A nuclear magnetic resonance study of the heme environment in beef liver catalase

Abstract

The effect of high-spin heme Fe in beef liver catalase [EC 1.11.1.6] on the longitudinal and transverse H+ relaxation rates of the solvent was used to probe the environment of the paramagnetic center. The longitudinal H+ relaxation rates [T1p] were measured as a function of temperature (5-31.degree. C), frequency (5-100 MHz) and pH. T1p was pH independent in the range 6-11, indicating that no significant difference occurs in the heme surrounding within this pH range. The ligands formate and acetate, which preserve the spin state of the heme Fe upon ligation, displace a water molecule from the 6th coordination position. This reaction is pH independent, while the binding measured by optical spectroscopy is pH dependent. The electron donors methanol and ethanol do not change the proton relaxation rates. The temperature and frequency dependencies indicate that the relaxation times are governed by the electronic relaxation time of the high-spin Fe3+ (.tau.s). .tau.s, which was frequency independent, could not be determined from the T1p/T2p [longitudinal ratio, but only from the frequency dependence of the T1p at low frequencies. The results of the least-squares fit of the data to the theory indicate that there is 1 iron-bound rapidly exchanging water molecule. For the Fe3+ .tau.s = 7 .times. 10-11s.