Supramolecular Self-Assembly of Escherichia coli Glutamine Synthetase: Effects of Pressure and Adenylylation State on Dodecamer Stacking

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Abstract
Escherichia coli glutamine synthetase is a dodecamer of identical subunits, consisting of two face-to-face hexameric rings. The enzymatic activity of GS is regulated by covalent attachment of an adenylyl group to each subunit, at the edge of the ring structure (Tyr-397). In the presence of Zn2+, Cu2+, Co2+, and other divalent metal ions, the free dodecamers self-organize into protein tabules [Miller et al. (1974) Arch. Biochem. Biophys. 163, 155-171]. Here, the temperature dependence and pressure dependence of the kinetics of Zn(2+)-induced self-assembly of GS tubules have been determined for the adenylylated and unadenylylated GS. The adenylylated enzyme exhibits a bimolecular rate constant for Zn(2+)-induced stacking that is 3-fold lower than for the unadenylylated GS at temperatures ranging from 0 to 25 degrees C. The enthalpy of activation, delta H++, for both adenylylated and unadenylylated GS increases from approximately 10 kcal/mol of dodecamer interface to 20 kcal/mol of dodecamer interface upon addition of 125 mM KCl to the reaction buffer. The delta H++ values for adenylylated and unadenylylated GS are nearly identical, at each concentration of KCl, suggesting that entropic factors are responsible for the differences in rate of stacking for these forms of GS. Hydrostatic pressure markedly inhibits the stacking reaction for both adenylylated and unadenylylated GS. The activation volumes, delta V++a, for stacking are increased from approximately 50 mL/mol of dodecamer interface in the absence of KCl to approximately 65 mL/mol of dodecamer interface in the presence of 125 mM KCl.(ABSTRACT TRUNCATED AT 250 WORDS)