It's the CO2 gas that expands in the dough and causes it to rise. The process is stopped when the oven is turned up. These higher temperatures kill the yeast. When you look at a slice of bread, all the little holes were actually trapped gas bubbles. This process occurs faster at a slightly warm temperature.
Enzymes are catalysts that make biochemical reactions occur at a faster rate.
These reactions may occur without the enzyme but at rates that are biologically too slow. Enzymes are proteins and they have a unique 3 dimensional shape. Each chemical reaction has its own enzyme. Argonne National Laboratory, Division of Educational Programs, Harold Myron, Ph.D., Division Director.