Chapter 7. Metabolism
Carbohydrate, Lipid, and Protein Metabolism
Carbohydrate Metabolism
The breakdown of glucose begins with glycolysis, which is a ten-step metabolic pathway, yielding two ATP per glucose molecule. Glycolysis takes place in the cytosol and does not require oxygen. In addition to ATP, the end-products of glycolysis include two three-carbon molecules, called pyruvate. Pyruvate can either be shuttled to the citric acid cycle to make more ATP or follow an anabolic pathway. In the absence of oxygen (anaerobic), pyruvate is converted to lactic acid, which is known as fast glycolysis. In the presence of oxygen (aerobic), and if a cell is in negative-energy balance, pyruvate is transported to the mitochondria where, with the help of pyruvate dehydrogenase, it first gets one of its carbons cleaved off, yielding acetyl-CoA. This process is known as slow glycolysis.
During intense exercise, lactic acid/lactate and other byproducts can build up in the muscle and contribute to fatigue, which is why we can’t sustain high-intensity exercise for a long period of time. The lactate diffuses into the bloodstream and is transported into the liver. When oxygen is available, the lactate is converted into pyruvate, which can then be used to synthesize glucose via gluconeogenesis. The process of converting lactate back into pyruvate/glucose is known as the Cori cycle.
In the citric acid cycle, acetyl-CoA is joined to a four-carbon molecule. In this multistep pathway, two carbons are lost as two molecules of carbon dioxide. The energy obtained from the breaking of chemical bonds in the citric acid cycle is transformed into two more ATP molecules (or equivalents thereof) and high-energy electrons that are carried by the molecules, nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH2). NADH and FADH2 carry the electrons to the inner membrane of the mitochondria where the third stage of energy release takes place, in what is called the electron transport chain. In this metabolic pathway, a sequential transfer of electrons between multiple proteins occurs and a lot of ATP is synthesized. The entire process of nutrient catabolism is chemically similar to burning, as carbon and hydrogen atoms are combusted (oxidized), producing carbon dioxide, water, and heat. However, the stepwise chemical reactions in nutrient catabolism pathways slow the oxidation of carbon atoms so that much of the energy is captured and not all is transformed into heat and light. Complete nutrient catabolism is between 30 and 40 percent efficient, and some of the energy is therefore released as heat. Heat is a vital product of nutrient catabolism and is involved in maintaining body temperature. If cells were too efficient at trapping nutrient energy into ATP, humans would not last to the next meal, as they would die of hypothermia (excessively low body temperature).
Lipid Metabolism
For fatty acids to be used for energy, they must be liberated from stored (e.g., muscle or adipose tissue) or circulating triglycerides. Hormone-sensitive lipase breaks down stored triglycerides, and lipoprotein lipase breaks down triglycerides in lipoproteins, allowing fatty acids to be liberated. The breakdown of fatty acids begins with the catabolic pathway known as β-oxidation, which takes place in the mitochondria. In this catabolic pathway, four enzymatic steps sequentially remove two-carbon molecules from long chains of fatty acids, yielding acetyl-CoA molecules. From here, lipid metabolism continues through the same stages 2 and 3 as carbohydrates.
Protein Metabolism
In the case of amino acids, once the nitrogen is removed from the amino acid through deamination, the remaining carbon skeleton can be enzymatically converted into pyruvate, acetyl-CoA, or some other intermediate of the citric acid cycle. Acetyl-CoA, a two-carbon molecule common to glucose, lipid, and protein metabolism, enters the second stage of energy metabolism, the citric acid cycle. From here, protein metabolism continues through the same stages 2 and 3 as carbohydrates.