 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 <unk> , these tertiary or <unk> 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 <unk> in chemical catalysis . In solution proteins also undergo variation in structure through thermal vibration and the collision with other molecules . 
