A hamburger or veggiburger contains three major sources of carbon skeletons for the person who eats it: carbohydrates, mostly as starch (a polysaccharide); lipids, mostly as triglycerides (three fatty acids attached to glycerol); and proteins (polymers of amino acids). Looking at Figure 7.17, you can see how each of these three types of macromolecules can be used in catabolism or anabolism.
catabolic interconversions. Polysaccharides, lipids, and proteins can all be broken down to provide energy:
► Polysaccharides are hydrolyzed to glucose. Glucose then passes through glycolysis and the citric acid cycle, where its energy is captured in NADH and ATP.
► Lipids are broken down into their substituents, glycerol and fatty acids. Glycerol is converted to dihydroxyace-tone phosphate, an intermediate in glycolysis, and fatty acids are converted to acetate and then acetyl CoA in the mitochondria. In both cases, further oxidation to CO2 and release of energy then occur.
► Proteins are hydrolyzed to their amino acid building blocks. The 20 different amino acids feed into glycolysis or the citric acid cycle at different points. A specific example is shown in Figure 7.18, in which an amino acid can be converted to an intermediate in the citric acid cycle.
anabolic interconversions. Many catabolic pathways can operate in reverse. That is, glycolytic and citric acid cycle intermediates, instead of being oxidized to form CO2, can be reduced and used to form glucose in a process called gluco-neogenesis (which means "new formation of glucose"). Likewise, acetyl CoA can be used to form fatty acids. The most common fatty acids have an even number of carbons: 14, 16, or 18. These molecules are formed by adding two-carbon acetyl CoA "units" one at a time until the appropriate chain length is reached. Amino acids can be formed by reversible reactions such as the one shown in Figure 7.18, and can then be polymerized into proteins.
Some intermediates of the citric acid cycle are used in the synthesis of various important cellular constituents. For example, a-ketoglutarate is a starting point for purines and ox-aloacetate for pyrimidines, both constituents of the nucleic acids DNA and RNA. a-Ketoglutarate is also a starting point for chlorophyll synthesis. Acetyl CoA is a building block for various pigments, plant growth substances, rubber, and the steroid hormones of animals, among other molecules.
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