How can the "shape" of a protein affect its function?

Protein function is directly related to the structure of that protein. A protein's specific shape determines its function. if the structure of the protein is altered because of a change in the structure of the amino acids, the protein becomes denatured and does not perform its function as expected.

So, basically, (very basically,) the "shape" of the protein is the way the molecules are arranged, and the way you arrange them has adverse affects on the properties and function of the end result