The orthorhombic crystal structures of mammalian catalase
Catalase (EC 220.127.116.11) is a heme-containing protein which catalyzes the decomposition of hydrogen peroxide. It is present in all areobic organisms to scanvenge the detrimental free radicals or reactive oxygen species (ROS). This enzyme is a tetrameric assembly as observed in a number of crystal structures solved to date, including those from E. coli, fungus, yeast and beef liver. The subunits are organized about three intersecting dyad axes and the molecule have a 222 point-group symmetry. In addition to the heme group, a catalase subunit either binds further to an NADPH molecule or contains an extra flavodoxin-like domain. The mammalian catalase binds NADPH and is one of the smaller catalase, that contains about 500 amino acid residues in each subunit. The crystal structures of bovine liver catalase (BLC) and human erythrocyte catalase (HEC) were determined to 2.3 and 2.75 Angstrom resolution, respectively, in collaboration with Prof. McPherson in Irvine, CA. and Prof. Safo in Richmond, VA. The coordinates are available from PDB with ID codes 4BLC and 1QQW, respectively. The unit cell dimensions are almost identical and the packing of the molecules are very similar in the BLC and HEC crystals. Interactions between the catalase tetramers are divided into two types. The stronger interactions occur between molecules related by the screw axis along the crystallographic b-axis (horizontal in the figure), while the weaker were between those related by the screw axis along the a-axis (vertical). Residues involved in the intermolecular interactions of BLC and HEC were similar yet they appear more conserved for the stronger category of interactions. Thus the BLC or HEC molecules in the orthorhombic crystals were organized by half-unit-cell layers. The stronger interactions hold molecules in the same layer together (colored red and blue in the figure) and the weaker interactions link adjacent layers of molecules (cyan and green). These are in agreement with the observations on the BLC crystals by atomic force microscopy (AFM), which suggest a growth mechanism by two-dimensional nucleation on the  face (same as the view direction of the figure) with the half-unit-cell layers. Similar mechanism may apply to the growth of the orthorhombic HEC crystals as well.
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For the tetragonal crystal structure of human erythrocyte catalase,
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See: Acta Cryst. D57, 1-7 Copyright © International Union of Crystallography
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