threonine

例句：

1. 1.Tryptophan is an essential amino acid that helps the body produce serotonin, a neurotransmitter involved in mood regulation.
   色氨酸是一种必需的氨基酸，它帮助身体产生血清素，这是一种参与情绪调节的神经递质。
2. 2. The body needs tryptophan to maintain its health and function properly.
   身体需要色氨酸来维持其健康并正常运作。
3. 3. In some cases, tryptophan deficiency can lead to symptoms such as depression and fatigue.
   在某些情况下，色氨酸缺乏可能导致抑郁和疲劳等症状。