Proteins are hydrolyzed to amino acids. Organisms synthesize new proteins from some of these amino acids. Excess amino acids are enzymatically converted to intermediates of glycolysis and the Krebs cycle. Common intermediates are pyruvate, acetyl CoA and a -ketoglutarate. This conversion process deaminates amino acids, and the resulting nitrogenous wastes are excreted.
Fats are excellent fuels because they are rich in hydrogens with high energy electrons.
Oxidation of one gram of fat produces twice as much ATP as a gram of carbohydrate. Fat sources may be from the diet or from storage cells in the body. Fats are digested into glycerol and fatty acids. Glycerol can be converted to glyceraldehyde phosphate, an intermediate of glycolysis. Most energy in fats is in fatty acids, which are converted into acetyl CoA by beta oxidation .