All chemical reactions speed up as temperature is raised.
As the temperature increases, more molecules have enough kinetic energy to undergo the reaction. Since enzymes are catalysts for chemical reactions, enzyme reactions also tend to proceed faster with increasing temperature. The positive effect of speeding up the reaction is now more than offset by the negative effect of denaturing more and more enzyme molecules. Many proteins are denatured by temperatures around 40-50deg. C, but some are still active at 70-80deg.C, and a few even withstand being boiled.