Microbes and Metabolism 27mammals and the fairly insoluble uric acid in the case of birds and most reptiles.
Bacteria may then convert the excreted ammonia, urea or uric acid into nitrite
and then oxidise it to nitrate which may then be taken up by plants. From there it
is included in anabolic processes such as amino acid synthesis to produce mate-
rial ingested by higher animals and the whole procedure of amino group transfer
repeats itself. This is the basis of the nitrogen cycle which forms a central part
of much of the sewage and effluent treatment described in Chapters 6 and 7.
Theα-keto acid resulting from deamination of the amino acid is degraded
by a series of reactions, the end product being dependent on the original amino
acid, but all will finally result as a glycolysis or TCA cycle intermediate. A
fascinating story of catabolism showing collaboration between mammals and
bacteria resident in the gut, is the degradation of haemoglobin, the component
of blood which carries oxygen and carbon dioxide. Haemoglobin comprises the
protein, globin, into which was inserted during synthesis, the haeme ring system
where the exchange between binding of oxygen or carbon dioxide takes place
in circulating blood. The first step of haemoglobin degradation, performed in
the mammalian system, is removal of the haeme ring structure releasing globin
which is subject to normal protein degradation. Haeme has its origins in the amino
acids in that the starting point for the ring structure is the amino acid, glycine.
The degradation pathways starts with removal of iron and release of carbon
monoxide to produce the linear structure, bilirubin. This is eventually excreted
into the gut where enteric (gut) bacteria degrade the bilirubin to urobilinogens
which are degraded further, some being excreted in the urine and others, such as
stercobilin, are excreted in the faeces. All these products are further metabolised
by microbes, for example, in the sewage treatment plant.
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 andβ-alanine orβ-aminoisobutyric acid both of which are
finally degraded to succinyl CoA which enters the TCA cycle.
Carbohydrates
The carbohydrates (see Figure 2.3) form a ready source of energy for most organ-
isms 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,