That amino acid is an analog of cysteine with this element in place of sulfur.
Thressa Stadtman is particularly known for studying the biochemistry of this element. This element's  dioxide is used for the oxidation of alkenes to al ylic alcohols and ketones to alpha-diketones. This  element is found in the active site of an enzyme subfamily that catalyzes the removal of an iodine from  T4 to form T3, which is active, or reverse T3 or T2, which are inactive; that enzyme subfamily is the  iodothyronine deiodinases. Glutathione peroxidases and thioredoxin reductases contain this element at  their active site. An amino acid with this element is coded for by the UGA codon, which is normal y a stop  codon. That amino acid is an analog of cysteine with this element in place of sulfur. For 10 points, identify  this chalcogen that is just heavier than sulfur.
Thressa Stadtman is particularly known for studying the biochemistry of this element.
selenium [or Se]