Selenocysteine is synthesized from
Alamine
Cysteine
Arginine
Histidine
Selenocysteine is the 21st amino acid. Biosynthesis of Selenocysteine is by replacing the oxygen by selenium in Serine which in turn formed from Cysteine.Ref: DM Vasudevan - Textbook of Biochemistry, 7th edition, page no: 214
Metabolism of protein and amino acid