PubMed · 3527493
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G H Rabbani. 1986. Cholera.. https://pubmed.ncbi.nlm.nih.gov/3527493/
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A few aminoacyl-tRNA synthetases are characterized by their ability to tightly bind a zinc atom. In the case of Escherichia coli methionyl-tRNA synthetase, a peptide of 21 residues (138--163) having a stable 3-D structure in solution is responsible for zinc binding [Fourmy, D., Meinnel, T., Mechulam, Y., & Blanquet, S. (1993) J. Mol. Biol. 231, 1066--1077; Fourmy, D., Dardel, F., & Blanquet, S. (1993) J. Mol. Biol. 231, 1078--1089]. This peptide, which belongs to a region connecting the two halves of the nucleotide-binding domain of methionyl-tRNA synthetase, is likely to form a modular domain close to the active center of the enzyme. In this study, two residues of the zinc-binding module, Asp138 and Arg139, are shown to contribute to the stabilization of the transition state of the reaction leading to the activation of methionine. Moreover, another residue, Phe135, located at the surface of the zinc-binding domain, is found to possibly guide the tRNA acceptor stem toward the active site of the enzyme during catalysis. The available data indicate an important functional role for the zinc-binding module of methionyl-tRNA synthetase, as well as for other modules connecting conserved secondary structure elements in the aminoacyl-tRNA synthetase family. The relation between the occurrence of such variable peptide modules and the expression of both substrate specificity and catalytic efficiency is discussed.
A galactose 3-O-sulfotransferase activity able to transfer a sulfate group from adenosine 3'-phosphate 5'-phosphosulfate to methyl galactosides or terminal N-acetyllactosamine-containing carbohydrate chains from human respiratory mucins was characterized in microsomal fractions prepared from human respiratory mucosa. The reaction products, methyl alpha- or beta-galactose 3-sulfate and three oligosaccharide alditols containing the sequence HSO3-3Gal beta 1-4GlcNAc beta 1-6GalNAc-itol were identified by high performance anion-exchange chromatography. Using methyl beta-galactoside as a substrate, the optimum activity was obtained with 0.1% Triton X-100, 30 mM NaF, 20 mM Mn2+, and 10 mM AMP in a 30 mM 2-(N-morpholino)ethanesulfonic acid buffer at pH 6.1. The apparent Km for methyl beta-galactoside and for adenosine 3'-phosphate 5'-phosphosulfate were observed at 0.69 x 10(-3) M and at 4 x 10(-6) M respectively. This sulfotransferase is different from that responsible for sulfatide synthesis.
Glutamine phosphoribosylpyrophosphate (PRPP) amidotransferase catalysis and regulation were studied using a new stable carbocyclic analog of PRPP, 1-alpha-pyrophosphoryl-2-alpha, 3-alpha-dihydroxy-4-beta-cyclopentane-methanol-5-phosphate (cPRPP). Although cPRPP competes with PRPP for binding to the catalytic C site of the Escherichia coli enzyme, two lines of evidence demonstrate that cPRPP, unlike PRPP, does not promote an active enzyme conformation. First, cPRPP was not able to "activate" Cys1 for reaction with glutamine or a glutamine affinity analog. The ring oxygen of PRPP may thus be necessary for the conformation change that activates Cys1 for catalysis. Second, binding of cPRPP to the C site blocks binding of AMP and GMP, nucleotide end product inhibitors, to this site. However, the binding of nucleotide to the allosteric site was essentially unaffected by cPRPP in the C site. Since it is expected that nucleotide inhibitors would bind with low affinity to the active enzyme conformation, the nucleotide binding data support the conclusion that cPRPP does not activate the enzyme.