Threonine is an α-amino acid containing a hydroxyl group, with the chemical structure HO-CH(NH₂)-CH(OH)-CH₃. It cannot be synthesized by the human body and must be obtained through diet, making it an essential amino acid. Threonine plays a critical role in protein synthesis and is a key component of many important proteins. This article aims to provide readers with a systematic framework of threonine metabolism, offering a valuable reference for research and applications in related fields, as well as providing insights for professionals in the fields of nutrition and biomedical sciences.
Biosynthesis of Threonine
The biosynthesis of threonine primarily occurs through the aspartate family pathway, involving multiple key enzymes and complex regulatory mechanisms.
Aspartate Family Pathway
Threonine biosynthesis occurs within the pathway of the aspartate family. This metabolic process initiates with aspartate and proceeds through multiple enzymatic reactions to yield threonine as the end product. The biochemical sequence involves several key transformations: Initially, the enzyme aspartate kinase catalyzes the phosphorylation of aspartate, generating aspartate-4-phosphate. Subsequently, this intermediate undergoes conversion to aspartate semialdehyde via the catalytic activity of aspartate semialdehyde dehydrogenase. The resulting compound is then subjected to reduction by homoserine dehydrogenase, forming homoserine. The final stage of synthesis comprises two sequential reactions—phosphorylation by homoserine kinase followed by conversion through threonine synthase—ultimately producing the amino acid threonine.
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Regulatory Mechanisms
Feedback inhibition plays a crucial role in regulating the synthesis pathway of threonine. At elevated concentrations within cells, threonine functions as a direct inhibitor of aspartate kinase activity. This inhibitory action leads to diminished formation of aspartate-4-phosphate, consequently reducing the metabolic flow through the entire biosynthetic sequence. Such a regulatory mechanism prevents excessive production of threonine, helping maintain proper amino acid equilibrium inside the cell. Beyond this inhibitory feedback loop, the production of threonine undergoes modulation via allosteric control mechanisms. For example, both homoserine dehydrogenase and aspartate kinase exhibit allosteric properties, with their enzymatic functions subject to modification when specific metabolites like isoleucine and threonine bind to them. The cell can thus flexibly adjust threonine production rates according to metabolic demands through this form of regulation, ensuring appropriate resource allocation within cellular metabolism.
Threonine Metabolic Pathways
Threonine is not only an essential amino acid for protein synthesis but also plays a crucial role in various metabolic pathways. Its metabolic pathways mainly include degradation pathways and interconversion with other amino acids. These pathways not only provide energy and metabolic intermediates to the cell but also participate in essential physiological processes such as one-carbon metabolism.
Figure 1. Threonine metabolism pathways.
Threonine Dehydrogenase Pathway
The degradation of threonine is primarily catalyzed by threonine dehydrogenase. The specific steps of this pathway are as follows: Threonine is oxidized by threonine dehydrogenase to form 2-amino-3-ketobutyrate. This compound is then further broken down by 2-amino-3-ketobutyrate coenzyme A lyase to produce glycine and acetyl-CoA. Acetyl-CoA enters the tricarboxylic acid (TCA) cycle to provide energy for the cell, while glycine participates in one-carbon metabolism or other biosynthetic pathways.
Threonine Deaminase Pathway
Threonine can also be degraded via the threonine deaminase pathway: Threonine is deaminated by threonine deaminase to produce α-ketobutyrate and ammonia. α-Ketobutyrate can be further converted into propionyl-CoA, which enters the TCA cycle or participates in other metabolic pathways. Ammonia is then removed from the body through the urea cycle, preventing its toxic accumulation.
Relationship Between Threonine and Serine
Threonine and serine are closely related metabolically. The glycine produced through threonine degradation can be further converted into serine. Glycine reacts with one-carbon units (e.g., 5,10-methylene tetrahydrofolate) in the presence of serine hydroxymethyltransferase (SHMT) to form serine. Serine is not only a component of protein synthesis but also involved in the biosynthesis of phospholipids, nucleotides, and other biomolecules.
Role of Threonine in One-Carbon Metabolism
The degradation product of threonine, glycine, plays an essential role in one-carbon metabolism. Glycine contributes one-carbon units (such as 5,10-methylene tetrahydrofolate) through the reaction catalyzed by serine hydroxymethyltransferase, participating in the synthesis of purines and thymidylates. one-carbon metabolism is a vital support system for cell proliferation and growth, and threonine indirectly participates in this process by providing glycine.
Threonine Metabolism and Disease
Threonine Deficiency and Hepatic Fatty Disease
Threonine plays a key role in lipid metabolism by promoting fatty acid metabolism and preventing fat accumulation in the liver, thereby helping to prevent the onset of fatty liver disease. Its specific mechanisms include: regulating the PPARγ signaling pathway to reduce triglyceride accumulation in the liver, thereby controlling lipid metabolism; accelerating fat breakdown and reducing fat deposition in the liver; and providing antioxidant protection to the liver, effectively shielding it from oxidative stress-induced damage. Threonine deficiency can lead to hepatic fat accumulation, which may eventually result in liver failure. Studies have shown that threonine deficiency increases mitochondrial uncoupling in the liver, leading to cellular dysfunction and liver disease (Ross-Inta, et al., 2009). Additionally, threonine deficiency can cause loss of appetite, mood fluctuations, and other health issues.
Role of Threonine in Tumor Growth
Research indicates that threonine promotes tumor growth and proliferation by enhancing the expression of the YRDC gene, which accelerates protein synthesis in tumor cells. Moreover, excessive threonine intake may further promote tumor cell survival and self-renewal by increasing the levels of tRNA modifications. Threonine metabolism influences protein synthesis in tumor cells by regulating tRNA modification levels. Studies have shown that threonine dynamically regulates the levels of N6-threonylcarbamoyladenosine (t6A) on tRNA, a modification critical for tRNA recognition of the ANN codon. YRDC is a key enzyme in tRNA modification, and its activity is regulated by threonine concentration. In glioblastoma stem cells (GSCs), the upregulation of YRDC expression promotes tRNA t6A modification, thereby accelerating protein synthesis.
Figure 2. Threonine is modified by t6A tRNA mediated by YRDC. (Wu, X., et al., 2024)
Threonine and Genetic Metabolic Disorders
Abnormal threonine metabolism is a genetic metabolic disorder, primarily caused by gene mutations leading to a deficiency or dysfunction of key enzymes in the metabolic pathway. This abnormality disrupts the normal metabolism of threonine, causing its accumulation in the body and interfering with other biochemical reactions. As a result, a range of clinical symptoms can arise, such as growth retardation, intellectual disabilities, and hypotonia. Additionally, patients may exhibit distinct facial features.
Threonine Metabolism and Autoimmune Diseases
Autoimmune diseases are conditions where the immune system erroneously recognizes and attacks the body's own tissues. Studies have found that abnormalities in threonine metabolism may be associated with the development of several autoimmune diseases. For example, in rheumatoid arthritis and systemic lupus erythematosus, alterations in threonine metabolism pathways lead to immune cell dysfunction. Metabolic disturbances in immune cells can alter the activity of T cells and B cells, resulting in a reduced immune tolerance and an excessive immune response, which in turn triggers autoimmune attacks. Furthermore, threonine metabolism may influence immune processes such as antigen presentation and cytokine secretion by affecting immune cell glucose and amino acid metabolism, thereby exacerbating the pathological progression of these diseases.
Threonine Metabolism and the Nervous System
Neurotransmitters are chemical substances that transmit signals between nerve cells, directly affecting the function of the nervous system. Threonine plays an important role in the health and function of the nervous system, particularly through its involvement in the synthesis of neurotransmitters like glycine and serine. Glycine is a crucial inhibitory neurotransmitter, primarily regulating the excitability and inhibition balance of the central nervous system by inhibiting neurons. Threonine serves as a precursor for glycine synthesis, undergoing a series of enzyme-catalyzed reactions to be converted into glycine. Therefore, the normal metabolism of threonine is essential for glycine production.
Serine is another important neurotransmitter, especially in brain functions related to cognition and neural plasticity. Threonine metabolism in the nervous system not only involves glycine synthesis but also serine synthesis. Serine plays a role in neurotransmitter synthesis by promoting neuronal activity and enhancing the transmission of neural signals.
Recent studies suggest that abnormalities in threonine metabolism may play a significant role in the onset and progression of neurodegenerative diseases. Threonine is involved in multiple metabolic pathways, including carbohydrate metabolism, lipid metabolism, and protein synthesis. Dysregulated threonine metabolism can lead to neuronal dysfunction and death, thereby accelerating the progression of neurodegenerative diseases. For example, threonine metabolism disorders may affect neuronal energy supply and oxidative stress responses, resulting in neuronal damage.
Threonine Metabolism and Microorganisms
Threonine Biosynthesis Pathway in Escherichia coli
In Escherichia coli, threonine biosynthesis is a complex and finely regulated metabolic process that involves a series of interconnected reactions, forming a unique metabolic network. The process begins with aspartate, which, under the catalysis of aspartate kinase, is converted into aspartyl-phosphate. This is then further transformed through a series of reactions into aspartate semialdehyde. From aspartate semialdehyde onward, the metabolic pathway diverges, with one branch leading to the formation of homoserine via the action of homoserine dehydrogenase. Homoserine is then catalyzed by homoserine kinase to ultimately synthesize threonine.
Several key enzymes play a decisive role in this biosynthesis pathway. Aspartate kinase, the first key enzyme in the pathway, catalyzes the reaction that initiates threonine synthesis and is crucial for the start of the entire process. Homoserine dehydrogenase and homoserine kinase are also indispensable, catalyzing subsequent critical reactions to ensure that the metabolic flow proceeds smoothly toward threonine synthesis. The activity and expression levels of these key enzymes directly affect the efficiency and yield of threonine biosynthesis.
Figure 3. Metabolic network of Thr synthesis. (Yang, Q., et al., 2022)
Threonine Metabolic Flux Analysis and Enzyme Control
Metabolic flux analysis plays a crucial role in threonine metabolism research. This method allows for the quantitative description of the flow rates of intracellular metabolites through various metabolic pathways, providing a clear understanding of the distribution and variation of metabolic flux within the threonine biosynthesis pathway. For instance, using stable isotope labeling techniques combined with mass spectrometry analysis, it is possible to accurately determine the labeling patterns of metabolites and calculate the metabolic flux. This helps to reveal the activity of the threonine metabolism pathway under different physiological conditions and the contribution of each reaction step to threonine synthesis.
Enzyme control plays a central role in threonine metabolic flux. The activity of key enzymes directly determines the rate of metabolic reactions, which in turn influences metabolic flux. By regulating the activity of key enzymes, threonine synthesis can be precisely controlled. For example, through genetic engineering techniques, the expression levels of key enzyme genes can be altered to either overexpress or underexpress them, thereby increasing or decreasing the reaction rates and guiding the metabolic flow toward threonine synthesis. Additionally, specific enzyme activators or inhibitors can be added to flexibly regulate threonine metabolic flux at the enzyme level to meet production or research requirements.
Optimization of Threonine Production
Metabolic engineering of Escherichia coli for threonine production is an important method for improving threonine yield. First, the genes encoding key enzymes in the threonine biosynthesis pathway in E. coli are modified. For example, site-directed mutagenesis can be used to modify the genes of aspartate kinase, homoserine dehydrogenase, and other key enzymes, changing the amino acid sequences to improve enzyme activity and stability, thus enhancing the efficiency of the catalytic reactions and promoting threonine synthesis. Next, metabolic pathways are optimized by relieving feedback inhibition in the synthesis pathway. The accumulation of excess threonine often results in feedback inhibition of key enzymes in the synthesis pathway. Through gene editing techniques, feedback regulatory genes can be knocked out or weakened, allowing the metabolic flow to continue smoothly toward threonine synthesis.
In the production process, metabolic regulation strategies are also crucial. On one hand, adjusting fermentation conditions, such as temperature, pH, and oxygen levels, optimizes the growth environment of E. coli, improving the cells' metabolic activity and promoting threonine synthesis. On the other hand, controlling the supply of nutrients is vital to ensure that the cells have sufficient carbon sources, nitrogen sources, and other nutrients required for threonine synthesis. Additionally, by utilizing gene expression regulatory elements, the expression levels of key enzyme genes can be precisely controlled according to the needs of cell growth and threonine synthesis, achieving efficient, stable, and sustainable threonine production.
Conclusion
The metabolic processes of threonine in humans and microorganisms represent a highly complex and precisely regulated biochemical network. It is not only a crucial component of protein synthesis but also plays a role in energy metabolism, one-carbon metabolism, neurotransmitter synthesis, and other physiological processes. As research into threonine metabolism mechanisms deepens, an increasing body of evidence suggests that abnormal threonine metabolism is closely linked to various diseases, including liver diseases, tumors, neurodegenerative disorders, and autoimmune diseases. Therefore, understanding threonine's biosynthesis, metabolic pathways, and its role in diseases holds significant scientific and practical value for developing new therapeutic strategies and improving health outcomes. Additionally, the application of metabolic engineering technologies has made it possible to optimize threonine production, providing new directions for industrial production. In the future, with ongoing research advancements, the field of threonine metabolism may offer more innovative solutions for treating metabolic diseases and developing new biotechnological products.
References
- Ross-Inta, C. M., Zhang, Y. F., Almendares, A., & Giulivi, C. (2009). Threonine-deficient diets induced changes in hepatic bioenergetics. American journal of physiology. Gastrointestinal and liver physiology, 296(5), G1130–G1139. https://doi.org/10.1152/ajpgi.90545.2008
- Wu, X., Yuan, H., Wu, Q., et al. (2024). Threonine fuels glioblastoma through YRDC-mediated codon-biased translational reprogramming. Nature cancer, 5(7), 1024–1044. https://doi.org/10.1038/s43018-024-00748-7
- Yang, Q., Cai, D., Chen, W., Chen, H., & Luo, W. (2022). Combined metabolic analyses for the biosynthesis pathway of l-threonine in Escherichia coli. Frontiers in bioengineering and biotechnology, 10, 1010931. https://doi.org/10.3389/fbioe.2022.1010931
