Enzymes are proteins occurring in all plant and animal tissue which act as organic catalysts to control the reactions which take place during metabolism. The reason yeast produces alcohol from sugars in base materials for beverages, turning it into wine, beer, or spirit, is that yeast produces enzymes which catalyze and control the reactions involved. Whether or not a given enzyme occurs in a given organism is determined by that organism's genetic makeup. (Wine/Chemistry & Flaws)