Serine protease, D- or L-serine arginine rich enzyme of serine threonine kinase with serine that is encoded by the codons UCU, UCC, UCA, UCG, AGU and AGC is an ɑ-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH+
3 form under biological conditions), a carboxyl group. It is non-essential in humans, meaning the body can synthesize it.