Electron paramagnetic resonance ATPases

Figure 2. Electron paramagnetic resonance spectra of Mr bound to the single catalytic site on (Na + K )-ATPase. The X-hand spectrum (9.5 GHz) is shown in A, while the K-band spectrum (35 GHz) of the same complex is shown in B. The enzyme-Mn2 complex was centrifuged out of 20mM Tes-TMA, pH 7.5, and then combined with buffer so that the final concentrations were 0.15mM (Ha 4-K )-A TPase, 0.1 mM MnCl, 20mM Tes-TMA, pH 7.5. T = 23°C.

The possibility of using the electron paramagnetic resonance properties of Gd3+ to probe its environment in and interactions with biological molecules has previously received little attention in the literature (40). However, the possibility exists that Gd2+ will be a sensitive EPR probe for characterizing macromolecular biological systems such as the Ca2+-ATPase. The EPR spectra of Gd3+, which has S = 7/2. in neutral water and in two different buffers are shown in Figure 13A. The linewidths were found to be independent of pH over the usable range of these buffers and independent of temperature between 4 and 30°C, The spectrum of Gd2+ in neutral water is centered around 3248 G, with a linewidth of 530 G. As shown, Gd3+ in Pipes buffer, but not in Tes buffer, yielded a spectrum similar to that of the aqueous Gd2+ solution. On this basis, all of our Gd3+ EPR and NMR studies... 

Three membrane-bound adenosine triphosphatase enzymes have been characterized using Mn(II) and Gd(III) electron paramagnetic resonance (EPR) and a variety of NMR techniques. Mn(II) EPR studies of both native and partially delipidated (Na+ + K+)-ATPase from sheep kidney indicate that the enzyme binds Mn2+ at a single, catalytic site with Kq = 0.21 x 10- M. The X-band EPR spectrum of the binary Mn(II)-ATPase complex exhibits a powder line shape consisting of a broad transition with partial resolution of the 55 n nuclear hyperfine structure, as well as a broad component to the low field side of the spectrum. ATP, ADP, AMP-PNP and Pj all broaden the spectrum, whereas AMP induces a substantial narrowing of the hyperfine lines of the spectrum. 

In order to characterize the active site structure of Ca ATPase from sarcoplasmic reticulum, we have employed Gd + as a paramagnetic probe of this system in a series of NMR and EPR investigations. Gadolinium and several other lanthanide ions have been used in recent years to characterize Ca + (and in some cases Mg2+) binding sites on proteins and enzymes using a variety of techniques, including water proton nuclear relaxation rate measurements (35,36,37), fluorescence (38) and electron spin resonance (39). In particular Dwek and Richards (35) as well as Cottam and his coworkers (36,37) have employed a series of nuclear relaxation measurements of both metal-bound water protons and substrate nuclei to characterize the interaction of Gd + with several enzyme systems. 

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