Effect of the fluctuations in electron density within a globular protein on the excess small‐angle X‐ray scattering

Abstract

AbstractExcess small angle X‐ray scattering in solvents of differing electron density has been calculated from the crystal structures obtained for rubredoxin, trypsin inhibitor, myogen, ferricytochrome c2, ribonuclease S, lysozyme, nuclease, myoglobin, α‐chymotrypsin, elastase, subtilisin, carboxypeptidase A, thermolysin, methemoglobin, deoxyhemoglobin, and a single polypeptide chain of M4 lactate dehydrogenase. The scattering curves for each protein can be reproduced by the sum of three curves, with the weighting of the three curves depending on the electron density of the solvent. The radius of gyration obtained from the small angle X‐ray scattering by globular proteins in aqueous solution will usually exceed the values defined by the shape of the macromolecule. Deviations for certain of the proteins cited are calculated to be as large as 6%. These deviations arise from the tendency for the amino acid residues with low electron density to be situated closer to the center of the protein than the amino acid residues of high electron density. An upper limit of 19% is obtained for the discrepancy between the radius of gyration defined by the shape of a spherical globular protein of typical amino acid composition and the apparent radius of gyration measured for that protein in water by small angle X‐ray scattering.