(1) Nucleotide analogs have been introduced as structural probes into active sites of dodecameric glutamine synthetase (GS) from E. coli. Various analogs of ATP substituted at the 6- or 8- position of the purine ring were further modified with spectrophotometric and fluorometric probes or an electron dense PT (II) marker. Fluorescence energy transfer measurements showed that active site nucleotide probes in the dodecamer are separated by approximately 56 angstrom units. The enzyme from S. typhimurium also has been labeled at active sites of adenylylation sites with mercaptonucleotide. Platinum (II) for crystallization and X-ray structural analysis in D. Eisenberg's laboratory at UCLA. Nucleotide probes also are being used for determining the relative intra-subunit distances between the active sites and Trp 57 and Trp 158. (2) Calorimetric studies of Zn2+-induced stacking of GS dodecamers have shown that Delta Cp is approximately equal to -850 cal/k.mol, suggesting a dominant role of water in the polymerization process. Thermodynamic parameters for binding various active-site ligands to E. coli GS also have been obtained by calorimetry and fluorometry. (3) The octameric GS from bovine brain was found to contain two essential divalent cation sites/subunit--a structural site and a higher affinity nucleotidemetal ion site which is filled after the first site is occupied by Mn(II) or Mg(II). Although the enzyme is active with either Mg(II) or Mn(II) in vitro, Mg(II) is bound to the brain enzyme in vivo. Allosteric sites for chloride and L- glutamate could regulate activity. (4) Regulatory subunits of E. coli aspartate transcarbamoylase bind Zn(II) with high affinity and kinetic constants for this interaction are being determined. (5) Possible roles of Zn(II) and Mn(II) ions in maintaining quaternary structures of arginases from yeast and bovine liver are being investigated.
