Figure 1. Many enzymes from intermediary metabolism form nondirectional filaments with dihedral symmetry.

Images show mainchain C, N, and C_α atoms only. The symmetry for the helical axes is shown in the oligomer (black shapes, second row); one, two-fold axis is shown on the oligomer (black oval, third row); and the helical axis is shown as a black line on the filaments (bottom row). A. Cytoskeletal proteins, like actin (shown - PDB ID 6DJM [60]) and tubulin assemble from monomers or asymmetrical dimers, which produces a directional filament. The filaments grow and shrink from a defined pole of the filament structure, and the longitudinal interfaces within the filaments are comprised of residues found on opposite sides of the protein. B. While glucokinase 1 filaments (6PDT) assemble from monomers, the repeating units in the filament (black outline) assemble with C2 symmetry around an axis (black oval) perpendicular to the helical axis, generating a filament with overall D1 symmetry. C. Symmetric C2 dimers can also assemble into filaments with D1 symmetry (Acetyl CoA-carboxylase: 6G2D). The symmetry axis perpendicular to the helical axis is shown in both the oligomer and the filament (black oval). D-H. Enzymes found in intermediary metabolism have dihedral symmetry within the oligomer which is propagated in the filament form of the enzyme (CTP synthase: 6PK4; PRPP synthetase: 8DBF; IMP dehydrogenase: 7RGL/7RGM; arginine decarboxylase: 7PK6; glutamine synthetase: 7W85).