What is the effect of excess heat or temperature on an enzyme?

The enzyme would be denatured.
In enzymes, each type has their own optimum temperature, it is the temperature that works best for them and they can work the fastest.
However, if the temperature goes too high, the enzyme active site would changed shapes and they're no longer functional. They cannot bind into substrates anymore. They're denatured.
Note that this only happens in too high temperature or too extreme pH values (same concept) , if the temperature is too low, the enzyme would only be inactive, warm it up and it'll return to its optimum condition.