Absolute dependence of phenylalanine and tyrosine biosynthetic enzyme on tryptophan in Candida maltosa

Metabolomics reveals differential levels of oral metabolites in HIV-infected patients: toward novel diagnostic targets

The objective of the current study was to characterize the profile of oral metabolites in HIV-infected patients using metabolomics. Oral wash samples were collected from 12 HIV-infected and 12 healthy individuals (matched for age, sex, and ethnicity), processed, and analyzed by metabolomics. We detected 198 identifiable and 85 nonidentifiable metabolites; 27 identifiable metabolites were differentially present (12 increased, 15 decreased) in HIV-infected patients. Elevated metabolites included p-cresol sulfate, nucleotides (e.g., allantoin), and amino acids (e.g., phenylalanine, tryptophan), whereas decreased oral metabolites included fucose, fumarate, and N-acetylglucosamine. Pathway network analysis revealed the largest multinode network in healthy versus HIV-infected patients to involve carbohydrate biosynthesis and degradation. HIV-infected patients on antiretroviral therapy (ART) showed the largest number (12) of statistically significant metabolite correlation differences compared with healthy controls. Interestingly, the oral phenlyalanine:tyrosine ratio increased in ART-naive HIV-infected patients (mean ± SEM = 2.58 ± 0.87) compared with healthy individuals (1.33 ± 0.10, p = 0.062) or ART-experienced patients (1.78 ± 0.30, p = 0.441). This is the first study to reveal differential levels of oral metabolites in HIV-infected patients compared withj healthy volunteers, and that oral phenlyalanine:tyrosine ratio may be a useful marker for noninvasive monitoring of the immune status during HIV infection.

Structures of open (R) and close (T) states of prephenate dehydratase (PDT)--implication of allosteric regulation by L-phenylalanine

The enzyme prephenate dehydratase (PDT) converts prephenate to phenylpyruvate in L-phenylalanine biosynthesis. PDT is allosterically regulated by L-Phe and other amino acids. We report the first crystal structures of PDT from Staphylococcus aureus in a relaxed (R) state and PDT from Chlorobium tepidum in a tense (T) state. The two enzymes show low sequence identity (27.3%) but the same prototypic architecture and domain organization. Both enzymes are tetramers (dimer of dimers) in crystal and solution while a PDT dimer can be regarded as a basic catalytic unit. The N-terminal PDT domain consists of two similar subdomains with a cleft in between, which hosts the highly conserved active site. In one PDT dimer two clefts are aligned to form an extended active site across the dimer interface. Similarly at the interface two ACT regulatory domains create two highly conserved pockets. Upon binding of the L-Phe inside the pockets, PDT transits from an open to a closed conformation.

Regulation of chorismate mutase activity of various yeast species by aromatic amino acids

The regulatory properties of chorismate mutase, its cellular localization and isoenzyme pattern were investigated in 23 yeast species. All yeasts contained only a single form of the enzyme, which is localized exclusively in the cytosol. The enzyme activity from all sources was activated 3-(Rhodotorula aurantiaca) to 185-fold (Candida maltosa) by tryptophan. The tryptophan concentration, which was necessary to obtain half maximum velocity was determined to be between 2 (Pichia guilliermondii) and 95 microM (Yarrowia lipolytica). Ten yeast species possessed an enzyme that was inhibited by both phenylalanine and tyrosine. The chorismate mutase from four strains was inhibited only by tyrosine and the enzyme from two species was inhibited by phenylalanine alone. The enzyme inhibition by phenylalanine and tyrosine was completely reversed by tryptophan. Six enzyme sources were not inhibited and the Y. lipolytica chorismate mutase was slightly activated by both amino acids.

Regulation of metabolic branch points of aromatic amino acid biosynthesis in Pichia guilliermondii

The regulatory properties of the enzymes involved in the aromatic amino acid biosynthesis of Pichia guilliermondii were investigated and compared with the regulatory pattern found in other yeast species. 3-Deoxy-D-arabino-heptulosonate-7-phosphate (DAHP) synthase, anthranilate synthase, chorismate mutase and prephenate dehydrogenase are key regulatory enzymes in P. guilliermondii. Two distinctly regulated isozymes of DAHP synthase, the initial pathway enzyme, which is inhibited by tyrosine or phenylalanine were separated by DEAE-cellulose chromatography and were characterized. Tryptophan is an excellent feedback inhibitor of anthranilate synthase, the first definite step in tryptophan biosynthesis. There are two controlled enzymes within the specific synthesis of phenylalanine and tyrosine, chorismate mutase and prephenate dehydrogenase. Chorismate mutase exhibits a balanced allosteric responsivity to phenylalanine and tyrosine, when these are used as inhibitor; tryptophan acts as an allosteric activator. Tyrosine is an effective inhibitor of prephenate dehydrogenase, whereas the activity of prephenate dehydratase is not affected by any of the aromatic amino acids. The synthesis of the enzymes in the yeast was not repressed by any single exogenous aromatic amino acids, nor by combinations of the same.