in 1958, daniel koshland suggested a modification to the lock and key model. since enzymes are rather flexible structures, the active site is reshaped by interactions with the substrate. as a result, the substrate does not simply bind to a rigid active site. the amino acid side-chains of the active site are bent into positions so the enzyme does its catalytic work. in some cases, such as glycosidases, the substrate molecule also changes shape slightly as it enters the active site.