate and butyrate respectively. Each of these reactions releases
sufficient energy to drive the phosphorylation of 1mol of ADP. Conversely, if the
electron transport chain is functioning, NADH may be oxidised by relinquishing
electrons to the cytochromes in the chain thus regenerating the oxidised cofactor.
In this scenario, pyruvate may enter the TCA cycle rather than a fermentation
route, thus a further mol of ATP is produced at substrate level during conver-
sion of succinyl CoA to succinate via GTP, which then transfers the terminal
phosphate to ATP. In addition, NADH and FADH 2 produced during the TCA
cycle thus generating up to 15mol of ATP per mol of pyruvate. An overall com-
parison may be made between glycolysis followed by reoxidation of NADH by
fermentation or, alternatively, glycolysis followed by entry into the TCA cycle
and reoxidation of cofactors via the electron transport chain. Remembering that
1mol of glucose generates 2mol of pyruvate during glycolysis, and that the 2mol
of NADH produced during glycolysis may also be reoxidised by transfer to the
electron transport chain and not through fermentation, the net result is that glu-
cose catabolised by the glycolysis-fermentation route results in the production
of 2mol of ATP whereas catabolism by the glycolysis-TCA cycle-electron trans-
port/oxidative phosphorylation route produces up to 32mol of ATP. The figure
of 36 was deduced by Lehninger (1975) but has since been revised to reflect the
tenets of the chemiosmotic theory described earlier.
Anaerobic respiration is less efficient than aerobic respiration. Oxidation of
the same amount of cofactor by methanogenesis rather than oxidative phospho-
rylation would produce fewer mol of ATP. Consequently, for a given amount
of ATP production, the flux of glucose through glycolysis followed by fermen-
tation would have to be approximately 16 times greater than through glycolysis
followed by oxidative phosphorylation, and the flux through methanogenesis is
somewhat intermediate. It is the metabolic capability of the organism and the
presence or absence of the appropriate inorganic electron acceptor which deter-
mines the fate of pyruvate on the grounds of energy considerations. On a practical
basis this may explain why anaerobic processes, such as the anaerobic digestion
of sewage sludge and municipal solid waste, are considerably less exothermic
than their aerobic counterparts. For a given quantity of carbon source, an aerobic
process will be able to extract in the order of 10 times the amount of energy than
that generated by an anaerobic process.
Regeneration of NAD + in plants
In addition to the processes discussed above for the production of NADH, plant
mitochondria operate an additional system whereby the required protons are