So far, this reading has focused on the metabolism of sugars and fats. Indeed, sugars and fats make up most organic molecules processed as fuel in cells. However, proteins can be metabolized to make ATP as well. Proteins are responsible for most of the structure and function in tissues, so animals wouldn’t want to metabolize them too extensively. In fact, animals employ several regulatory mechanisms to spare proteins from metabolism. When proteins undergo catabolism, they are broken down into individual amino acids. Amino acids differ with respect to the "R group. The "R" group will determine where in the metabolic cycles that the amino acid products will enter. Notice in figure 13 that there are several metabolic entry points for amino acids in the biochemical pathways we have discussed.
The conversion of pyruvate to acetyl CoA is an irreversible reaction. This means that when fatty acids are metabolized to form acetyl CoA, it is not possible to turn the acetyl CoA back to pyruvate or any earlier glycolytic product. Also, acetyl CoA is 2 carbons long and 2 carbons are lost in the early reactions of the Citric Acid Cycle. For both reasons, it is not possible to use fatty acids to make glucose. To make glucose from scratch (Gluconeogenesis), cells must use a substrate that is not acetyl CoA and will not go through CO2 expelling steps. In the figure below, we see that some amino acids can enter the metabolic pathways in places that meet these requirements. Therefore, amino acids are the best choice for a raw material to make glucose. When amino acids enter the metabolic pathways for the purpose of making glucose, the reactions of glycolysis more or less run-in reverse to synthesize a new glucose molecule. The liver is particularly good at doing this. Gluconeogenesis is stimulated by hormones that are released when blood sugars become low.
Image created by JS at BYU-Idaho Fall 2013.
This illustration shows the metabolic entry point of carbohydrates, fatty acids, and amino acids (from proteins). Notice that many of the reactions are reversible.