how does an enzyme speed up (catalyze) a chemical reaction? by stopping all chemical reactions in the body…

Answer:

by lowering the activation energy

Brief Explanation:

Enzymes are biological catalysts. They speed - up reactions by reducing the activation energy ($E_a$), as shown in the graph where $E_a$ (with catalyst) is lower than $E_a$ (no catalyst). Stopping all reactions is not their function.