Proton magnetic resonance relaxation in Pseudomonas aeruginosa cytochrome oxidase solutions

We have measured the temperature and frequency dependence of solvent proton magnetic relaxation rates in solutions of Pseudomonas aeruginosa cytochrome oxidase (EC 1.9.3.2) in its native low spin oxidized, its reduced, and its carbonyl reduced derivative. In solutions of the native oxidized enzyme, a large paramagnetic enhancement of the proton NMR relaxation rates, propagated to the solvent by the fast exchange mechanism, is observed. The ratio (T1/T2)pmg = 1.25 +/- 0.10 at 24 MHz demonstrates that dipole-dipole interaction of the neighboring paramagnets is the dominant relaxation mechanism. Measurements of proton relaxation in solutions of cytochrome oxidase from which the hemes D have been extracted demonstrates that hemes C do not contribute to the observed paramagnetic effects. The electron spin relaxation time of the ferric hemes D of 3.2 +/- 0.4 ns is calculated from the frequency dispersion data. This is the longest value reported for hemoprotein solutions so far. These features of a low spin ferric hemoprotein are similar to those found recently both for the microbial and for the microsomal cytochrome P-450. The calculated distances between the exchanging proton(s) and heme D iron ions demonstrate the high accessibility of the environment of heme D from the solvent side, also for molecules not penetrating the inner coordination sphere.