Zusammenfassung
Electron paramagnetic resonance spectroscopy has been used to obtain information on the structure and stability of the products of GTP cleavage at the active site of elongation factor Tu (EF-Tu) from Bacillus stearothermophilus. Using stereospecifically labelled (Sp)-(Rp)-[beta-17O]GTP (prepared by modification of a previously published procedure which is now also suitable for guanine ...
Zusammenfassung
Electron paramagnetic resonance spectroscopy has been used to obtain information on the structure and stability of the products of GTP cleavage at the active site of elongation factor Tu (EF-Tu) from Bacillus stearothermophilus. Using stereospecifically labelled (Sp)-(Rp)-[beta-17O]GTP (prepared by modification of a previously published procedure which is now also suitable for guanine nucleotides), it was found that only one of the two possible diastereomers (Sp) led to detectable line-broadening of the EPR spectrum of Mn2+ at the active site of EF-Tu (linewidth 1.5 mT), whereas the Rp isomer caused the same linewidth as unlabelled nucleotide (1.3 mT). From our earlier work and from a demonstration that the lifetime of the state giving the broadened spectrum is too long to be assigned to the EF-Tu.GDP.Mn complex [the rate constant for decay as measured by displacement of GDP by the fluorescent 2'(3')-O-(N-methylanthraniloyl)-GDP is 6.2 x 10(-3) s-1 at 25 degrees C and pH 6.8], we conclude that the broadened signal arises from the EF-Tu.Mn.GDP.Pi complex, the predominant steady-state species. During the hydrolysis of GTP the Mn2+ remains bound to the beta-phosphate oxygen of GDP which arises from the beta pro-S oxygen of GTP, possibly until GDP dissociates and certainly until Pi dissociates. Addition of elongation factor Ts (EF-Ts) to this intermediate leads to rapid reduction of the linewidth to that expected for random distribution of interactions of one 17O and two 16O atoms of GDP with Mn2+, and is not distinguishable from that exhibited by (Rp)-[beta-17O]GTP in the corresponding complex in the presence of EF-Ts.
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