21st Amino acid is
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
Selenocysteine
Valine
Tyrosine
Hydroxyproline
Metabolism of protein and amino acid