Microbes and Metabolism - Environmental Biotechnology: Theory and Application - page 25

reaction requires the addition of acetyl CoA to glyoxalate to produce malic
acid and thus rejoin the TCA cycle. The purpose of this shunt is to permit the
organism to use acetyl CoA, which is the major breakdown product of fatty
acids, as its sole carbon source.
Macromolecules - description and degradation
Lipids
This class of macromolecules (see Figure 2.2) includes the neutral lipids which
are triacylglycerols commonly referred to as fats and oils. Triacylglycerols are
found in reservoirs in micro-organisms as fat droplets, enclosed within a 'bag',
called a vesicle, while in higher animals, there is dedicated adipose tissue, com-
prising mainly cells full of fat. These various fat stores are plundered when energy
is required by the organism as the degradation of triacylglycerols is a highly exer-
gonic reaction and therefore a ready source of cellular energy. Gram for gram,
the catabolism of these fats releases much more energy than the catabolism of
sugar which explains in part why energy stores are fat rather than sugar. If this
were not the case the equivalent space taken up by a sugar to store the same
amount of energy would be much greater. In addition, sugar is osmotically active
which could present a problem for water relations within a cell, should sugar be
the major energy store.
Triacylglycerols comprise a glycerol backbone onto which fatty acids are ester-
ified to each of the three available positions. They are insoluble in an aqueous
environment due to the non-polar 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 mono-acylglycerols have in the past been described
as tri-, di- or mono-glycerides. Although these are inaccurate descriptions of the
chemistry of these compounds the terms tri-, di- and mono-glycerides 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 by β -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 non-polar