  The beautiful structures of single and multi-domain proteins are clearly
ordered in some fashion but cannot be readily classified using group theory
methods that are successfully used to describe periodic crystals. For this
reason, protein structures are considered to be aperiodic, and may have evolved
this way for functional purposes, especially in instances that require a
combination of softness and rigidity within the same molecule. By analyzing the
solved protein structures, we show that orientational symmetry is broken in the
aperiodic arrangement of the secondary structural elements (SSEs), which we
deduce by calculating the nematic order parameter, $P_{2}$. We find that the
folded structures are nematic droplets with a broad distribution of $P_{2}$. We
argue that non-zero values of $P_{2}$, leads to an arrangement of the SSEs that
can resist mechanical forces, which is a requirement for allosteric proteins.
Such proteins, which resist mechanical forces in some regions while being
flexible in others, transmit signals from one region of the protein to another
(action at a distance) in response to binding of ligands (oxygen, ATP or other
small molecules).

Soft Condensed Matter;Biomolecules