Small-angle neutron scattering study of Bence-Jones protein Mcg: comparison of structures in solution and in crystal

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Abstract
Ig and Ig fragments are composed of globular domains linked by extended polypeptide segments. The molecular flexibility inherent in this arrangement allows for significant potential differences between structures observed in the crystalline state and those attained in solution. Small-angle neutron scattering measurements in dilute solution were performed on the Mcg Bence-Jones protein dimer [from the urine of a patient with multiple myeloma], for which accurate atomic coordinates were determined by crystallographic methods. The measured radius of gyration (Rg) in H2O buffer is 24.0 .+-. 0.4 .ANG. and in D2O buffer is 23.3 .+-. 0.1 .ANG.; the calculated value of Rv (Rg in vacuo) is 24.0 .ANG.. The values compare well with the calculated Rg value of 23.6 .ANG. when refined coordinates of the trigonal crystal form of the Mcg Bence-Jones protein are used. On the basis of a match point of 44.2% D2O concentration, the experimental partial specific volume is 0.74 cm3/g. The experimentally derived MW of 47,000 is in very good agreement with that (45,500) calculated from the amino acid composition. For comparisons with different Fab (antigen binding fragments) exhibiting various elbow bends due to the flexibility of the switch peptide between variable and constant domains of the Ig chains, calculation of the Rg value of the Mcg dimer was performed as a function of the elbow bend. The Rg varied from 22.8-26.0 .ANG. as the elbow bend was opened from 100.degree. to 180.degree.; the maximum radius of gyration of the particle was 26.5 .ANG. with the switch peptide stretched by separating the variable and constant domains by an additional 1.5 .ANG. at an elbow bend of 180.degree.