L-threonine Makes up Proteins

L-Threonine is one of the essential amino acids in the human body, playing a key role in protein metabolism. Its interactions with protein metabolism are reflected in several key processes:

I. Protein Synthesis

1. Participation as a Raw Material:  

L-Threonine is a fundamental building block of proteins. Within cells, on ribosomes, amino acids like L-Threonine are carried by tRNA and assembled into polypeptide chains following the sequence dictated by mRNA templates. These polypeptide chains then fold into proteins with specific structures and functions. When dietary intake of L-Threonine is insufficient, protein synthesis is limited due to a lack of raw materials, leading to decreased protein levels in the body and impairing normal physiological functions.

2. Regulation of Gene Expression Related to Protein Synthesis:  

L-Threonine can regulate protein synthesis by influencing the expression of genes associated with this process. For instance, it can activate the mechanistic target of rapamycin (mTOR) signaling pathway. mTOR phosphorylates and activates various transcription factors and ribosomal proteins, promoting the transcription and translation of genes related to protein synthesis, thereby enhancing protein production.

II. Protein Degradation

1. Prevention of Muscle Protein Breakdown:  

Under stress conditions such as starvation, trauma, or infection, protein degradation increases to provide energy and amino acids for other tissues, with muscle proteins being particularly susceptible to breakdown. L-Threonine can regulate certain intracellular signaling pathways—such as activating the insulin-like growth factor-1 (IGF-1)/Akt signaling pathway—to inhibit the activity of the ubiquitin-proteasome system and the autophagy-lysosome system in muscle cells. This reduces muscle protein degradation and helps maintain muscle mass and function.

2. Maintaining Amino Acid Pool Balance:  

Amino acids released from protein degradation enter the intracellular amino acid pool, where L-Threonine interacts with other amino acids to maintain metabolic balance. When the levels of other amino acids fluctuate, L-Threonine metabolism adjusts accordingly to ensure the stability of the amino acid network. For example, under certain disease conditions where other amino acids are heavily consumed, L-Threonine can undergo transamination and be converted into other amino acids to meet the body’s needs.

III. Nitrogen Balance Regulation

1. Maintaining Positive Nitrogen Balance:  

Nitrogen balance is an important indicator of the body’s protein metabolic status. A positive nitrogen balance occurs when nitrogen intake exceeds nitrogen excretion, indicating that protein synthesis surpasses degradation. As a nitrogen-containing compound, L-Threonine plays a vital role in both protein synthesis and degradation, helping to maintain nitrogen balance. Adequate intake promotes protein synthesis, reduces nitrogen excretion, and supports physiological processes such as growth, development, and tissue repair.

2. Relationship with the Urea Cycle:  

During protein metabolism, amino acids undergo deamination, producing ammonia, which must be converted into urea through the urea cycle to maintain nitrogen homeostasis. The ammonia generated from L-Threonine metabolism also participates in this cycle. If L-Threonine metabolism becomes abnormal, it may disrupt the urea cycle, leading to ammonia accumulation, which can be toxic to the body. Conversely, impairments in the urea cycle can affect L-Threonine metabolism by disrupting amino acid breakdown, resulting in the accumulation of amino acids in the body.