Answer by  
                                                                                          nepenthe (39)
                                                            
                          
                          
                                                          Enzymes are proteins that work as catalysts: the speed up reactions. When combining with a substrate, an enzyme speeds up the metabolic process and thus lowers the activation energy. Because they are catalysts, enzymes are never used up by a reaction. Enzymes can be deactivated by temperature, acids (like stomach acid), or poisons.