Final answer:
Amino acids get oxidized during amino acid catabolism within the citric acid cycle, primarily in the liver, where the amino group is removed and the carbon skeleton is converted into intermediates that enter the cycle.
Step-by-step explanation:
The level at which amino acids get oxidized/burned off is during the amino acid catabolism process in the citric acid cycle (TCA cycle). Initially, the amino group is removed, usually by transamination, and the resulting α-keto acid undergoes specific pathways. One example is phenylalanine, which is transformed after a series of reactions into fumarate and acetoacetate, with fumarate participating directly in the TCA cycle. Oxidative deamination also plays a key role, where glutamate loses its amino group and is reverted to α-ketoglutarate. The principal site of amino acid metabolism is the liver, with other tissues also contributing.
When energy sources like carbohydrates and fats are low, for example, during fasting, the amino acids are oxidized as an alternate energy source. Complete oxidation of the carbon skeleton of amino acids means their conversion into acetyl-CoA and subsequent metabolism by the TCA cycle, leading to the production of ATP, CO2, and H2O.