Proteins (enzymes) have different levels of structure: primary, secondary, tertiary, and sometimes quaternary. In 2nd and 3rd, they form bonds within their protein components, hence their amino acids bend and loop to form bonds with amino acids on other areas of the protein. These links can be hydrogen bonds, sulfide bonds, etc. Temperatures above optimal can break these linkages, rendering the enzyme inactive or unable to catalyze the reaction(s).
