title: Enzymological features of aromatic amino acid biosynthesis reflect the phylogeny of mycoplasmas.
abstract: Acholeplasma laidlawii possesses a biochemical pathway for tyrosine and phenylalanine biosynthesis, while Mycoplasma iowae and Mycoplasma gallinarum do not. The detection of 7-phospho-2-dehydro-3-deoxy-D-arabino-heptonate (DAHP) synthase (EC 4.1.2.15), dehydro-shikimate reductase (EC 1.1.1.25) and 3-enol-pyruvoylshikimate-5-phosphate synthase (EC 2.5.1.19) activities in cell-free extracts established the presence in A. laidlawii of a functional shikimate pathway. L-Phenylalanine synthesis occurs solely through the phenylpyruvate route via prephenate dehydratase (EC 4.2.1.51), no arogenate dehydratase activity being found. Although arogenate dehydrogenase was detected, L-tyrosine synthesis appears to occur mainly through the 4-hydroxyphenylpyruvate route, via prephenate dehydrogenase (EC 1.3.1.12), which utilized NAD+ as a preferred coenzyme substrate. L-Tyrosine was found to be the key regulatory molecule governing aromatic biosynthesis. DAHP synthase was feedback inhibited by L-tyrosine, but not by L-phenylalanine or L-tryptophan; L-tyrosine was a potent feedback inhibitor of prephenate dehydrogenase and an allosteric activator of prephenate dehydratase. Chorismate mutase (EC 5.4.99.5) was sensitive to product inhibition by prephenate. Prephenate dehydratase was feedback inhibited by L-phenylalanine. It was also activated by hydrophobic amino acids (L-valine, L-isoleucine and L-methionine), similar to results previously found in a number of other genera that share the Gram-positive line of phylogenetic descent. Aromatic-pathway-encoded cistrons present in saprophytic large-genome mycoplasmas may have been eliminated in the parasitic small-genome mycoplasmas.
mesh_terms: D010723:3-Deoxy-7-Phosphoheptulonate Synthase / Q000378:metabolism; D000128:Acholeplasma laidlawii / Q000201:enzymology; D000494:Allosteric Regulation; D000596:Amino Acids / Q000096:biosynthesis*; D008715:Methionine / Q000378:metabolism; D009174:Mycoplasma / Q000145:classification / Q000201:enzymology*; D010649:Phenylalanine / Q000378:metabolism; D010802:Phylogeny; D014443:Tyrosine / Q000378:metabolism
chemical_list: D000596:Amino Acids; D014443:Tyrosine; D010649:Phenylalanine; D008715:Methionine; D010723:3-Deoxy-7-Phosphoheptulonate Synthase