For enzymes used in animal feeds the degrading site of the substrate plays a major role in the effect to be achieved. There is a fundamental difference between exo- (external) and endo- (internal) enzymes. Exo-enzymes only break down the terminal structural building blocks of the molecular strand whilst endo-enzymes degrade bonds within the molecular strand (see figure 1). Endo-enzymes are thus able to effectively break down large and long-chain molecules into smaller fragments. This is mainly significant for the effect on the digesta viscosity through enzymes which break down non-starch polysaccharides.

Figure 1 Schematic diagram of the sites attacked by exo-enzymes and endo-enzymes on a branched molecular strand

In addition to the specific degradation site in the molecule, the effectiveness of an enzyme is further largely determined by the prevalent conditions at the site of the reaction. The reaction conditions include the pH value, temperature, water content and presence of activators or inhibitors as well as the substrate concentration. Depending on their origin, i.e. of the producing strain, enzymes vary quite considerably in their activity depending on the reaction conditions.

Enzymes exhibit a high specificity, which means that each enzyme only breaks down highly specific substrates. This is what is known as the so-called “lock and key” principle. The following diagram (figure 2) shows the mode of action of enzymes:

Figure 2 Mode of action of enzymes

Substrate A forms an enzyme substrate complex with the enzyme from which the reaction products B and C are released much faster than would be possible without enzymes. An example of enzymatic breakdown is that of trypsin breaking down a polypeptide (substrate A) into several smaller peptides (products B and C). At the end of the reaction the unchanged enzyme is available to break down yet another polypeptide.

In summary, the reaction conditions of feed enzymes must be adapted to those conditions which are prevalent in the digestive tract of animals. They must either be able to act under the acidic pH conditions prevalent in the stomach or must be able to resist both the low pH and the proteolytic action of the pepsin in the stomach in order to be able to act in the following sections of the digestive tract. This prerequisite must be considered when selecting enzymes for use in animal nutrition.