ABSTRACT
The amino acid sequence of ATP phosphoribosyltransferase [1-(5'-phosphoribosyl)-ATP:pyrophosphate phosphoribosyltransferase, EC 2.4.2.17] of Salmonella typhimurium has been determined. The amino acid sequence analysis was carried out with a combination of manual and automated methods. It was complemented by DNA sequence analysis (done in another laboratory) of the hisG gene, which codes for it. The subunit polypeptide chain contains 299 amino acid residues and has a molecular weight of 33,216. The amino-terminal segment of the protein is relatively basic in character and has limited sequence homologies with the lac repressor and histidinol dehydrogenase. In addition, the protein contains a 40-residue segment that has 13 residues identical with the sequence surrounding the active-site cysteine of glyceraldehyde-3-phosphate dehydrogenase.
Subject(s)
ATP Phosphoribosyltransferase , Pentosyltransferases , Salmonella typhimurium/enzymology , Amino Acid Sequence , Molecular Weight , Peptide Fragments/analysis , Repressor ProteinsABSTRACT
Previous studies have shown that chlorite serves as a halogenation substrate for horseradish peroxidase. In its substrate role, chlorite serves both as a halogen donor and as a source of oxidizing equivalents in the chlorination reaction. We now show that a new spectral intermediate, which we have termed Compound X, can be detected as the initial product of the reaction of chlorite with horseradish peroxidase. The reaction of chlorite with horseradish peroxidase to form Compound X is a relatively fast reaction especially at acidic pH values. The second order rate constant (Kf) for the formation of Compound X at pH 4.5 (optimum pH) is 0.9 X 10(6) M-1 S-1. Compound X, in the absence of a halogen acceptor, decomposes to Compound I and chloride ion. The first order rate constant (Kd) for the decay of Compound X to Compound I is 0.2 s-1 at pH 4.5. The pH optimum for enzymatic chlorination with chlorite compares favorably with the pH profile for the lifetime of Compound X (Kf/Kd). These observations indicate that Compound X is the halogenating intermediate in the chlorite reaction and that the rate of enzymatic chlorination is directly related to the stability of Compound X. We propose an -OCl ligand on a ferric heme as the most likely structure for Compound X.
