Biochemistry - The Chemical Reactions of Living Cells Volume 1 - D. Metzler 1980

How molecules join together
Macromolecular packaging
Oligomers containing both heterologous and isologous bonds

The packing of four subunits to form a square-like Structure can occur via either isologous or heterologous contacts. Large aggregates may involve both types of contacts. For example, the association of two heterologous trimers such as those shown in Fig. 4-6 leads to The formation of a hexamer with dihedral (D₃) Symmetry; a heterologous "square tetramer" can dimerize to form a dihedral (D₄) octamer [43].

The enzyme Glutamine Synthetase is a double ring composed of 12 subunits. It is likely that the upper ring is positioned relative to the lower one in such a way that the resulting structure possesses dihedral symmetry (D₆) with one 6-fold axis of symmetry and six perpendicular 2-fold axes.

Isologous bonds can also form within helices. Actin monomers form both chains and dimers. Thus, the subunits in the structure shown in Fig. 4-7 may be composed of isologous dimers formed by pairs located on opposite sides of the axis. The reader might now find it interesting to consider independently whether bacterial flagella could be composed of isologous dimers.