21st amino acid Selenocysteine

Selenocysteine is known as the 21st amino acid because of its unique presence and role in some proteins, despite not being coded directly by the DNA in the traditional sense. It is incorporated into proteins via a special mechanism that involves the recoding of the UGA stop codon, typically signaling the end of protein synthesis. This recoding requires a specific selenocysteine insertion sequence (SECIS) element in the mRNA. Selenocysteine is crucial for the catalytic activity of several enzymes, including thioredoxin reductase. Thioredoxin reductase is a selenium-containing enzyme that plays a vital role in cellular redox homeostasis by reducing thioredoxin, thereby contributing to the regulation of cell growth and protecting against oxidative stress. This function is essential for maintaining the balance of reactive oxygen species within cells and supports various critical biological processes. Problem: The rare amino acid selenocysteine is recognized as the 21st amino acid. It is incorporated into proteins by a specific tRNA recognizing the UGA codon, usually a stop signal. This amino acid is essential for the function of which of the following enzymes? A) Alcohol dehydrogenase B) Glucokinase C) Thioredoxin reductase D) Hexokinase