Microbes and Metabolism - Environmental Biotechnology: Theory and Application - page 28

Nucleic acids
Degradation of nucleic acids (see Figure 2.4) is also a source of ammonium ion.
The purines are broken down to release CO 2 and uric acid which is reduced to
allantoin. This is then hydrolysed to produce urea and glyoxylate which can enter
the TCA cycle by the glyoxylate pathway present in plants and bacteria but not
mammals. The urea thus produced may be further hydrolysed to ammonium ion
or ammonia with the release of carbon dioxide. The form in which the nitrogen
derived from the purines is excreted again depends upon the organism.
Pyrimidines are hydrolysed to produce ammonia which enters the nitrogen
cycle, carbon dioxide and -aminoisobutyric acid both of which are

finally degraded to succinyl CoA which enters the TCA cycle.

β -alanine or β

Carbohydrates
The carbohydrates (see Figure 2.3) form a ready source of energy for most
organisms as they lead, by a very short route, into the central metabolic pathways
from which energy to fuel metabolic processes is derived. When several sugar
units, such as glucose, are joined together to form macromolecules, they are
called polysaccharides. Examples of these are glycogen in animals, and cellulose
in plants. In nature, the sugars usually occur as ring structures and many have
the general formula, C(H 2 O) n , where carbon and water are present in equal
proportion. Catabolism of glucose has been described earlier in this chapter.
As stated earlier, the resulting metabolite from a given carbon source, or the
presence of specific enzymes, can be diagnostic of an organism. Whether or not
the enzymes of a particular route are present can help to identify a microbe, and
carbohydrate metabolism is frequently the basis of micro-organism identification
in a public health laboratory. Glucose enters the glycolytic pathway to pyruvate,
the remainder of which is determined in part by the energy requirements of the
cell and in part by the availability of oxygen. If the organism or cell normally
exists in an aerobic environment, there is oxygen available and the pyruvate is not
required as a starting point for the synthesis of another molecule, then it is likely
to enter the TCA cycle. If no oxygen is available, fermentation, defined later in
this chapter, is the likely route. The function of fermentation is to balance the
chemical reductions and oxidations performed in the initial stages of glycolysis.

Production of Cellular Energy

Cellular energy is present mainly in the form of ATP and to a lesser extent,
GTP (Figure 2.4) which are high energy molecules, so called because a large
amount of chemical energy is released on hydrolysis of the phosphate groups.
The energy to make these molecules is derived from the catabolism of a food,
or from photosynthesis. A food source is commonly carbohydrate, lipid or to a
lesser extent, protein but if a compound considered to be a contaminant can enter