The molecular shape of dopamine beta-hydroxylase from chromaffin granules of bovine adrenal medulla.

The structural features of the soluble dopamine beta-hydroxylase from chromaffin granules of bovine adrenal medulla were studied using negative staining and platinum shadowing electron microscopic methods. The enzyme was shown to be highly asymmetric as suggested in earlier hydrodynamic studies. The tetramer of the enzyme appeared as four subunits arranged in the shape of a planar rose with an estimated width of 15 nm. A minimum thickness of 3.0 nm for the enzyme monomer was calculated from the shadow length of unidirectionally shadowed molecules. A model composed of four oblate ellipsoid monomers in a tetrameric rose arrangement is proposed for the shape of the dopamine beta-hydroxylase molecule. Two monomers associate edge to edge to form an in-plane dimer and two dimers associate side-by-side with their respective long axes at a slight angle to form a tetramer. Theoretical calculations based on the model are consistent with previous hydrodynamic studies.