While our bodies require many minerals for optimal health, some are needed in only minuscule amounts. Among these trace elements is selenium, which is incorporated into a special class of proteins called selenoproteins. These proteins are defined by the presence of selenium, which is necessary for their biological activity. Found throughout the body, selenoproteins perform a wide array of functions important for maintaining human health.
The Role of Selenium in Protein Synthesis
The creation of selenoproteins involves a unique biological mechanism. This process uses the amino acid selenocysteine, which contains a selenium atom and is often called the 21st amino acid. Its incorporation into a protein chain requires the cell to reinterpret a specific genetic signal.
Normally, the genetic codon UGA signals the cell to stop protein synthesis. For selenoproteins, however, the cell reinterprets this UGA codon as an instruction to insert selenocysteine. This recoding is guided by a sequence in the messenger RNA (mRNA) known as the selenocysteine insertion sequence (SECIS) element. The process begins with the amino acid serine attached to a transfer RNA (tRNA) molecule, which is then converted into selenocysteine and delivered to the ribosome to be added to the protein.
This synthesis occurs on the tRNA molecule itself. The availability of selenium in the body directly influences this process. This highlights the direct link between dietary intake and the proper formation of these proteins.
Key Functions of Selenoproteins
Selenoproteins are involved in many physiological processes, with their primary functions stemming from antioxidant capabilities. They protect cells from oxidative damage caused by free radicals, which are unstable molecules that can harm cellular structures. By neutralizing these compounds, selenoproteins help prevent oxidative stress.
A prominent family of selenoproteins central to this protective function is the glutathione peroxidases (GPXs). These enzymes work to reduce harmful hydrogen peroxide and lipid peroxides into harmless substances, acting as a first line of defense against oxidative damage. Another group, the thioredoxin reductases (TXNRDs), are involved in redox regulation, which controls the balance of molecules within cells. Together, these antioxidant selenoproteins form a network that shields cells, proteins, and DNA from damage.
Beyond antioxidant defense, selenoproteins are important for thyroid hormone regulation. The thyroid gland produces hormones that regulate metabolism, but these must be converted into their active form to function properly. This conversion is carried out by a family of selenoproteins known as iodothyronine deiodinases (DIOs). These enzymes remove iodine atoms from the thyroid hormone thyroxine (T4) to produce the more active triiodothyronine (T3), thereby controlling metabolic rate and energy production.
Selenoproteins also influence the immune system by modulating inflammation and supporting a robust immune response. They are involved in regulating the production of inflammatory cytokines and preserving the function of immune cells like neutrophils from oxidative stress. By managing inflammation and supporting immune cell activity, these proteins contribute to the body’s ability to fight off infections.
Selenoproteins and Human Health
The amount of selenium in the body directly impacts health, as an imbalance can lead to problems. The body requires a narrow window for optimal selenium levels to ensure these proteins function correctly.
A deficiency in selenium impairs the production of selenoproteins, leaving the body vulnerable to health issues. In regions where soil selenium content is very low, severe deficiency is linked to specific diseases. One such condition is Keshan disease, a form of cardiomyopathy that affects the heart muscle, leading to cardiac complications. Another is Kashin-Beck disease, a disorder of the bones and joints that causes cartilage breakdown and joint deformities. While other factors may contribute, selenium deficiency is a primary cause.
Milder selenium deficiency can result in a weakened immune system and may contribute to thyroid dysfunction due to the impaired activity of deiodinase enzymes. Conversely, an excess of selenium can be toxic, a condition known as selenosis. This typically results from over-supplementation rather than dietary intake, although it can occur in areas with extremely high soil selenium levels. Symptoms of chronic selenosis include hair and nail loss or brittleness, gastrointestinal disturbances, skin rashes, and neurological issues such as fatigue and irritability. A distinctive sign of acute selenium toxicity is a garlic-like odor on the breath.
Dietary Selenium for Selenoprotein Production
Ensuring adequate production of selenoproteins relies on obtaining sufficient selenium from the diet. The Recommended Dietary Allowance (RDA) for adults is 55 micrograms (mcg) per day, an amount most people can achieve through a balanced diet. The concentration of selenium in plant-based foods is highly dependent on the selenium content of the soil in which they were grown, leading to geographical variations.
Some of the richest dietary sources of selenium are animal products and seafood.
- Fish such as tuna, halibut, and sardines
- Shellfish
- Poultry, beef, and organ meats like liver
- Eggs and dairy products
For those following plant-based diets, Brazil nuts are famously high in selenium, with just a single nut often exceeding the daily requirement. Grains and cereals can also be good sources, though their content varies with soil conditions.