PubMed Health⌕ Search

Biomedical subjects

S M Parsons

Publications and source records attributed to S M Parsons.

At least 109 records · Page 6Linked to original sources
PubMed1977

Inhibition of histidyl-tRNA-adenosine triphosphate phosphoribosyltransferase complex formation by histidine and by guanosine tetraphosphate.

Formation of the complex between the first enzyme of histidine biosynthesis from Salmonella typhimurium, ATP phosphoribosyltransferase [1-(5'-phosphoribosyl)-ATP: pyrophosphate phosphoribosyltransferase; EC 2.4.2.17], and histidyl-tRNA is shown to be inhibited by L-histidine and by guanosine-5'-diphosphate-3'-diphosphate in the presence of histidine. Higher histodine levels make guanosine tetraphosphate a more effective inhibitor. Relatively high concentrations of guanosine-5'-triphosphate also inhibit complex formation, but this inhibition is not enhanced by histidine. The possible implications of these observations with respect to the gene regulatory activity of this enzyme are discussed.

ATP Phosphoribosyltransferase↗
PubMed1975

Evidence against a covalent intermediate in the adenosine triphosphate phosphoribosyltransferase reaction of histidine biosynthesis.

14C-Labeled 5-phospho-alpha-D-ribose-1-diphosphate (PRibPP) was synthesized and its interaction with adenosine triphosphate phosphoribosyltransferase was examined by gel filtration in a search for a form of this substrate covalently bound to the enzyme. Wide variation in solvent conditions gave little labeling of the enzyme. Heavy labeling was found only in the presence of the second substrate, ATP, and this was shown to arise from tightly but noncovalently bound product. Previous reports of a covalent intermediate in this enzymatic reaction probably were due to contaminating ATP in 5-phospho-alpha-D-ribose-1-diphosphate. Feedback inhibition of the enzyme by histidine was shown to occur at the step giving product or at some earlier step in the mechanism.

ATP Phosphoribosyltransferase↗
PubMed1975

Stereochemical course of the adenosine triphosphate phosphoribosyltransferase reaction in histidine biosynthesis.

The product of the first reaction in histidine biosynthesis is shown by optical rotation measurements on three derivatives to have inverted, beta stereochemistry at the newly formed bond. This is in contrast to alpha linkage expected on the basis of previously observed exchange, specificity, and covalent intermediate phenomena. The postulate double displacement mechanism for adenosine triphosphate phosphoribosyltransferase must be modified to account for the product stereochemistry.

Adenosine Triphosphate↗
PubMed1975

A unique reactive residue in adenosine triphosphate phosphoribosyltransferase sensitive to five conformation and dissociation states.

Adenosine triphosphate phosphoribosyltransferase is inactivated rapidly by bulky alkylating reagents in a biphasic reaction. The initial inactivation rate is dependent upon an optimal concentration of histidine and is more rapid at low enzyme concentrations and low ionic strength. A histidine-free dimer form of the enzyme is the proposed reactive species. The dimer is shown by ultraviolet difference spectroscopy to bind histidine about 1 order of magnitude more weakly than the hexameric form of the enzyme. Alkylated enzyme is similar to native enzyme in dissociation and histidine-binding properties. Native enzyme must exist at significant levels in at least five different conformational and dissociative states to account for the inactivation behavior.

Acetamides↗
PubMed1975

Composition of the first enzyme of histidine biosynthesis isolated from wild-type and mutant operator strains of Salmonella typhimurium.

The first enzyme of histidine biosynthesis in Salmonella typhimurium, adenosine triphosphate phosphoribosyltransferase (EC 2.4.2.17), has been purified from two bacterial strains containing histidine operator deletions and compared to the eenzyme from a strain that has a normal operator. The enzymes isolated in different ways also are compared. Evidence as to the separateness of the operator and first structural gene or covalent modification of the first enzyme was sought. Specific activity, histidine feedback inhibition, amino acid analysis, discontinuous-gel electrophoresis, and gel filtration of the native enzyme, and Ouchterlony double-immunodiffusion tests were carried out. The purified enzyme contains little phosphorous and has five cysteine residues per subunit, which all are readily titratable. No evidence for differences in the enzyme preparations was obtained. Thus, no evidence for overlap of the histidine operator with the first structural gene was obtained.

Amino Acids↗