Short chains of amino acids are linked together to form peptides. If two amino acids form the chain, then it is referred to as a dipeptide and if the chain is formed by three amino acids, it becomes a tripeptide and so on and so forth. When the number of amino acids forming the chain reaches ten or more, the substance now becomes a polypeptide and very large polypeptides are known as proteins.
There is no particular size at which a large polypeptide will turn into a small protein, but in most cases, polypeptides have molecular weights in the realms of a few thousands while proteins have molecular weights of tens of thousands.
Enzymes are responsible for digesting protein molecules in diet. The enzymes break the molecules at the molecular bonds into smaller and smaller lengths. It thus implies that amino acids and peptides are the final results of protein digestion. Amino acids are absorbed in the gut and so are certain dipeptides and tripeptides. This is because the gut has a specific carrier system used to transport the peptides through the walls of the small intestines into the blood and to the rest of the body.
Over a century ago, the dipeptide known as carnosine was identified in the muscles and it is formed from acids histidine and alanine. But not until recently that its properties and significance was researched and revealed. The dipeptide is also present in the brain where it may sometimes act as a neurotransmitter.
In muscles, it is believed that the dipeptide is responsible for making the contractile filaments more receptive to calcium ions and also in controlling the acidity levels of the contractile filaments. The peptide is also known to bind strongly with zinc and this is essential for the absorption of trace elements from the gut.
Glutathione, a tripeptide is known to be an important co-factor for various enzymes, thus increasing their activity.