Two-Angstrom Crystal Structure of Oxidized Chromatium High Potential Iron Protein

Studies on NO2-Tyr82and NH2-Tyr82 derivatives of adrenodoxin. Effects of chemical modification on electron transferring activity

Bovine apoadrenodoxin was treated with tetranitromethane to introduce a nitro group into the tyrosyl residue at position 82 of this protein. The degrees of nitration under the best conditions were estimated to be 90% and nearly 100% on the basis of amino acid analysis and the spectrophotometric method, respectively. An amino derivative was prepared by reducing the nitro group with sodium dithionite. The apoadrenodoxin derivatives could be reconstituted to have an iron-sulfur chromophore similar to the native adrenodoxin which contains a 1:1 molar ratio of labile sulfur to iron content and displays absorption peaks at 414 and 450 nm. The enzymatic acitivies of these reconstituted nitro and amino derivatives toward cytochrome c reduction in the presence of adrenodoxin reductase and NADPH were 19 and 7% of native adrenodoxin, respectively. We studied the kinetics of the direct reduction of the reconstituted amino derivative in the presence of NADPH and adrenodoxin reductase under anaerobic conditons. The initial rate of reduction for the amino derivative was 7% of the native adrenodoxin, which is in good agreement with its activity toward cytochrome c reduction. From these results, it is concluded that by modifying the tyrosyl residue at position 82 of the adrenodoxin polypeptide, the electron-transferring activity of the molecule is largely diminished.

Comparison of the predicted and observed secondary structure of T4 phage lysozyme

Predictions of the secondary structure of T4 phage lysozyme, made by a number of investigators on the basis of the amino acid sequence, are compared with the structure of the protein determined experimentally by X-ray crystallography. Within the amino terminal half of the molecule the locations of helices predicted by a number of methods agree moderately well with the observed structure, however within the carboxyl half of the molecule the overall agreement is poor. For eleven different helix predictions, the coefficients giving the correlation between prediction and observation range from 0.14 to 0.42. The accuracy of the predictions for both beta-sheet regions and for turns are generally lower than for the helices, and in a number of instances the agreement between prediction and observation is no better than would be expected for a random selection of residues. The structural predictions for T4 phage lysozyme are much less successful than was the case for adenylate kinase (Schulz et al. (1974) Nature 250, 140-142). No one method of prediction is clearly superior to all others, and although empirical predictions based on larger numbers of known protein structure tend to be more accurate than those based on a limited sample, the improvement in accuracy is not dramatic, suggesting that the accuracy of current empirical predictive methods will not be substantially increased simply by the inclusion of more data from additional protein structure determinations.

Magnetic studies of the four-iron high-potential, non-heme protein from Chromatium vinosum

Extensive EPR studies on high-potential, iron-sulfur protein from Chromatium vinosum indicate that the singular spectrum of this four-iron, non-heme protein consists of a superposition of three distinct signals; namely, two principal signals of equal weight, one reflecting axial and the other rhombic symmetry, and a third nearly isotropic minority component. In addition, magnetic susceptibility experiments on two oxidation states of the protein from 4.2 to approx. 260 degrees K indicate antiferromagnetic exchange coupling between iron atoms. Possible origins of the complex EPR signals are discussed, and a preferred model that is consistent with EPR, magnetic susceptibility, NMR, X-ray, and Mössbauer data is presented.