Protein is an important body-building raw material. The body prefers to use it for that purpose rather than to “cash it in” to make energy. It has glucose and fats to do that. However a small percentage (about 10%) is used to make energy and even more in times of starvation. So far you have learned that proteins are digested in the small intestine by proteolases such as tyripsin and chymotrypsin to make amino acids. These amino acids are then absorbed directly into the blood stream for transport to the liver. What happens next? Well, in the liver, amino acids may be used to make proteins or other biomolecules that require nitrogen. Amino acids that the liver does not use to construct other molecules are sent to the blood and then to tissue cells to be also utilized for protein synthesis and synthesis of nitrogen-containing biomolecules for their building and repair functions. Once those needs are satisfied, leftover amino acids are degraded in the liver to make energy via a process called oxidative deamination.
Oxidative deamination is a process in which the amino group of the amino acid is stripped off with the help of enzymes to produce a carbon skeleton. The resulting carbon skeleton is then either converted to pyruvate, an intermediate of the TCA cycle (acetoacetyl CoA, α-ketoglutarate, succinyl CoA, fumarate, and oxaloacetate), or to acetyl CoA. Amino acids that are converted to pyruvate or intermediates of the TCA cycle are called glucogenic amino acids since they can be used to make glucose. Amino acids converted to acetyl CoA are called ketogenic amino acids since this pathway can lead to the production of ketones instead of glucose. The amount of ATP produced by the amino acid will depend on where they enter the energy production system. Of course, the higher up they are, the more ATP that can be derived from them. The various fates of the 20 amino acids are summarized in the diagram below. Ketogenic amino acids are in red and the glucogenic amino acids in blue. Do you want to remember the ketogenic ones? Try this: TT PILL or “The toxic pill” for tryptophan, tyrosine, phenylalanine, isoleucine, leucine and lysine. If you know the ketogenic amino acids you will know that the rest are glucogenic.
Reference: Thompson,& J., Manore, M., Vaughan, L. (2020). The science of nutrition (5th ed.). New York. Pearson