asparagine synthetase (glutamine-hydrolyzing)
The ASNS gene provides instructions for making an enzyme called asparagine synthetase. This enzyme is found in cells throughout the body, where it converts the protein building block (amino acid) aspartic acid to the amino acid asparagine. Another amino acid called glutamine helps in the conversion and is itself converted to the amino acid glutamic acid during the process. It is thought that asparagine synthetase helps to maintain the normal balance of these four amino acids in the body.
Asparagine is needed to produce many proteins but also plays other roles. Asparagine helps to break down toxic ammonia within cells, is important for protein modification, and is needed for making a certain molecule that transmits signals in the brain (a neurotransmitter).
Although asparagine can be obtained through the diet, the amino acid cannot cross the protective barrier that allows only certain substances to pass between blood vessels and the brain (the blood-brain barrier). As a result, brain cells rely solely on asparagine synthetase to produce asparagine.
At least 15 mutations in the ASNS gene have been found to cause asparagine synthetase deficiency, a severe condition that causes neurological problems soon after birth. In most cases, these mutations replace single amino acids in the enzyme. The altered enzymes have little or no function. Asparagine from the diet likely makes up for the enzyme's inability to produce the amino acid in cells. However, because asparagine cannot cross the blood-brain barrier, brain cells in people with asparagine synthetase deficiency have a shortage (deficiency) of this amino acid. The exact effect of asparagine synthetase deficiency on brain cells is unknown, but because of the severe features of this condition, it is clear that asparagine is necessary for normal brain development. A lack of asparagine in developing brain cells leads to poor brain development and the severe neurological problems in individuals with asparagine synthetase deficiency. It is unclear whether a lack of glutamic acid or a buildup of glutamine impacts the signs and symptoms of this condition.
- aspartate ammonia ligase
- glutamine-dependent asparagine synthetase
- TS11 cell cycle control protein