Why might a protein function incorrectly if its shape were changed?

If a proteins shape was changed it may be able to do its function to do being unable to fit in all blood vessels/arteries and the shape change may of hindered its capability of carrying nutrients it needs to transport, etc.

for example:

If a hemoglobin (red blood cell) shape changed, (this is normally called sickle cell) the red blood cell would be to unable to travel through the body with ease and easily clog bloodstreams. Also, the hemoglobin will not be able to transport as much oxygen throughout the body.