Conformational Heterogeneity and Spin-Labeled −SH Groups: Pulsed EPR of Na,K-ATPase

Membranous Na,K-ATPase from shark salt gland and from pig kidney was spin-labeled on class I −SH groups in the presence of glycerol, or on class II −SH groups in the absence of glycerol. The class I-labeled preparations retain full enzymatic activity, whereas the class II-labeled preparations are at least partially inactivated. This provides an excellent testbed on which to demonstrate how advanced electron paramagnetic resonance (EPR) can provide novel information on specific residues in unique environments in a complex, membrane-bound transport system. The polarity of the environment, and the librational dynamics and conformational exchange, of the spin-labeled groups were studied with pulsed EPR by using electron spin echo envelope modulation (ESEEM) spectroscopy and spin−echo detected (ED) EPR spectroscopy, respectively. 2H-ESEEM spectra of membranes dispersed in D2O reveal that class I groups of the shark enzyme are more exposed to water than are those of the pig enzyme or class II groups of either species, consistent with the more superficial membrane location in the former case. Spin−echo decay curves indicate conformational heterogeneity at low temperatures (