Protein becomes denatured and cease to perform the work if structural changes occur because protein is highly specific in its function. It is stated that the function of the protein is determined by its structure. The proteins are the product of the continuous process of replication, transcription, translation and post-translational modifications. The information stored in it highly specific.
Protein binds with other molecules and perform various physiological functions as it has specific complementary attachment sites, the alteration in the structure would not let it bind to the target and hence desired result will not come out.