because it alters the enzymes active site, which makes it unable to catalyze the reaction it's intended to.
This "Affect" occurs in many Forms - for Elucidation - pick one. 2-D, 3-D, 4-D; alone or combined; phosphorylated or non-phosphorylated [Active and Inactive Forms]; dimerized and tri-merized and...
Enzyme structure is important because the shape of an enzyme allows only certain molecules to bind to an enzyme's active site. JESSE FRANCE
The substrates interact with R-Groups at the enzyme's active site.
IT IS OFTEN THE SHAPE OF THE MOLECULE THAT DETERMINES ITS PHYSICAL AND CHEMICAL PROPERTIES. [Structure determines function]