 Proteins are not entirely rigid molecules . In addition to these levels of structure , proteins may shift between several related structures while they perform their functions . In the context of these functional rearrangements , these tertiary or quaternary structures are usually referred to as " conformations " , and transitions between them are called conformational changes . Such changes are often induced by the binding of a substrate molecule to an enzyme 's active site , or the physical region of the protein that participates in chemical catalysis . In solution proteins also undergo variation in structure through thermal vibration and the collision with other molecules . 
 Proteins <a href='negative_concord' title='1'><mark>are</mark></a> <a href='negative_concord' title='2'><mark>not</mark></a>entirely rigid molecules . In addition to these levels of structure , proteins may shift between several related structures while they perform their functions . In the context of these functional rearrangements , these tertiary or quaternary structures are usually referred to as " conformations " , and transitions between them are called conformational changes . Such changes are often induced by the binding of a substrate molecule to an enzyme <a href='null_genetive' title='3'><mark>'s</mark></a>active site , or the physical region of the protein <a href='null_relcl' title='4'><mark>that</mark></a><a href='uninflect' title='5'><mark>participates</mark></a> in chemical catalysis . In solution proteins also undergo variation in structure through thermal vibration and the collision with other molecules . 
Proteins ain't entirely rigid molecules . In addition to dem levels of structure , proteins may shift between several related structures while they perform their functions . In the context of these functional rearrangements , these tertiary or quaternary structures are usually referred to as " conformations " , and transitions between em are called conformational changes . Such changes r often induced by the binding of a substrate molecule to an enzyme  active site , or the physical region of da protein participate in chemical catalysis . In solution proteins also undergo variation in structure through thermal vibration and the collision with other molecules .