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Glucose can be made from intermediates in glycolysis and the citric acid cycle by gluconeogenesis. The metabolic pathways for the production and breakdown of lipids and amino acids are linked to those of glucose metabolism. Reaction products regulate key enzymes in the various pathways.
learning outcomes
You should be able to:
Describe catabolic and anabolic pathways that link carbohydrate, lipid, and protein metabolism.
Trace the path of an atom as it moves through various metabolic pathways within a cell.
Explain how allosteric changes in an enzyme can be used to regulate flow through a metabolic pathway.
Give examples of the pathways relating to glycolysis and the citric acid cycle for catabolism of a lipid and the anabolism of a protein.
A catabolic interconversion of a lipid: Fatty acids are catabolized to acetyl CoA. This enters the citric acid cycle and is oxidized to CO2, providing reduction energy to form NADH and FAHD. An anabolic interconversion of a protein: Amino acids are converted to intermediates in glycolysis and the citric acid cycle. These intermediates can be used in anabolism to form glucose, fats, or other amino acids to make proteins.
How does phosphofructokinase serve as a control point for glycolysis?
Phosphofructokinase acts early in glycolysis and is a focal point for turning off or activating the rest of the energy pathways. It is turned off by later products (ATP, citrate) and turned on when energy is needed (ADP).
You eat a burger that contains polysaccharides, proteins, and lipids. How could carbon atoms in the proteins’ amino acids and the glucose in the polysaccharides end up as fats?
Some amino acids are converted to intermediates of glycolysis. Once they enter glycolysis, these intermediates are further metabolized to a glycolytic intermediate that can be converted to glycerol, which is incorporated into triglycerides. Glycolysis and pyruvate oxidation produce acetyl CoA, which is converted to fatty acids and incorporated into lipids. Glucose is converted in glycolysis to acetyl CoA, which is then converted to fatty acids as above.