DIGESTION IN PROTEIN

There is a process by which the body converts the ingested foods into its simpler constituents that can be easily absorbed and assimilated. This above mentioned process gives us an idea of what is digestion. Proteins are defined as the group of complex organic macromolecules containing carbon, oxygen, hydrogen, nitrogen and sulfur and are composed of one or more amino acid chains. Proteins are components of enzymes, hormones and antibodies, and therefore are very important for an organism's survival. Some factors influence the rate of protein digestion in the body and they include;

1.      The concentration of the enzyme.

2.      The amount of protein food needing action.

3.      The acidity of the food and of the stomach.

4.      The temperature of the food; time.

5.      The presence of any digestion inhibitors, such as antacids.

Cooking and chewing help, but protein digestion does not begin in the mouth, as carbohydrate metabolism does. The hydrochloric acid in the stomach is required to break the protein bonds. The protein-containing foods are broken apart, separating out the protein, then the proteins are broken into their constituent parts, the amino acids.  The digestion of proteins takes place in two organs, stomach and small intestine.

Digestion of Protein in Stomach

Digestion of protein does not start with chewing of food in the mouth. It begins in the stomach. The stomach is especially designed for the purpose of digestion of foods. Its walls are composed of strong muscles. These muscles mix and churn the ingested food. They do it with the help of rhythmic contractions, occurring at the average rate of 3 per min. The lining of the stomach contains glands. Their function is to secrete gastric juice. It is a colorless and strong acidic liquid at a pH of 1-3. The main components of gastric juice are digestive enzymes, hydrochloric acid and mucus.

Hydrochloric acid produced in the stomach is a very strong acid. It is produced by the type of epithelial cells called parietal cells present in the lining of the stomach. HCl is so strong that it can easily digest the stomach itself. But such a destructive process is prevented from occurring by another secretion of the stomach called mucus. It protects the delicate cell lining of the stomach as well as moistens the food present there. However, the cells in the stomach lining keep getting destroyed by hydrochloric acid. It gets replaced by newer cells. According to studies, the lining of the stomach gets completely replaced every third day.

Protein digestion in the stomach occurs mainly by the action of hydrochloric acid (HCl) and enzyme called pepsin. The enzyme pepsin forms in the stomach when its precursor pepsinogen reacts with HCl. Pepsin and HCl breaks the protein bonds. The foods containing proteins are separated from each other. The proteins get separated out, which is necessary for the action of enzymes. The enzymes needed for digesting proteins are proteinases and proteases. These enzymes break down the molecules of proteins into its constituents, amino acids by a depolymerisation process called hydrolysis. It is described as a chemical reaction wherein a water molecule breaks down into hydrogen cations and hydroxide anions.

The rate of action of these protein digestive enzymes is influenced by a number of factors. Some of them are concentration and amount of the enzyme, amount of protein food needed to be digested, temperature of the food, acidity of the food, acidity of the stomach and presence of antacids or other inhibitors of digestion. The task of enzymes is to breakdown of protein molecules into simpler structures called peptones and proteose. They leave the stomach and enter the small intestine with the help of peristalsis movement of the body. It is called chyme. The entire process of protein digestion in the stomach takes about 4 hours.

Digestion of Protein in Small Intestine

The chyme first enters duodenum, which is a part of small intestine. It is a C-shaped structure about 25 centimeters long. The chyme is very acidic but here it mixes with an alkaline secretion and becomes neutral. Pancreas secrete digestive enzyme, trypsin and chymotrypsin, which reach the duodenum through bloodstream and aid in the breakdown of proteins. They break the complex protein molecules into its constituents, amino acids. They accomplish this task of breaking down by hydrolysis, described above.

The walls of the small intestine are covered with numerous finger like projections, known as villi. They increase the surface area of the small intestine by about 600 times. Each villus contains a network of blood capillaries and lymph vessels. The amino acids pass through the capillary walls, and get carried away by the blood flowing through the network. In this manner, the amino acids thus produced get absorbed, reach different body parts and finally get converted to human proteins. The human body uses proteins for building and maintaining its structures, sometimes for energy generation as well.

Amino Acids Put To Use

Once in the blood, the amino acids are carried by both the red blood cells and by the liquid part of the blood, called the plasma. The amino acids are thereby distributed to all the body tissues, where the various body cells take what they need to repair and reform the protein structures they need.

The blood contains amino acids at all times. Fasting does not clear them, and a high protein diet does not materially increase them. The body has a constant need for protein amino acids, and it keeps a fairly uniform balance.

Taking The Protein From The Muscles

The body's skeletal muscles act as an emergency source of protein if insufficient amounts are eaten. The body can break down its own muscle tissue, and transport the amino acids gathered from that muscle destruction to the more vital organs, if necessary. (As an aside, recall that we know that people on very low fat diets are also, frequently and by default, on low protein diets. This is because most of the rich sources of protein in foods are also in sources of dietary fat. These dieters lose their muscle mass because their bodies cannibalize their own muscles as a source of the proteins that they need, but are not eating.)

Problems Arising From Incomplete or Improper Protein Digestion

Sometimes, instead of being properly broken down into amino acids, small amounts of whole or partial proteins are absorbed into the blood. The body wants amino acids, not whole proteins, and whole proteins are viewed by the system as an enemy. This is where we get the phrase foreign protein. The presence of protein instead of amino acids may lead to food allergies, to a shock reaction called anaphylaxis (anna-phil-AXIS), to other symptoms typical of an allergy, such as sneezing, breathing difficulties, skin rashes, headaches, nausea, or even, in severe cases, death. And these problems result from just a very small amount of the food protein, which doesn't belong there.

Sometimes protein substances containing nitrogen may reach the large intestine. This may be undigested or partly digested food residues, unabsorbed amino acids, unused protein enzymes, or the protein of dead bacteria. These protein substances will likely be attacked by microorganisms (bacteria) that live in the intestinal tract, and be decomposed by the process called putrefaction (pew-tra-FAC-tion). This often results in diarrhea.

Waste Products of Protein Metabolism

The destruction of proteins in the body gives rise to two classes of waste products: nitrogenous (ny-TRA-gin-us), those containing nitrogen, and non-nitrogenous (non-ny-TRA-gin-us), those that don't contain nitrogen. The non-nitrogenous types of waste products are carbon dioxide and water. Nitrogenous waste products only relate to proteins since only proteins contain nitrogen.

The nitrogenous waste products are known as urea (yur-RE-ah), uric acid (yur-ick acid), creatinine (cree-AT-tin-neen), and hippuric acid (hip-PURE-ick acid). Urea is the major nitrogenous waste product, making up some 80% of it. Urea is formed in the liver, and is excreted by the kidneys in the urine along with the other types of protein waste products.

SUMMARY

Most proteins are decomposed to single amino acids in digestion. Digestion of protein typically begins in the stomach when pepsinogen is converted to pepsin by the action of hydrochloric acid, and continued by trypsin and chymotrypsin in the intestine. Before the absorption in the small intestine, most proteins are already reduced to single amino acid or peptides of several amino acids. Most of peptides longer than four amino acids are not absorbed. Absorption into the intestinal absorptive cells is not the end. There most of peptides are broken into single amino acids.

Absorption of the amino acids and their derivatives into which dietary protein is degraded is done by the gastrointestinal tract. The absorption rates of individual amino acids are highly dependent on the protein source; for example, the digestibilities of many amino acids in humans, the difference between soy and milk proteins and between individual milk proteins, beta-lactoglobulin and casein. For milk proteins, about 50% of the ingested protein is absorbed between the stomach and the jejunum and 90% is absorbed by the time the digested food reaches the ileum. Biological value (BV) is a measure of the proportion of absorbed protein from a food which becomes incorporated into the proteins of the organism's body.

REFERENCES

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