Microbes and Metabolism - Environmental Biotechnology: Theory and Application - page 23

the diversity and to maximise the opportunity for microbes to metabolise the
offending carbon source.
Metabolic Pathways of Particular Relevance
to Environmental Biotechnology
Having established that the overall strategy of environmental biotechnology is
to make use of the metabolic pathways in micro organisms to break down or
metabolise organic material, this chapter now examines those pathways in some
detail. Metabolic pathways operating in the overall direction of synthesis are
termed anabolic while those operating in the direction of breakdown or degrada-
tion are described as catabolic: the terms catabolism and anabolism being applied
to describe the degradative or synthetic processes respectively.
It has been mentioned already in this chapter and it will become clear from
the forthcoming discussion, that the eventual fate of the carbon skeletons of
biological macromolecules is entry into the central metabolic pathways.
Glycolysis
As the name implies, glycolysis is a process describing the splitting of a phosphate
derivative of glucose, a sugar containing six carbon atoms, eventually to produce
two pyruvate molecules, each having three carbon atoms. There are at least four
pathways involved in the catabolism of glucose. These are the Embden-Meyerhof
(Figure 2.1), which is the one most typically associated with glycolysis, the
Entner-Doudoroff and the phosphoketolase pathways and the pentose phosphate
cycle, which allows rearrangement into sugars containing 3, 4, 5, 6 or 7 carbon
atoms. The pathways differ from each other in some of the reactions in the
first half up to the point of lysis to two three-carbon molecules, after which
point the remainder of the pathways are identical. These routes are characterised
by the particular enzymes present in the first half of these pathways catalysing
the steps between glucose and the production of dihydroxyacetone phosphate
in equilibrium with glyceraldehyde 3-phosphate. All these pathways have the
capacity to produce ATP and so function in the production of cellular energy.
The need for four different routes for glucose catabolism, therefore, lies in the
necessity for the supply of different carbon skeletons for anabolic processes and
also for the provision of points of entry to glycolysis for catabolites from the
vast array of functioning catabolic pathways. Not all of these pathways operate
in all organisms. Even when several are encoded in the DNA, exactly which of
these are active in an organism at any time, depends on its current metabolic
demands and the prevailing conditions in which the microbe is living.
The point of convergence of all four pathways is at the triose phosphates
which is the point where glycerol as glycerol phosphate enters glycolysis and
so marks the link between catabolism of simple lipids and the central metabolic
pathways. The addition of glycerol to the pool of trioses is compensated for
by the action of triose phosphate isomerase maintaining the equilibrium between