A large number of compounds are called lipoproteins, and others called proteolipids, that occur in living cells are lipids combined with proteins by linkages that are very little understood. Lipids are also often combined with carbohydrate complexes or polysaccharides (lipopolysaccharide).
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Lipoproteins are important constituents of the cell membranes of many types of cell, including important species of bacteria. They also occur in the outer coverings of some viruses that infect animals. In viruses the lipoproteins confer special and distinctive properties that will be mentioned in the section on viruses.
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In certain bacteria such as typhoid bacilli the lipopolysaccharides of the cell walls, complexed with polypeptides, have important pathogenic properties to be mentioned later.
One of the most important fatty-acid compounds is poly-beta- hydroxybutyric acid, a polymer of p-hydroxybutyric acid:
The units are joined by ester linkages between the alcoholic hydroxyl group of one molecule and the carboxyl group of the next, a molecule of water being eliminated (anhydrosynthesis) as in all typical ester groupings.
Enzymes involved in such polymerizations are sometimes called synthases or synthetases. Poly-beta-hydroxybutyric acid is found as granules and globules in the cells of a number of important and common species of bacteria.
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It serves a purpose like that of fat or starch, or inorganic metaphosphates (volutin), i.e., as a reserve food. Like starch, fat and volutin, it is insoluble and compactly stored as a polymer but readily hydrolyzed to release soluble, readily metabolizable, individual units: beta-hydroxybutyric acid, from its polymer.
Similarly, soluble glucose is released from insoluble starch, soluble phosphorous compounds from insoluble volutin, fatty acids and glycerol from water- insoluble fats and oils.
In the synthesis of lipid polymers, energy for the linkages is derived from ATP as in polysaccharide synthesis, though not by direct formation of energy-rich phosphate bonds with the monomers.
In lipid polymers the energy appears in an energy-rich acyl (R—C—R) bond introduced into the fatty acid by combination with coenzyme A (CoA). The acyl group is then transferred from the fatty acid, leaving the acyl-bond energy. The energy is used in forming the ester linkage between the activated fatty acid and the alcohol (or alcoholic group) with which it combines.