AminoAcidsAre Important Fuels in Addition to Being the Building Blocks of Protein
Amino acids are important fuels. Whereas these monomers are the building blocks of proteins, amino acids are also carbon- containing compounds that can provide energy to the body.
In addition, they are important substrates for gluconeogenesis, indicating that most amino acids can be converted to glucose when the available glucose supply is limited. Although it is sometimes said that there is no storage site of amino acids in the body, the protein of skeletal muscle could be considered to have an amino acid storage function in addition to its locomotor functions.
FIGURE 32-2 Physiological ketone bodies.
FIGURE 32-3 Metabolism during the absorptive period is characterized by the movement of potential fuels into depot sites and the utilization of glucose (G) as a fuel. AA, Amino acid;
TG, triglyceride.
ruminants, ketone bodies are not only products of fatty-acid metabolism, but also products of normal digestion. Elevated serum concentrations of ketone bodies are characteristic of several diseases associated with abnormalities of fuel homeostasis. This fact might lead one to conclude that ketone bodies are abnormal, or even toxic, metabolites. In fact, when present in physiological concentrations, ketone bodies are important fuels that occupy an integral part of the scheme of fuel homeostasis. Figure 32-2 illustrates the chemical structure of the three major ketone bodies.