<P> Besides the regulations described above, each amino acids terminal pathway can be regulated . These terminal pathways progress from chorismate to the final end product, either tyrosine, phenylalanine, or tryptophan . Each one of these pathways is regulated in a similar fashion to the common pathway; with feedback inhibition on the first committed step of the pathway . </P> <P> Tyrosine and phenylalanine share the same initial step in their terminal pathways, chorismate converted to prephenate which is converted to an amino acid - specific intermediate . This process is mediated by a phenylalanine (PheA) or tyrosine (TyrA) specific chorismate mutase - prephenate dehydrogenase . The reason for the amino acid - specific enzymes is because PheA uses a simple dehydrogenase to convert prephenate to phenylpyruvate, while TyrA uses a NAD - dependent dehydrogenase to make 4 - hydroxylphenylpyruvate . Both PheA and TyrA are feedback inhibited by their respective amino acids . Tyrosine can also be inhibited at the transcriptional level by the TyrR repressor . TyrR binds to the TyrR boxes on the operon near the promoter of the gene that it wants to repress . </P> <P> In the terminal - tryptophan synthesis pathway, the initial step converts chorismate to anthranilate using anthranilate synthase . This enzyme requires either ammonia or glutamine as the amino group donor . Anthranilate synthase is regulated by the gene products of trpE and trpG . trpE encodes the first subunit, which binds to chorismate and moves the amino group from the donor to chorismate . trpG encodes the second subunit, which binds glutamine and uses it as the amino group donor so that the amine group can transfer to the chorismate . Anthranilate synthase is also regulated by feedback inhibition . The finished product of tryptophan, once produced in great enough quantities, is able to act as the co-repressor to the TrpR repressor which represses expression of the trp operon . </P> <P> The oxaloacetate / aspartate family of amino acids is composed of lysine, asparagine, methionine, threonine, and isoleucine . Aspartate can be converted into lysine, asparagine, methionine and threonine . Threonine also gives rise to isoleucine . All of these amino acids contain different mechanisms for their regulation, some being more complex than others . All the enzymes in this biosynthetic pathway are subject to regulation via feedback inhibition and / or repression at the genetic level . As is typical in highly branched metabolic pathways, there is additional regulation at each branch point of the pathway . This type of regulatory scheme allows control over the total flux of the aspartate pathway in addition to the total flux of individual amino acids . The aspartate pathway uses L - aspartic acid as the precursor for the biosynthesis of one fourth of the building block amino acids . Without this pathway, protein synthesis would not be possible . </P>

Most amino acid degradation takes place in the