The specificity of an enzyme is dependant on the structure of the enzyme for a few reasons:
1) Depending on the structure of the enzyme, and therefore the shape of the enzymes active site, a substrate may be more or less effective at binding to the active site. As the active site is generally shaped for a particular substrate that works only with that particular enzyme, the shape of the active site as a result of the enzyme structure can influence which substrates the enzyme can and cannot bind with.
(thats the main reason, so I am ending it with that)
1) Depending on the structure of the enzyme, and therefore the shape of the enzymes active site, a substrate may be more or less effective at binding to the active site. As the active site is generally shaped for a particular substrate that works only with that particular enzyme, the shape of the active site as a result of the enzyme structure can influence which substrates the enzyme can and cannot bind with.
(thats the main reason, so I am ending it with that)