Microbes and Metabolism - Environmental Biotechnology: Theory and Application - page 24

glyceraldehyde 3-phosphate and dihydroxyacetone phosphate which normally lies
far in favour of the latter. This is perhaps surprising since it is glyceraldehyde
3-phosphate which is the precursor for the subsequent step. The next stage is
the introduction of a second phosphate group to glyceraldehyde 3-phosphate
with an accompanying oxidation, to produce glyceraldehyde 1,3-diphosphate. The
oxidation involves the transfer of hydrogen to the coenzyme, NAD, to produce
its reduced form, NADH. In order for glycolysis to continue operating, it is
essential for the cell or organism to regenerate the NAD + which is achieved
either by transfer of the hydrogens to the cytochromes of an electron transport
chain whose operation is associated with the synthesis of ATP, or to an organic
molecule such as pyruvate in which case the opportunity to synthesise ATP is lost.
This latter method is the first step of many different fermentation routes. These
occur when operation of electron transport chains is not possible and so become
the only route for the essential regeneration of NAD + . Looking at the Embden-
Meyerhof pathway, this is also the third stage at which a phosphorylation has
occurred. In this case, the phosphate was derived from an inorganic source, in a
reaction which conserves the energy of oxidation.
The next step in glycolysis is to transfer the new phosphate group to ADP,
thus producing ATP and 3-phosphoglycerate, which is therefore the first sub-
strate level site of ATP synthesis. After rearrangement to 2-phosphoglycerate
and dehydration to phosphoenolpyruvic acid, the second phosphate is removed
to produce pyruvic acid and ATP, and so is the second site of substrate level ATP
synthesis. As mentioned above, depending on the activity of the electron trans-
port chains and the energy requirements of the cell balanced against the need for
certain metabolic intermediates, pyruvate, or its derivatives may now be reduced
by accepting the hydrogen from NADH and so continue on a fermentation route
or it may be decarboxylated to an acetyl group and enter the TCA cycle. The
overall energy balance of glycolysis is discussed later when considering chemical
cellular energy production in more detail.

TCA cycle
Pyruvate decarboxylation produces the acetyl group bound to Coenzyme A,
ready to enter the TCA cycle otherwise named Kreb's Citric Acid Cycle in
tribute to the scientist who discovered it. Not only is this cycle a source of
reduced cofactors which 'fuel' electron transport and thus, the synthesis of ATP,
but it is also a great meeting point of metabolic pathways. Cycle intermediates
are constantly being removed or replenished. During anaerobic fermentation,
many of the reactions seen in the TCA cycle are in operation even though they
are not linked to electron transport.

Glyoxalate cycle
This is principally the TCA cycle, with two additional steps forming a 'short
circuit',

involving the formation of glyoxalate from isocitrate. The second