Protein structure
From diffraction data to protein structure
A basic scheme of protein crystal structure can be determined by measuring diffraction patterns (a) at specific wavelengths guided by the fluorescence absorption scan around an absorption edge (b) can be combined to produce an electron density image (c) and can then be interpreted to yield a model of the protein, shown here as a "ribbon diagram" (d). The absorption edge at 0.9797 Å for selenium corresponds to the energy (12632 eV) needed to eject an electron from the K shell in a selenium atom. The electron density image shown here is ~ 50 Å across and the electron density is high in regions where the contours are close together. In the model the alpha helices (red cylinders) and beta sheets (green arrows) can be clearly seen. The blue spheres roughly represent the locations of the seleno-methionine residues. All data are for the active form of the enzyme hydroxymethylbilane synthase and were taken by Alfonse Haedener of the University of Basle and JRH at Daresbury and the ESRF. In preparation for Acta Crystallographica Section D (co-ordinate file 1AH5 deposited at the Protein Data Bank, Brookhaven, US).