All of the following amino acids undergo transamination except?
**Core Concept**
Transamination is a reversible chemical reaction in which an amino group is transferred from an amino acid to a keto acid, resulting in the formation of a new amino acid and a new keto acid. This process is catalyzed by transaminase enzymes and is an important mechanism for the synthesis and degradation of amino acids in the body.
**Why the Correct Answer is Right**
Transamination typically involves amino acids with an amino group (-NH2) and a keto group (-CO) in their side chains. However, not all amino acids undergo transamination. For example, cysteine, which has a thiol (-SH) group instead of a keto group, cannot undergo transamination.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is a common amino acid that undergoes transamination. Transamination reactions are essential for the synthesis and degradation of many amino acids, including alanine, aspartate, and glutamate.
**Option B:** This option is incorrect because it is another amino acid that can undergo transamination. The transamination reaction allows for the interconversion of these amino acids and their corresponding keto acids.
**Option C:** This option is incorrect because, although it is not commonly transaminated, it still can be transaminated under certain conditions.
**Clinical Pearl / High-Yield Fact**
Cysteine is an exception to the general rule of transamination because of its thiol group, which is not compatible with the transamination reaction. This knowledge is essential for understanding the metabolism of amino acids and the regulation of nitrogen balance in the body.
**Correct Answer: D. Cysteine. Cysteine is an exception to the general rule of transamination due to its thiol group.**