Which step comes first in the catabolism of amino acids to urea?

In the catabolism of amino acids, the first step is the deamination of the amino acid. During this process, the amino group (amine) is removed from the amino acid, resulting in the formation of an α-keto acid and ammonia. This is a crucial step because it allows the body to utilize the carbon skeleton of the amino acid for energy production or conversion into glucose or fatty acids, while the ammonia generated must be processed further, often into urea.

Following deamination, the ammonia can enter pathways like the urea cycle, where it is ultimately converted into urea for excretion. This makes the deamination step pivotal as it initiates the overall process of amino acid catabolism leading to urea production, which the body needs to eliminate excess nitrogen.

Other options reflect subsequent steps or processes that follow deamination, such as the conversion of the resulting keto acid or the processing of urea in the kidneys, rather than the initial removal of the amine group itself.