Heating slightly is often enough to render a protein nonfunctional.?

Proteins are amino acids joined by peptide bonds, the reason they become nonfunctional at relatively low temps is because the structure of the protein is why it works so slight changes in the structure will cause an enzyme to become denatured and no longer catalyze a reaction.

Starch is alpha glucose molecules joined by glycosidic bonds to form chains, and branched molecules. Because its use is a storage of simple sugars heating it might break bonds in the starch but this will not affect as there will just be smaller branches of starch.

Hope that makes sense.

Proteins have three different forms.  Primary form is the order in which the peptides are linked together.  Secondary is a few different shapes the peptide chain can be bent into.  Tertiary form is still further different shapes.  The protein is held together mainly by weak hydrogen bonds.  So it is easily changed, and loses one or all of its forms.