Protein Secondary Structure Section A
DETERMINATION OF PROTEIN STRUCTURE IN 3-D: X-RAY CRYSTALLOGRAPHY
Proteins are space-filling 3-D molecules. The bonds that stabilize the shape of the protein 3-D structure are very complex.
Figure 1. X-ray Structure of Glycogen Phosphorylase. (From Voet/Biochemistry ©1990 John Wiley)
Crystallization of proteins is difficult. The protein must be very pure (ie homogeneous) and then a supersaturated solution is prepared at about 10 mg protein/ml and allowed to stand until crystals form. To see the bonds holding the protein in its 3-D shape, the best method available is X-ray diffraction analysis of protein crystals. In a crystal, the protein molecules form a very regular lattice. This protein lattice is like a 3-D mosaic. Several different shapes of crystal are found.
Figure 2. Protein Crystals. (From Unknown Source who owns copyright)
A single crystal is placed in an X-ray beam. The X-rays are bent by running into the atoms of protein and the pattern of the diffracted X-rays is collected on a piece of film. The diffraction pattern is converted into an electron density map by a computer. Then a mathematical process called Fourier Transformation is used to convert the map into a structural model of the protein.
Figure 3. Schematic of an X-ray Diffraction Structure Determination of a Molecule.
Figure 4. Electron Density Map of the (top) Heme-Iron in Hemoglobin and (bottom) His Ligands to Heme in Hemoglobin.
©Wilbur H. Campbell, 1995; wcampbel@mtu.edu