16 Environmental Biotechnology
At the core of metabolism are the central metabolic pathways of glycolysis and
the tricarboxylic acid (TCA) cycle on which a vast array of metabolic pathways
eventually converge or from which they diverge. Glycolysis is the conversion
of the six-carbon phosphorylated sugar, glucose 6-phosphate, to the three-carbon
organic acid, pyruvic acid, and can be viewed as pivotal in central metabolism
since from this point, pyruvate may enter various pathways determined by the
energy and synthetic needs of the cell at that time. A related pathway, sharing
some but not all of the reactions of glycolysis, and which operates in the opposite
direction is called gluconeogenesis. Pyruvate can continue into the TCA cycle
whose main function is to produce and receive metabolic intermediates and to
produce energy, or into one of the many fermentation routes.
The principles of glycolysis are universal to all organisms known to date,
although the detail differs between species. An outline of glycolysis, the TCA,
and its close relative the glyoxalate, cycles is given in Figure 2.1, together with an
indication of the key points at which the products of macromolecule catabolism,
Figure 2.1 Glycolysis, the TCA and glyoxalate cycles