How does denaturation affect the function of proteins?
The function of proteins requires fairly exact 3D structures in order to perform a reaction (lets say an enzymatic reaction). The protein needs to have the correct structure in order to first recognize the substrate (thing that is going to be reacted upon), and to actually do the chemical reaction.
An analogy is like an operating room - the room is where the reaction takes place. If we boil the protein, that will unwind and denature the protein, which is a lot like knocking the floor and ceiling and walls out of the operating room. The operation can no longer take place (there is no more operating table), and its the same for an enzymatic reaction in a protein when the protein is denatured.