Homo sapiens (human)
Part of the initial process of digesting food already takes place in the mouth, before the food reaches the stomach. Plants for instance contain starch, extended networks of carbohydrates, that provide mechanical stability. The chewing process, through mechanical shearing forces, breaks down some of this network. Enzymes then take this process further by digesting these carbohydrates through chemical reactions. Proteins that take part in the digestive process, include amylases, which can be found both in salivary fluids in the mouth and in pancreatic fluids in the stomach. Careful, prolonged chewing not only breaks food stuff by mechanical force, it also exposes it for prolonged time to the amylase in the saliva, thus reducing the amount of work that needs to be done in the stomach. Salivary amylase has been implicated in three different functions: one, the enzymatic cleavage of starch, two, binding to certain bacteria in the mouth with high affinity, and three, being found in the the plaque formed on teeth.
Human salivary amylase, as shown here, is a monomeric protein, consisting of a single long polypeptide chain. It is divided into three spatially separated domains. One of them however is rather short without any peculiar internal order. Therefore the picture clearly shows only two of them, a large globular domain containing a high amount of helices, and a clearly distinct domain near the C-terminus of the protein, consisting exclusively of beta strands. In this beta domain the active site of the protein is located, where digestion of starch takes place. The enzyme also has one calcium ion bound at the bottom of the large domain and one chlorine ion in the center of this domain.
Protein Data Bank (PDB)
author: Arno Paehler