Allosteric interpretation of Mg2+ ion binding to the denaturable Escherichia coli tRNAGlu2

Abstract

The Mg2+ binding properties of the denaturable tRNAGlu2 from E. coli in 0.1 M Na+, pH 7, are characterized by equilibrium dialysis. At 34.degree. C, where the native and denatured conformers are in equilibrium, Mg2+ binding is cooperative. By trapping the tRNA completely in the native conformation at 4.degree. C it is shown that native tRNAGlu2 possesses 1 strong binding site, K1 [association constant] = 7.5 .times. 104 M-1 and .apprx. 36 weak sites with K2 = 8.3 .times. 102 M-1. A significantly lower affinity for the denatured conformer is indicated. Mg2+ effects an allosteric transition from the low affinity denatured conformational state to the high affinity native state and appropriate equations were developed to fit the Mg2+ binding data with physically meaningful parameters. The previously reported cooperative cation binding to tRNA probably arises from a cation induced conformational change to the native tRNA conformation and does not reflect the inherent Mg2+ binding properties of the native conformer.