Structure of insulin: results of joint neutron and X-ray refinement.
Neutron diffraction data for porcine 2Zn insulin were collected to 2.2 A resolution from a single crystal deuterated by slow exchange of mother liquor. A joint neutron/X-ray restrained-least-squares refinement was undertaken using the neutron data, as well as the 1.5 A resolution X-ray data collected previously. The final R factors were 0.182 for the X-ray data and 0.191 for the neutron data. Resulting atomic coordinates were compared with the initial X-ray model, showing a total r.m.s. shift of 0.36 A for the protein and 0.6 A for the solvent. Protonation of a number of individual amino acids was investigated by analysis of the neutron maps. No D atoms were found between the carboxylates of Glu B13 which make an intermolecular contact, suggesting nonbonded interaction rather than the predicted hydrogen bond. Amide hydrogen exchange was investigated in a refinement of their atomic occupancies. Regions of unexchanged amide groups were found in the center of the B helices. The results of this study emphasize the limited amount of information available in neutron diffraction studies of proteins at resolution lower than 2 A.
Wlodawer
A
A
Center for Chemical Physics, National Bureau of Standards, Gaithersburg, MD 20899.
Savage
H
H
Dodson
G
G
D000818:Animals; D006859:Hydrogen; D007328:Insulin; D008968:Molecular Conformation; D015394:Molecular Structure; D009502:Neutrons; D013552:Swine; D014961:X-Ray Diffraction
D007328:Insulin; D006859:Hydrogen