| Abstrakt: |
Active site ligand interactions with dodecameric glutamine synthetase from Escherichia coli were studied spectrally, using the resolved L-S- and L-R-diastereoisomers of the substrate analog L-methionine-SR-sulfoximine. direct measurements of the reversible binding of the S-isomer to unadenylylated manganese-enzyme show a stoichiometry of 1 eq/subunit and negative cooperativity with a Hill coefficient of 0.7. The affinity of this enzyme complex is greatest for the S-isomer alone ([S]0.5 = 35 microM), least with the R-isomer alone ([S]0.5 = 0.38 mM), and intermediate (but closer to that for the S-isomer) for an equimolar mixture of S- and R-isomers ([S]0.5 = 61 microM). The affinity for the S-isomer is enhanced greater than 35-fold by ADP and is decreased approximately 3-fold by adenylylation of the enzyme. Shrake, A., Whitley, E. J. Jr., and Ginsburg, A. ((1980) J. Biol. Chem. 255, 581-589) reported that UV spectral perturbations markedly differ for binding commercial L-methionine-SR-sulfoximine to unadenylylated and adenylylated manganese enzymes. However, essentially the same saturating protein difference spectrum is produced by binding the resolved S- and R-diastereoisomers, and equimolar mixture of S- and R-isomers, and the commercial S- and R-isomeric mixture to a particular enzyme complex. Since neither the subunit interactions that give rise to the observed negative cooperativity of binding nor the affinity differences in binding the S- and R-isomers are reflected in protein difference spectra, spectral perturbations derive from a conformational change that is solely a marked for the occupancy of the single subunit site by either isomer. |
