When you eat proteins in food, your body has to break them down into amino acids before they can be used by your cells. Most of the time, amino acids are recycled and used to make new proteins, not oxidized for fuel.
However, if there are more amino acids than the body needs, or if cells are starving, some amino acids will broken down for energy via cellular respiration. In order to enter cellular respiration, amino acids must first have their amino group removed. This step makes ammonia left parenthesis, N, H, start subscript, 3, end subscript, right parenthesis as a waste product, and in humans and other mammals, the ammonia is converted to urea and removed from the body in urine.
Once they’ve been deaminated, different amino acids enter the cellular respiration pathways at different stages. The chemical properties of each amino acid determine what intermediate it can be most easily converted into.
For example, the amino acid glutamate, which has a carboxylic acid side chain, gets converted into the citric acid cycle intermediate α-ketoglutarate. This point of entry for glutamate makes sense because both molecules have a similar structure with two carboxyl groups, as shown below.
Fats, known more formally as triglycerides, can be broken down into two components that enter the cellular respiration pathways at different stages. A triglyceride is made up of a three-carbon molecule called glycerol, and of three fatty acid tails attached to the glycerol. Glycerol can be converted to glyceraldehyde-3-phosphate, an intermediate of glycolysis, and continue through the remainder of the cellular respiration breakdown pathway.
Fatty acids, on the other hand, must be broken down in a process called beta-oxidation, which takes place in the matrix of the mitochondria. In beta-oxidation, the fatty acid tails are broken down into a series of two-carbon units that combine with coenzyme A, forming acetyl CoA. This acetyl CoA feeds smoothly into the citric acid cycle.
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