Amino acid catabolism releases nitrogen in the form of ammonia. In the liver, the urea cycle prepares ammonia for excretion. Which amino acid could undergo deamidation to produce ammonia for the urea cycle?

Question

Amino acid catabolism releases nitrogen in the form of ammonia.  In the liver, the urea cycle prepares ammonia for excretion.  Which amino acid could undergo deamidation to produce ammonia for the urea cycle?

Quizplus · Accepted Answer

11ecba2d_0e56_8ad8_a3bf_039c191c7a0a_MD0008_00 Amino acids all have a common backbone structure that consists of an amino group bonded to a carbon atom (the α-carbon), which is bonded to a carboxylic acid. The α-carbon is also bonded to a side chain, called an R-group, which gives the amino acid its identity. Amino acid catabolism (degradation) involves the removal of nitrogen from backbones and certain R-groups in the form of ammonia, which is detoxified and prepared for excretion by the urea cycle: The nitrogen of amino acid backbones is removed by transamination and deamination (removal of an amine to produce a ketone), producing α-keto acids in the process. The side chains of asparagine and glutamine contain amides, which are catabolized by deamidation rather than deamination. Deamidation releases ammonia from an amide instead of an amine, producing a carboxylic acid instead of a ketone. Of the choices given, only glutamine can produce ammonia through deamidation. (Choices B, C, and D) Arginine, glycine, and aspartate each contain an amino group in their backbones, but they do not have any amido groups in their R-groups. Therefore, these amino acids cannot undergo deamidation. Educational objective: The urea cycle processes ammonia for excretion in the urine. The ammonia that enters the cycle is derived from the amino groups of amino acid backbones (deamination) and from the amide side chains of glutamine and asparagine (deamidation).

Amino Acid Catabolism Releases Nitrogen in the Form of Ammonia