Microbes and Metabolism 25environment due to the nonpolar nature of the fatty acids forming ‘tails’ on the
triacylglycerol. However, diacylglycerols and monoacylglycerols which are ester-
ified at only two or one position respectively, may form themselves into micelles
due to their polar head, and so may exhibit apparent solubility by forming an
emulsion. The tri-, di- and monoacylglycerols have in the past been described
as tri-, di- or monoglycerides. Although these are inaccurate descriptions of the
chemistry of these compounds the terms tri-, di- and monoglycerides are still
in common usage. Chemically, fats and oils are identical. If the compound in
question is a liquid at room temperature, frequently it is termed an oil, if solid it
is described as a fat. The melting point of these compounds is determined to a
large extent by the fatty acid content, where in general, saturated fatty acids, due
to their ability to pack together in an orderly manner, confer a higher melting
point than unsaturated fatty acids.
Their catabolism is by hydrolysis of the fatty acids from the glycerol backbone,
followed by oxidation of the fatty acids byβ-oxidation. This process releases
glycerol which may then be further degraded by feeding into the central path-
ways of glycolysis, and several units of the acetyl group attached to the carrier
Coenzyme A (Figure 2.2), which may feed into the central metabolic pathways
just prior to entry into the TCA cycle (Figure 2.1).
Compound lipids include the phosphoglycerides which are a major component
of cell membranes. These can have very bulky polar head groups and nonpolar
tails which allow them to act as surfactants and in this specific context, biosur-
factants. The most common surfactants are glycolipids (Figure 2.7), which do not
have a glycerol backbone, but have sugar molecules forming a polar head and
fatty acids forming nonpolar tails, in an overall structure similar to that shown for
phospholipids in Figure 2.2. Derived lipids include fat soluble vitamins, natural
rubber, cholesterol and steroid hormones. It is interesting to note here that bacteria
do not synthesise steroids, and yet some, for example,Comamonas testosteroni,
are able to degrade specific members of the group; testosterone in the case given
(Horinouchiet al. 2001). However, oestrogen and its synthetic analogues used
in the contraceptive pill, are virtually recalcitrant to decomposition by bacteria.
This is proving a problem in waterways especially in Canada where the level of
such endocrine disrupters has become so high in some lakes that the feminisation
of fish is becoming a concern (McMaster 2001). This subject, and similar more
recent findings for the UK, are explored further in Chapter 3.
Proteins
The first catabolic step in protein degradation (see Figure 2.5) is enzymatic
hydrolysis of the peptide bond formed during protein synthesis resulting in the
release of short pieces, or peptides, and eventually after further degradation,
amino acids. The primary step in amino acid catabolism is to remove the amino
group thus producing anα-keto acid. This is usually achieved by transfer of
the amino group to the TCA cycle intermediate,α-ketoglutarate, resulting in the