Threonine is an essential amino acid that can perform several functions in metabolism due to its hydroxyl group. It is a component of most proteins in the body, with a particularly high proportion present in connective tissue. Threonine occurs in four stereoisomeric forms, with only L-threonine with the (2S,3R) configuration being considered for protein construction.
What is threonine?
Threonine represents an essential proteinogenic amino acid. The human body cannot synthesize it. Therefore, it is mandatory to be supplied with food. Therefore, when there is a deficiency of threonine, health problems occur. Threonine is a simply structured alpha amino acid with two stereogenic centers. For this reason, four different stereoisomers can form. However, only L-threonine with the stereoisomeric configuration (2S,3R) is relevant for protein assembly. In the following, this molecule will be further described and, for simplicity, will be referred to only as threonine. Threonine is a polar amino acid capable of phosphorylation in enzymes due to its hydroxyl group. Therefore, it is often a component of enzymes. Threonine was discovered as the last proteinogenic amino acid by the American biochemist William Cumming Rose in the 1930s. While feeding rats, he realized that the 19 amino acids known up to that time were not sufficient for their growth. After a systematic search for the missing growth factor, Rose was able to isolate and describe the previously unknown amino acid threonine from fibrin.
Function, action, and roles
Threonine performs a variety of functions in the organism. However, many of the functions have not yet been studied in detail. However, it is known that threonine plays a major role in growth and uric acid metabolism. It has been found, for example, that if excess threonine is supplied, too much uric acid is produced in the body, which in extreme cases can also lead to gout. For its optimal mode of action, sufficient magnesium, vitamin B6 and vitamin B3 must also be present in the body. Furthermore, threonine is involved in the formation of many proteins. However, it is particularly abundant in the collagen of connective tissue. In addition to connective tissue, it is also a component of mucins. Mucins are glycoproteins that are very rich in threonine and are the most important components in the mucus of the mucous membranes. They protect certain organs, such as the stomach, from aggressive chemical substances. In the case of the stomach, this is the hydrochloric acid containing gastric acid. However, they also provide protection for other organs equipped with mucous membranes against attack by infectious germs and reactive chemicals. The threonine contained in the mucin with the functional hydroxyl group also plays an important role. The hydroxyl group is the attachment point for esterifications with acids and compounds containing acid groups. Thus, the phosphate groups of phosphoric acid can also be bound here. Within enzymes, threonine is therefore responsible for the transfer of phosphate groups, i.e. for the many phosphorylation reactions. Furthermore, threonine is also an important component of antibodies. Here it is present glycosylated with sugar residues, which is particularly crucial for the correct function of antibodies. Threonine also plays an important role in the formation of the neurotransmitter glycine. Glycine is a breakdown product of threonine.
Formation, occurrence, properties, and optimal levels
Threonine, as mentioned above, is an essential amino acid and therefore must be supplied in the diet. It is formed biochemically from L-aspartate in plants and microorganisms. Threonine is found in animal and plant foods. Particularly rich in threonine are chicken eggs, salmon, chicken breast, beef, cow’s milk, walnuts, whole wheat and corn flour, unshelled rice or dried peas. In the human organism, it is obtained by the breakdown of these proteins and incorporated into the body’s own proteins. In all organisms, threonine is degraded to either glycine and acetaldehyde or propionyl-CoA. The daily requirement for an adult person is approximately 16 milligrams per kilogram of body weight. This is 1 to 2 grams of threonine daily, depending on weight.
Diseases and disorders
Because threonine is an essential amino acid, deficiency symptoms can develop if the intake is too low. Threonine deficiency occurs when a diet is unbalanced with foods that contain little threonine.Thus, a lack of threonine is noticeable through fatigue, loss of appetite, weight loss, fatty liver or deficient bone growth. Especially if the threonine deficiency occurs in childhood, the child’s growth can be massively impaired. Furthermore, the immune system is weakened, since threonine is an important component of antibodies. This results in an increased susceptibility to infections. The weakened immune system also increases the risk of cancer. Furthermore, the mucous membranes can no longer properly fulfill their protective function. They become more susceptible to infections and exposure to aggressive chemicals. Since the breakdown product of threonine is, among other things, the neurotransmitter glycine, the nerve functions are also affected by threonine. If this amino acid is lacking, neurological symptoms may occur. When there is a severe excess of threonine, uric acid is formed to a greater extent. However, the effect of moderately elevated threonine concentrations is also to increase uric acid excretion by the kidneys, which in turn contributes to lowering uric acid levels. If this balance of threonine action is disturbed, this can lead to the development of gout. There is an increased need for threonine in infections, nervous system disorders (for example, spasms in multiple sclerosis), ALS (amyotrophic lateral sclerosis), anxiety, irritability, liver disease, schizophrenia, and many other conditions. Threonine, via its breakdown product glycine, dampens hyperactive nerve responses and helps improve neuromuscular control.
