Why does the rate of an enzyme reaction not just always increase with temperature? Why does it fall after a point?

Enzymes have an area on them known as the ACTIVE SITE. This active site is shaped to fit the substrate that will bind to it -complementary fit. Eventually, the bonds in the active site of the enzymes are going to break because the heat energy is high enough to break it. This means the active site of the enzyme changes shape- it no longer has the complementary fit to the substrate. When the active site changes shape and no longer fits the substrate we say the enzyme is DENATURED. This means less enzymes are available to work and this makes the reaction slower. Eventually, when all enzymes are denatured the reaction stops completely!