Peptides – Amino Acids: Polypeptide Hormones, Neuropeptides and Their Effect on Digestion

What are the Peptides?

They are short chains of amino acids linked together. If there are two amino acids, then the peptide is a dipeptide. Similarly, there are tripeptides, tetrapeptides, etc.

If the number of amino acids in the chain reaches around ten, such substances are called polypeptides, while the larger ones are called proteins.

There is no particular size agreed upon in which a large polypeptide becomes a small protein, but generally the polypeptides have molecular weights of a few thousand, while the proteins have molecular weights of tens of thousands.

Depending on which amino acids are involved, between seven and ten amino acids will add approximately 1000 to the molecular weight.

Protein molecules in the diet are digested by enzymes (which in turn are specialized proteins), which break them down into increasingly smaller lengths, and breakage occurs at the peptide bonds.

Peptides and amino acids are, therefore, the final cleavage products of protein digestion.

Amino acids are the main degradation product of proteins that are absorbed in the intestine, but some tripeptides are also absorbed, with specific carrier systems in the cells that line the small intestine to transport these small peptides from the lumen to the blood.

The dipeptide carnosine, formed from the amino acids alanine and histidine, was identified in the muscle a century ago, but only recently has research revealed its properties and the probable variety and importance of its functions.

It is known that it is also present in the brain, where it can act as a neurotransmitter.

In muscle it is likely to be important to make the contractile filaments more sensitive to calcium ions and to control the internal acidity of these fibers.

It has been suggested that it can also be a scavenger of free radicals. Its strong union with zinc can be important in the joint absorption of the intestine of this essential trace element; and physiologically significant interactions between carnosine, zinc and histamine are being discovered.

The tripeptide glutathione (glutamic acid-cysteine-glycine) is an important cofactor for many enzymes, which increases its activity.

Polypeptide hormones

The polypeptides control or trigger a large number of bodily functions, acting near or at a distance from the site where they are produced and released.

The following table gives some examples, which provide the production site, the number of amino acids and an indication of the functions that polypeptides promote.

The proteins generally fold to form particular three-dimensional shapes (which determine their actions), but the polypeptides are not so structurally restricted, so in solution they can take many conformations.

For example, oxytocin and vasopressin have about a thousand different conformations in solution, all in dynamic equilibrium with each other.

A part of the polypeptide binds to the receptor, while the adjacent parts rotate and rotate until the correct form is reached. Therefore, the polypeptides use a “zipper” mechanism to bind to the membrane receptors.


There are many different peptides in the neurons, released together with other neurotransmitters.

Some peptides that were originally identified as hormones, which were thought to be produced at a particular site and that acted in certain places, have been found more recently elsewhere and have other functions.

The body uses the same peptide for different purposes. For example, of cholecystokinin (CCK), a 33 amino acid polypeptide known for many decades as a hormone that originated in the duodenum and caused the emptying of the gallbladder.

Since the 1980s it has been revealed that it is a modulator of neuronal activity, produced by many nerve cells, disseminated in the nervous system.

Likewise, corticotropin-releasing factor (CRF), with 41 amino acids, was originally created and released by a group of neurons in the hypothalamus, which passes to the pituitary gland and stimulates the secretion of ACTH (adrenocorticotrophic hormone).

But it has also been discovered that it is a neuromodulator produced by neurons in many parts of the brain.

A family of peptides called opioid peptides or endorphins, which are found in the brain and other parts of the body, are responsible for the modulation of the sensation of pain.

A group of these, the pentapeptide enkephalins, are released as neurotransmitters by nerve cells in certain parts of the brain and spinal cord.

They bind to opioid receptors (the membrane receptors in which opioid drugs act) in other nerve cells in the pathways that mediate pain, and therefore act as “endogenous” (internally generated) analgesics.

The digestion and absorption of protein

Wake up in the morning and eat an egg for breakfast. Let’s follow the trip of this protein through the body.

The egg is swallowed, the enzymes enter and break the protein into amino acids. Free amino acids recombine in various ways forming what is known as “specialized proteins”.

Specialized proteins can become “various things”, that is, enzymes, antibodies, hormones. They can also end up as a structural protein, such as collagen that can occur in the connective tissue.

Large protein molecules are broken down in different ways. The digestion with proteins begins in the stomach. An enzyme called pepsin is the well-known stomach protein digestive enzyme.

When pepsin acts on a protein molecule, it eliminates the bonds that hold the protein molecule together. These peptide bonds, once broken, result in chains of amino acids that bind to each other and which are called “polypeptides”.

As polypeptides, they move to the small intestine to complete the digestion process. In the small intestine, pancreatic enzymes called trypsin, chymotrypsin and carboxypeptidase complete this decomposition. These proteins are introduced through the duodenum through the pancreatic duct.

These pancreatic enzymes are aided by additional enzymes that are found within the microvilli of the small intestine.

The peptide bonds continue to decompose, which results in what we call “peptides” affectionately.

Enzymes will continue to break down polypeptides and peptides into amino acids. The amino acids are small and have the ability to be absorbed through the lining of the small intestine and enter the bloodstream again.

The digested nutrients that leave the digestive tract will be directed to the liver before it enters the bloodstream.