<prompt> 21st Amino acid isSelenocysteineValineTyrosineHydroxyproline  <response>: Selenocysteine, the 21st Amino AcidSelenocysteine is present at the active site of several human enzymes that catalyze redox reactions. Examples include thioredoxin reductase, glutathione peroxidase, and the deiodinase that conves thyroxine to triiodothyronine.Ref: Harper;s Biochemistry; 30th edition; Chapter 27 Biosynthesis of the Nutritionally Nonessential Amino Acids 