Cj. Halkides et al., HIGH-FREQUENCY (139.5 GHZ) ELECTRON-PARAMAGNETIC-RESONANCE SPECTROSCOPY OF THE GTP FORM OF P21 RAS WITH SELECTIVE O-17 LABELING OF THREONINE, Biochemistry, 35(37), 1996, pp. 12194-12200
Electron paramagnetic resonance spectroscopy at 139.5 GHz has been use
d to study p21 I-ns complexed with Mn(II) and guanosine 5'-(beta,gamma
-imidotriphosphate), an analog of GTP. The p21 sample studied was sele
ctively labeled with [O-17(gamma)]threonine to a final enrichment of 3
0%, A Mn(II)-O-17 hyperfine interaction was observed, but the value of
the coupling constant, 0.11 +/- 0.04 mT, is the smallest such value y
et reported, Ab initio calculations indicate that this value is consis
tent with direct coordination of the threonine hydroxyl group and prov
ide an estimate for the Mn(II)-O-17 bond length of 2.7 Angstrom. The m
easured hyperfine coupling constant and associated bond length starkly
contrast with typical values for Mn(II)-O-17 coordination complexes,
namely similar to 0.25 mT and similar to 2.2 Angstrom, respectively. T
his contrast underscores the peculiar weakness of this Mn(II)-O intera
ction in p21 and persuasively argues that the nucleotide-induced confo
rmational change, which is known to encompass the region of p21 involv
ing Thr35, is not driven by Mn(II) coordination of the Thr35 hydroxyl
group.