Enzymes are complex proteins that cause a specific chemical change in other substances, without being changed themselves. For example, they can change starches, proteins, and sugars into substances the body can use. Blood clotting is another example of enzymes at work.
Enzymes are essential to all bodily functions. They are found in the mouth (saliva), stomach (gastric juice), intestines (pancreatic juice, intestinal juice, and intestinal mucosa), blood, and every other organ and cell in the body
An important function of enzymes is in the digestive systems of animals. Enzymes such as amylases and proteases break down large molecules (starch or proteins, respectively) into smaller ones, so they can be absorbed by the intestines. Starch molecules, for example, are too large to be absorbed from the intestine, but enzymes hydrolyse the starch chains into smaller molecules such as maltose and eventually glucose, which can then be absorbed. Different enzymes digest different food substances. In ruminants which have a herbivorous diets, microorganisms in the gut produce another enzyme, cellulase to break down the cellulose cell walls of plant fiber
Enzymes determine what steps occur in these pathways. Without enzymes, metabolism would neither progress through the same steps, nor be fast enough to serve the needs of the cell. Indeed, a metabolic pathway such as glycolysis could not exist independently of enzymes. Glucose, for example, can react directly with ATP to become phosphorylated at one or more of its carbons. In the absence of enzymes, this occurs so slowly as to be insignificant