Selenium-dependent enzymes are proteins that require the trace element selenium for their activity. These enzymes play crucial roles in various biological processes, including antioxidant defense and thyroid hormone metabolism. They contain a unique amino acid called selenocysteine, which is essential for their function. One of the most well-known selenium-dependent enzymes is glutathione peroxidase, which helps protect cells from oxidative damage. Another important enzyme is iodothyronine deiodinase, which is involved in the activation and deactivation of thyroid hormones. Adequate selenium intake is vital for the proper functioning of these enzymes and overall health.