<P> In humans, non-protein amino acids also have important roles as metabolic intermediates, such as in the biosynthesis of the neurotransmitter gamma - amino - butyric acid (GABA). Many amino acids are used to synthesize other molecules, for example: </P> <Ul> <Li> Tryptophan is a precursor of the neurotransmitter serotonin . </Li> <Li> Tyrosine (and its precursor phenylalanine) are precursors of the catecholamine neurotransmitters dopamine, epinephrine and norepinephrine and various trace amines . </Li> <Li> Phenylalanine is a precursor of phenethylamine and tyrosine in humans . In plants, it is a precursor of various phenylpropanoids, which are important in plant metabolism . </Li> <Li> Glycine is a precursor of porphyrins such as heme . </Li> <Li> Arginine is a precursor of nitric oxide . </Li> <Li> Ornithine and S - adenosylmethionine are precursors of polyamines . </Li> <Li> Aspartate, glycine, and glutamine are precursors of nucleotides . However, not all of the functions of other abundant non-standard amino acids are known . </Li> </Ul> <Li> Tryptophan is a precursor of the neurotransmitter serotonin . </Li> <Li> Tyrosine (and its precursor phenylalanine) are precursors of the catecholamine neurotransmitters dopamine, epinephrine and norepinephrine and various trace amines . </Li>

Which of the following are common to all 20 amino acids