Proton-metal distance determination in cobalt(II) stellacyanin by proton nuclear magnetic resonance relaxation measurements including Curie-spin effects: a proposed structure of the metal-binding region

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Abstract
1H nuclear magnetic resonance (1H NMR) experiments on Co(II)-substituted stellacyanin have been performed. Large paramagnetic hyperfine shifts are observed, the whole spectrum covering a range of 190 ppm. Experiments were mainly performed at 270 MHz from which temperature and pHI dependencies of the out-shifted resonances are reported, as well as determinations of the longitudinal (T1) and transverse (T2) relaxation times. These relaxation times are, among other things, dependent on the individual proton-metal distance, and the aim of this work has been to determine these distances, by use of the Solomon-Bloembergen equations modified to include the so-called "Curie spin". The application of this method to a protein has not been reported earlier. Experiments were also performed at 100, 400, and 500 MHz in order to estimate the size of the Curie spin from the field dependence of the line widths. Furthermore, determination of the values for the rotational correlation time, .tau.r, and the effective magnetic moment, .mu.eff, was necessary for the present approach. With apostellacyanin, .tau.r was found to be (6.0 .+-. 0.4) .times. 10-8 s. From the paramagnetic susceptibility of Co(II) stellacyanin, the value (4.53 .+-. 0.03).beta. was determined for .mu.eff. The proposed assignments of several paramagnetically out-shifted resonances, the proton-metal distances obtained, and the known peptide sequence of stellacyanin have allowed us to build a three-dimensional model of the metal site and its surrounding structure consistent with all the experimental data. It is revealed that both histidine ligands bind the metal with their 3-nitrogens. Also, we find strong indications that a second sulfur atom is actually bindign the metal, this being the long-sought-after fourth ligand. The model suggests that this sulfur belongs to Cys-59, which togehter with Cys-93 constitutes the disulfide bridge known to be present in the structure. A potential fifth ligand, an amide oxygen from Asn-47, is also found.

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