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Selenoprotein – Function, Role and Selenium

Selenoproteins are proteins that contain the trace element selenium in the form of selenocysteine. They play a key role in antioxidant defense and thyroid hormone metabolism.

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Things worth knowing about "Selenoprotein"

Selenoproteins are proteins that contain the trace element selenium in the form of selenocysteine. They play a key role in antioxidant defense and thyroid hormone metabolism.

What Are Selenoproteins?

Selenoproteins are a unique class of proteins that incorporate the rare amino acid selenocysteine, often referred to as the 21st proteinogenic amino acid. Selenocysteine contains the essential trace element selenium. To date, 25 selenoprotein-encoding genes have been identified in the human genome, each contributing to vital physiological processes.

Biological Functions

Selenoproteins fulfill a wide range of regulatory and protective roles in the human body. Their key functions include:

Antioxidant defense: Enzymes such as the glutathione peroxidases (GPx) neutralize reactive oxygen species (free radicals) and protect cells from oxidative damage.
Thyroid hormone metabolism: Iodothyronine deiodinases activate and inactivate thyroid hormones (e.g., converting the prohormone T4 into the active form T3).
Redox homeostasis: Thioredoxin reductases (TrxR) maintain the cellular redox balance and support DNA synthesis.
Selenium transport: Selenoprotein P (SELENOP) is the primary selenium carrier in the bloodstream, delivering selenium to target organs including the brain and testes.
Immune function: Selenoproteins protect immune cells from oxidative stress, supporting a healthy immune response.
Reproduction: Phospholipid hydroperoxide glutathione peroxidase (GPx4) is essential for sperm motility and male fertility.

Key Selenoproteins at a Glance

Glutathione Peroxidases (GPx1-GPx8)

This enzyme family reduces hydrogen peroxide and lipid hydroperoxides, acting as the first line of defense against oxidative stress. Different GPx isoforms are active in various tissues, including the liver, kidney, lungs, and red blood cells.

Thioredoxin Reductases (TrxR1-TrxR3)

These enzymes regenerate the thioredoxin antioxidant system and are involved in cell proliferation, apoptosis regulation, and the synthesis of deoxyribonucleotides. They are also a target of interest in cancer research.

Iodothyronine Deiodinases (DIO1-DIO3)

These enzymes catalyze the activation and inactivation of thyroid hormones. Adequate selenium intake is therefore essential for proper thyroid function and hormone regulation.

Selenoprotein P (SELENOP)

Selenoprotein P is the only known selenoprotein containing multiple selenocysteine residues. It is primarily synthesized in the liver and transports selenium via the blood to various organs, particularly the brain and reproductive tissues.

Selenium as the Foundation of Selenoprotein Synthesis

The biosynthesis of selenoproteins depends on an adequate dietary intake of selenium. Good food sources of selenium include:

Brazil nuts (exceptionally high selenium content)
Seafood and fish
Meat and organ meats
Eggs and dairy products
Cereals and grains (selenium content varies by soil composition)

The World Health Organization (WHO) recommends a daily selenium intake of approximately 55 µg per day for adults. Requirements may increase during pregnancy and breastfeeding.

Consequences of Selenium Deficiency on Selenoproteins

When selenium intake is insufficient, the synthesis and activity of selenoproteins are impaired. The body follows a strict hierarchy in selenium distribution, prioritizing the brain and vital organs while peripheral tissues are affected first. Consequences of selenium deficiency can include:

Increased oxidative stress due to reduced GPx activity
Impaired thyroid function (e.g., hypothyroidism)
Weakened immune defenses
Keshan disease (selenium deficiency-related cardiomyopathy)
Kashin-Beck disease (a joint and bone disorder)

Selenoproteins and Health

Research into selenoproteins is a dynamic and growing scientific field. Studies suggest that optimal selenoprotein activity may be associated with a reduced risk of cardiovascular disease, neurodegenerative conditions, and certain types of cancer. However, it is important to note that excessive selenium intake can be toxic, causing a condition known as selenosis. Therefore, a balanced diet is generally preferable to high-dose supplementation.

References

Labunskyy VM, Hatfield DL, Gladyshev VN. Selenoproteins: molecular pathways and physiological roles. Physiological Reviews. 2014;94(3):739-777.
Schomburg L. Dietary selenium and human health. Nutrients. 2020;12(2):332.
World Health Organization (WHO). Trace Elements in Human Nutrition and Health. Geneva: WHO Press; 1996.

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