How is protein digested

Protein digestion

mouth

In the saliva there are enzymes for digesting carbohydrates (amylases), but not enzymes with which proteins can be digested.

stomach

The stomach is important for protein digestion. The parietal cells of the gastric mucosa produce hydrochloric acid and other cells of the gastric mucosa form the precursor of the enzyme pepsin for protein digestion.

Task of hydrochloric acid

The 0.3% hydrochloric acid in gastric juice (gastric acid) has several tasks:

  1. It kills a large part of the germs that have entered with the food (but not all),
  2. It denatures and swells the food proteins so that they can be more easily attacked by the digestive enzyme in the stomach.
  3. It activates the digestive enzyme pepsin (see next section).
Abandonment of pepsin

Pepsin is a protease, or more precisely, an endopeptidase. That is, it attacks peptide bonds in the middle of the protein. By the way, enzymes that attack proteins from the ends are called exopeptidases.

Pepsin (Greek pepsis = Digestion) consists of exactly 327 amino acids. The pH optimum of pepsin is between 1 and 4, i.e. in the very acidic range. Pepsin mainly breaks down proteins where aromatic amino acids occur in the chain[3].

Pepsin is not a single compound, but the name for a whole group of enzymes. In addition to pepsin A and pepsin C, five to six other pepsins are suspected in gastric juice[5].

The stomach is also made up of proteins. Why aren't the stomach proteins attacked and digested by the pepsins?

There are two reasons for this: The first reason is that the inner wall of the stomach is surrounded by a thick mucous membrane that protects the stomach from its own enzymes and hydrochloric acid. The second reason is that the glands of the gastric mucosa do not produce "finished" pepsins, but rather harmless precursors, the pepsinogens. The pepsinogens are only converted into the effective pepsins at a pH value of less than 2.

When pepsins break down food proteins, oligopeptides (2-10 amino acids) and polypeptides (11-100 amino acids) are created from the proteins.

For experts

Infants have another protease in gastric juice, namely gastricin. The task of gastricin is to prepare the milk protein casein, which is contained in breast milk, for digestion by pepsin. The water-soluble casein (also called caseinogen) is converted into a water-insoluble form (called paracasein) and can then be digested more easily[1][2].

Small intestine

The small intestine is the main place of protein digestion. In the duodenum, the first section of the small intestine, the chyme (stomach pulp) is mixed with secretions from the pancreas and the actual small intestine. The stomach acid is neutralized because the juice from the small intestine is alkaline (pH> 7). This also causes the pepsins to lose their effectiveness; the high pH value denatures and breaks them down.

There are mainly three types of proteases active in the small intestine: the two endoproteases trypsin and chymotrypsin and four exoproteases.

In the case of exoproteases, a distinction is made again between carboxypeptidases and aminopeptidases. The carboxypeptidases attack the protein from the carboxy end, i.e. where there is still an intact COOH group. The aminopeptidases attack the protein from the other end, i.e. where there is an intact NH2-Group is located. The two carboxypeptidases are called carboxypeptidase A and carboxypeptidase B. Accordingly, the two aminopeptidases are referred to as aminopeptidase A and aminopeptidase B.[2]

Trypsin and chymotrypsin split the oligo- and polypeptides of gastric porridge into di-, tri-, tetra-, pentapeptides and so on, i.e. into even smaller fragments. In contrast, the four exopeptidases split off individual amino acids.

Colon

The large intestine is of no importance for protein digestion. At least you won't find anything about it in the specialist literature.

Absorption

With a mixed diet, about 90 - 125 g of amino acids are absorbed by the enterocytes (cells in the wall of the small intestine) every day[5]. This absorption is an active transport, since the enterocytes already contain a high concentration of amino acids. Di- and tri-peptides can also be included in a similar manner. You will then in hydrolyzed the enterocytes to amino acids.

More precisely, the active transport is a symport in which the amino acids coexist with Na+-Ions get into the cells (Symport: two different molecules or ions are transported in the same direction at the same time)[4]. Di- and tripeptides are also transported into the cells in a similar way, but not together with sodium ions, but together with protons[2].

The amino acids then pass from the enterocytes into the blood by passive transport (diffusion). Most of these amino acids are first transported to the liver via the portal vein.

In some cases, whole proteins can even get into the blood via special cells in the small intestine. This is important for the immune system, since foreign (but harmless) proteins stimulate the formation of immune cells and thus strengthen the immune system (basically this is a kind of "training" for the immune system so that it can react better to actually dangerous proteins). This is particularly important in infants so that the immunoglobulins in breast milk get into the infant's blood without being broken down[2].

Availability of proteins

If you eat protein-containing foods, it does not automatically mean that the proteins contained in the food can be digested easily. Many factors influence the so-called availability of proteins.

Spatial structure

It starts with the spatial structure of the food proteins. Tender meat is easier to digest than tendons or cartilage. How come The structural proteins in tendons, cartilages, muscles and so on are elongated and strongly networked with each other. With the degree of cross-linking, however, the availability decreases; strongly cross-linked proteins are more difficult to break down into poly-, oligo- and dipeptides because the digestive enzymes do not have such a large target area.

Type of preparation

Raw meat, raw legumes, and other raw, protein-rich foods are more difficult to digest than cooked, baked, or fried foods. This is because heat destroys protein structure (see Denaturing Proteins). Denatured proteins offer the digestive enzymes a larger target. However, one should not overdo it with roasting and baking, because at a certain temperature complex chemical compounds form from the amino acids and the carbohydrates contained in the food, so-called Maillard products, which are no longer digestible.

Human "hardware"

The condition of the human "hardware" - what is meant here is the digestive system (one would actually have to speak of "wetware") - has an influence on how well food proteins can be utilized.

Take Celiac Disease, for example

Celiac disease or gluten intolerance is a disease of the small intestine; the symptoms are triggered by the gluten contained in cereals. Celiac disease is caused by a (possibly genetically determined) enzyme deficiency in the mucous membrane of the small intestine, but the immune system is also involved in the clinical picture[6].

One of the consequences of celiac disease is that certain nutrients cannot be absorbed or can only be absorbed to a limited extent. The "malabsorption-associated symptoms of celiac disease" include, for example, anemia due to iron deficiency, osteoporosis due to vitamin D and calcium deficiency and edema due to protein deficiency[2]Which brings us back to the subject of "availability".

Defective amino acid transporters

When the food proteins are completely broken down into amino acids, these must be actively transported into the cells of the small intestine wall. Certain transport proteins in the membrane of these cells serve this purpose. In Hartnup's disease, for example, the transport protein for neutral amino acids is defective[7], while in cystinuria the amino acids cystine, arginine and lysine can no longer be correctly absorbed[1].

Vegetable and animal proteins

In general, vegetable proteins are less readily available than animal proteins. The availability of vegetable proteins is on average 80%, whereas in animal proteins it is significantly higher, up to 98%. Exceptions prove the rule.

Protein metabolism

As already mentioned above, 90 to 125 g of amino acids are absorbed in the small intestine every day[5]. The amino acids enter the bloodstream immediately and are transported to the places of consumption, i.e. to the cells. In the cells of a healthy adult, approx. 400 g of proteins are produced from 500 g of amino acids every day[5] (Protein biosynthesis).

The amino acids that get into the blood through absorption, the amino acids that are formed through the breakdown of the body's own proteins and the amino acids that are formed through the conversion of carbohydrates and fats, enter the so-called amino acid pool. This amino acid pool is then available to the cells to build up new endogenous proteins; it comprises approx. 600 g amino acids.

Sources for the Amino Acid Pool
  • Absorption of amino acids
  • Breakdown of the body's own proteins
  • New synthesis of non-essential amino acids
Breakdown of the body's own proteins

The absorption of amino acids has already been discussed above, so it will not be discussed further here.

The breakdown of the body's own proteins takes place constantly in the cells, namely in the lysosomes. Excess proteins that are no longer needed are hydrolytically split by certain enzymes, which - similar to protein digestion in the stomach and small intestine - result in short-chain peptides and finally individual amino acids. Excess amino acids can also be broken down further. On the one hand, they can be used to generate energy, 1 g of protein contains around 17 kJ of energy. On the other hand, valuable building blocks for the synthesis of other compounds can be obtained from the amino acids.

If amino acids are broken down to generate energy, the amino group must first be split off. In a chemist's test tube, the amino group would be released as toxic ammonia. Of course, that doesn't work in a person's cells. So the amino groups are incorporated into another - non-toxic - compound, namely urea. Urea is not entirely non-toxic either, so it has to be excreted via the kidneys - but we will come to that later on a separate series of pages.

When the amino group has been split off, a carbon chain remains. This carbon chain can then be fed into the cell's energy metabolism. Depending on the degraded amino acid, it can be converted into acetyl-CoA, pyruvate or another intermediate product of the citric acid cycle. It is also possible to build up fatty acids or glucose from the amino acid carbon chains[5].

Build up of endogenous proteins

This topic is described in detail on the biology pages, so here are just a few keywords:

DNA -> transcription -> mRNA -> ribosomes -> protein synthesis -> protein processing

For the subject of nutrition, it is perhaps also important to know that there are essential and non-essential amino acids. If protein biosynthesis in the cells requires amino acids such as lysine or threonine, these amino acids cannot be produced from other amino acids or sugars or fats themselves, but must be taken in with food and then digested and absorbed.

The so-called protein turnover in 24 hours is interesting: 125 g of amino acids are produced from 100 g of dietary proteins through digestion and absorption. In addition, 400 g of the body's own proteins are broken down per day, which results in 500 g of amino acids. Together, that's 625 g of amino acids - the amino acid pool. On average, 125 g of these 625 g of amino acids are used for energy production, which produces carbon dioxide, water and ammonia. However, the ammonia is immediately bound in the form of urea and thus rendered harmless. 500 g of the amino acids in the pool are used for the new synthesis of endogenous proteins[5].