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How protein denaturation affects digestion?

How protein denaturation affects digestion?


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Which one of these is a) easier to digest, b) more nutritious (in whatever sense): 1. scrambled egg, 2. raw egg.

Bascially is "denatured protein are worse than not denatured" a myth, or not?


This article specifically looks at the question you're asking. Just jump down to the conclusion if it's too technical.

To summarize, cooking the egg (or heat pre-treatment as they call it in the paper) leads to a significant increase in protein uptake.

I'd call that myth busted via radiolabeling.


Why is protein denaturation important?

Read rest of the answer. Thereof, why is protein denaturation important in digestion?

As you have read, denaturing is an important part of the digestive process. This unfolds the proteins and makes the protein bonds more accessible by the enzymes so that proteins can be efficiently broken down. When you cook a protein, you denature it in a similar way to how the HCL in your stomach does.

Similarly, what is the purpose of denaturing proteins? If a protein loses its shape, it ceases to perform that function. The process that causes a protein to lose its shape is known as denaturation. Denaturation is usually caused by external stress on the protein, such as solvents, inorganic salts, exposure to acids or bases, and by heat.

Also to know, what does denaturation mean and why is it important?

Denature means lose their structure and unfold due to acid or temperature. hydrogen bonds in between amino acids are disrupted and falls apart because of that. Once its shape is messed up, it can't do what it usually does. When it becomes denatured, it can't break down the molecules and speed up the processes.

Are denatured proteins good for you?

You may have read that denatured protein is bad for you, and that you want to avoid denaturing your protein as much as possible. You denature proteins when you digest them, and in some cases, buying denatured (think pre-digested) protein can help you absorb the amino acids better.


Track down unseasoned meat tenderizer, which can be hard to come by in the store. Avoid the seasoned version as the color of the seasoning makes it more difficult to observe your results. You may have to order it online — it is available on Amazon if you can’t find it in your local grocery store.

To make this most relevant to an egg sandwich, consider also getting some rennet, and using it in similar ways to the meat tenderizer in this experiment. This can even be a great inquiry step for students to design their own conditions for testing the effects of rennet and providing the necessary controls.


Dietary Protein Denaturation and Digestion

Have you every wondered why we cook eggs and meat? Or what is the difference between denatured whey protein and undenatured (native)? I have. Especially about the whey protein – denatured vs. native.

I wanted to find some answers for myself and to also put together a nice article that (hopefully) sheds some light as far as what exactly dietary protein denaturation means and is it better for us or not (as far as nutrition and possible health benefits are concerned). Moreover, in which cases is it better to aim for denaturation and in which for the raw, native form of the dietary proteins.

Below is the result of what I’ve found. I have to warn you, though. This article will seem slightly more academical than engaging, so it will require (slightly) more attention and focus. Not too bad, though..

Shall we begin? Before I get to what happens when proteins get denatured, you will have to undergo a crash course on proteins (just like I had to).

1) Amino Acids – building blocs of proteins
2) Peptides – short chains of amino acids (fewer than 50 individual amino acids)
3) Proteins – long chains of of multiple peptide formations (polypeptides)

How do you get from an amino acid to a protein? By a process called polymerization.

Polymerization of amino acids (1):

1) Primary protein structure – individual amino acids bound together by peptide bonds
2) Secondary protein structure – helix – a curled (like a coiled spring) tri-dimensional peptide structure
3) Tertiary protein structure – a tri-dimensional structure formed by the multiple folding of helices (plural of helix)
4) Quaternary protein structure (2) – multiple secondary and tertiary protein structures, some times combined with other molecules, like sugars (glycoproteins) or fats (lipoproteins) (3)

Protein denaturation is the unfolding of the quaternary, tertiary and secondary protein structures. When the unfolded proteins bond to each other that’s called coagulation. These bonds are usually permanent (like what happens when you cook an egg white). Most often the denaturation of proteins is caused by physical (heat) and chemical (acids, alcohol) reactions.

I am going to make the assumption that we are all interested mostly in the denaturation of dietary proteins that are most common in the human diet – particularly dairy proteins, meat and egg proteins.

Generally, it is a well established fact that denaturing of dietary proteins makes them more available to digestion, and in the case of milk proteins – more bio-available (4) .

Denaturing of milk proteins

There are two major proteins in milk – casein (80 percent) and whey (20 percent).

Casein undergoes little if any denaturing by heat or acid because it doesn’t have the usual secondary and tertiary protein structures. In the stomach, under the action of the stomach acids, casein forms glue-like cloths, which are difficult to digest. For that reason it stays in the stomach longer, which gives it the slow-release properties that are popular among bodybuilders.

We now know that casein releases certain bio-active peptides under the digestive powers of the proteolitic (protein-digesting) enzymes in the stomach, like pepsin, trypsin and chymotrypsin. These enzymes break down the longer protein fractions to 2 – 20 amino acids-long peptides that exhibit multiple health benefits (5).

Whey is also very popular among those who value its properties to release amino acids in the blood stream in a very short period of time, making it an ideal protein for recovery, muscle building and re-building.

Whey protein also exhibits health benefits due to its native bioactive fractions, among them beta-lactoglobulin, alpha-lactalbumin, bovine serum albumin and lactoferrin. All these bioactive fractions are shown to have cardiovascular, digestive, endocrine, immune and nervous system- modulating effects… But, only when they are in their undenatured form.

This is where I have to ask for your more acute attention..

Whey protein is derived in one of two ways – trough the cheese-making process and by direct processing of skim milk to micellar casein and native whey concentrates and isolates (via micro-filtration/ultra-filtration). The first process denatures the native whey fractions, not because the milk is twice pasteurized during the cheese-making (these biologically-active fractions are only partially lost during pasteurization) (6), but mainly because of the subsequent lowering of the Ph level to as low as 3.0 mostly by adding citric acid (at this level the most of the native fractions are destroyed, except lactoferrin). The lactic acid is added after the whey has been separated from the curd, as a part of the whey concentrate processing (7).

The processing of native way from skim milk (not from cheese) provides mostly undenatured whey protein. It undergoes one pasteurization process, but there is no need to lower the Ph to levels where the native fractions are denatured.

The interesting thing is that undenatured whey protein will provide health benefits despite the Ph level in the stomach, which is even lower than that in cheese making (Ph between 1 – 3). The reason is the digestive juices in the stomach (pepsin, trypsin and chymotrypsin), when presented with these native bioactive fractions will further break them down into smaller bioactive peptides that the body derives health benefits from. For example, from further digestion of beta-lactoglobulin the body will make Ala-Leu-Pro-Met-His-Ile-Arg (or ALPMHIR, yeah… I know), which is proven to have blood pressure-lowering properties (ACE inhibitor) (5).

Something has to said also about the cysteine-cystine ratio (cystine forms when two cysteine amino acids bind together via a disulfide bond) in native and denatured whey protein. In native whey the this ratio is more favorable to the production of glutathione (a major antioxidant) by the body. In denatured whey this ratio is skewed in favor of cystine, which makes it more difficult for the body to utilize this important amino acid (it has to break down cystine to cysteine first).

To sum it up, cheese making denatures whey, but it doesn’t denature casein. The manufacturing of native whey straight from skim milk produces undenatured whey protein.

It is important for whey protein to be manufactured in a way that doesn’t denature it before it enters the body. Once in the body, the undenatured whey is ‘denatured’ but in a way that provides the health benefits undenatured whey is well known for. Simply, the body ‘unlocks’ the bioactive substances via the action of its digestive enzymes. This is true for both whey and casein.

If the native protein fractions in whey are denatured before they enter the stomach, the body can still use the proteins by breaking them down into very short peptides and individual amino acids.

Denaturing of meat proteins

Usually we eat meat protein that’s already denatured by heat – like cooked meat. The two parts of protein that are denatured in this process are collagen, which is the connective tissue that separates the bundles of muscle fibers, and the proteins inside the muscle fibers themselves.

When meat is cooked the collagen turns into gelatin. Collagen is tough and gelatin is tender. So, cooking the meat naturally makes it more tender. But if meat is overcooked a lot of the water that is naturally trapped in muscle fibers escapes and the meat becomes tough again (3).

Denaturing of the meet proteins (mainly myosin and collagen) trough cooking makes them partially more bioavailable. The stomach proteolitic digestive enzyme pepsin can digest these proteins better, but there is no change of the digestibility level for the other two proteolitic enzymes – trypsin and chymotrypsin (8). In other words, cooked meat is still easier to digest than raw meat, although raw meat can contribute its own enzymes to aid digestion.

When is meat cooked? At 120°F the protein myosin begins to denature (coagulate). At 140°F another protein called myoglobin denatures and turns from red to tan-colored color (hemichrome). The connective tissue protein collagen starts denaturing between 140-150°F. It dissolves into gelatin at between 160-170°F.

Denaturing of egg proteins

In eggs both – white and yolk – contain protein. About 60 percent of the egg protein is in the white and 40 percent in the yolk.

The egg yolk contains two types of proteins – phosvitins and lipovitellins – phosphorus and iron-storing proteins. Albumen is the common name of the egg white proteins (not albumin). But there are a few different egg white proteins with their own names.

The most abundant protein in the white is ovalbumin – about 50 percent. It denatures at 176°F when the egg is fresh. This temperature slightly increases when the egg is several days old. Ovalbumin in its raw form inhibits the action of protein-digesting enzymes.

Ovotransferin is the second most abundant protein in egg whites – 12 percent. It denatures before ovalbumin at 145°F so it is the protein that determines the final shape of a cooked egg since it sets first.

Ovomucoid takes 11 percent of the total protein and ovomucin – 2 percent. Both proteins do not gel upon heating but they help with tightening and strengthening up the structure of the cooked egg white. When ovalbumin and ovotransferin denature and coagulate ovomucoid and ovomucin get incorporated in the final set structure.

There is one more protein that is present in minuscule amounts in the egg white – avidin (less than 0.1 percent). Interesting thing about this protein is that it binds to biotin – an essential B vitamin – preventing the body from absorbing it. This property of avidin is removed upon denaturing/cooking of the egg protein.

Heat-denatured egg proteins are digested at about 90 percent in the human body whereas raw (undenatured) eggs are digested at only about 50 percent (9). This is most likely due to anti-nutrients (digestive enzyme inhibitors, vitamin and iron absorption inhibitors) that the egg has developed during millions of years of evolution to protect the nourishing egg from being used for food by predators. Heat treatment/denaturing for the most part disables these anti-nutrients.

That’s it in a nut shell. If you find anything missing that I need to add in this article, let me know and I will try to make it as complete as possible.


Protein Denaturation: Unraveling the Fold

When a cake is baked, the proteins are denatured. Denaturation refers to the physical changes that take place in a protein exposed to abnormal conditions in the environment. Heat, acid, high salt concentrations, alcohol, and mechanical agitation can cause proteins to denature. When a protein denatures, its complicated folded structure unravels, and it becomes just a long strand of amino acids again. Weak chemical forces that hold tertiary and secondary protein structures together are broken when a protein is exposed to unnatural conditions. Because proteins’ function is dependent on their shape, denatured proteins are no longer functional. During cooking the applied heat causes proteins to vibrate. This destroys the weak bonds holding proteins in their complex shape (though this does not happen to the stronger peptide bonds ). The unraveled protein strands then stick together, forming an aggregate (or network).

Figure 6.6 Protein Denaturation

When a protein is exposed to a different environment, such as increased temperature, it unfolds into a single strand of amino acids.

Learning Activities

Technology Note: The second edition of the Human Nutrition Open Educational Resource (OER) textbook features interactive learning activities. These activities are available in the web-based textbook and not available in the downloadable versions (EPUB, Digital PDF, Print_PDF, or Open Document).

Learning activities may be used across various mobile devices, however, for the best user experience it is strongly recommended that users complete these activities using a desktop or laptop computer and in Google Chrome.

A class of compounds composed of linked amino acids. They contain carbon, hydrogen, nitrogen, oxygen, and sometimes other atoms in specific configurations.

The loss of a protein’s functional three-dimensional structure due to environmental influences such as temperature or pH.

The molecules from which proteins are built, each protein being composed of a specific sequence of linked amino acids.

The chemical linkage between amino acids into the structure of a protein chain.


Denaturation and Protein Folding

Each protein has its own unique sequence and shape that chemical interactions hold together. If the protein is subject to changes in temperature, pH, or exposure to chemicals, the protein structure may change, losing its shape without losing its primary sequence in what scientists call denaturation. Denaturation is often reversible because the polypeptide’s primary structure is conserved in the process if the denaturing agent is removed, allowing the protein to resume its function. Sometimes denaturation is irreversible, leading to loss of function. One example of irreversible protein denaturation is frying an egg. The albumin protein in the liquid egg white denatures when placed in a hot pan. Not all proteins denature at high temperatures. For instance, bacteria that survive in hot springs have proteins that function at temperatures close to boiling. The stomach is also very acidic, has a low pH, and denatures proteins as part of the digestion process however, the stomach’s digestive enzymes retain their activity under these conditions.

Protein folding is critical to its function. Scientists originally thought that the proteins themselves were responsible for the folding process. Only recently researchers discovered that often they receive assistance in the folding process from protein helpers, or chaperones (or chaperonins) that associate with the target protein during the folding process. They act by preventing polypeptide aggregation that comprise the complete protein structure, and they disassociate from the protein once the target protein is folded.

– What is the state in which proteins are properly folded and/or assembled that makes it operative and functional?

A classic example of denaturing in proteins comes from egg whites, which are typically largely egg albumins in water. Fresh from the eggs, egg whites are transparent and liquid. Cooking the thermally unstable whites turns them opaque, forming an interconnected solid mass. The same transformation can be effected with a denaturing chemical. Pouring egg whites into a beaker of acetone will also turn egg whites translucent and solid.

Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e

Research Article: Hysteresis in Pressure-Driven DNA Denaturation

Date Published: April 9, 2012 Publisher: Public Library of Science Author(s): Enrique Hernández-Lemus, Luz Adriana Nicasio-Collazo, Ramón Castañeda-Priego, Alejandro Raul Hernandez Montoya. http://doi.org/10.1371/journal.pone.0033789 Abstract: In the past, a great deal of attention has been drawn to thermal driven denaturation processes. In recent years, however, the discovery of stress-induced denaturation, observed at the one-molecule level, has revealed … Continue reading

Research Article: Denaturation of proteins by surfactants studied by the Taylor dispersion analysis

Date Published: April 20, 2017 Publisher: Public Library of Science Author(s): Aldona Jelińska, Anna Zagożdżon, Marcin Górecki, Agnieszka Wisniewska, Jadwiga Frelek, Robert Holyst, Eugene A. Permyakov. http://doi.org/10.1371/journal.pone.0175838 Abstract: We showed that the Taylor Dispersion Analysis (TDA) is a fast and easy to use method for the study of denaturation proteins. We applied TDA to study … Continue reading

Research Article: Kinetics of Thermal Denaturation and Aggregation of Bovine Serum Albumin

Date Published: April 21, 2016 Publisher: Public Library of Science Author(s): Vera A. Borzova, Kira A. Markossian, Natalia A. Chebotareva, Sergey Yu. Kleymenov, Nikolay B. Poliansky, Konstantin O. Muranov, Vita A. Stein-Margolina, Vladimir V. Shubin, Denis I. Markov, Boris I. Kurganov, Eugene A. Permyakov. http://doi.org/10.1371/journal.pone.0153495 Abstract: Thermal aggregation of bovine serum albumin (BSA) has been … Continue reading

Research Article: On the Effect of Sodium Chloride and Sodium Sulfate on Cold Denaturation

Date Published: July 21, 2015 Publisher: Public Library of Science Author(s): Andrea Pica, Giuseppe Graziano, Piero Andrea Temussi. http://doi.org/10.1371/journal.pone.0133550 Abstract: Both sodium chloride and sodium sulfate are able to stabilize yeast frataxin, causing an overall increase of its thermodynamic stability curve, with a decrease in the cold denaturation temperature and an increase in the hot … Continue reading

Research Article: Polar or Apolar—The Role of Polarity for Urea-Induced Protein Denaturation

Date Published: November 14, 2008 Publisher: Public Library of Science Author(s): Martin C. Stumpe, Helmut Grubmüller, Vijay S. Pande Abstract: Urea-induced protein denaturation is widely used to study protein folding and stability however, the molecular mechanism and driving forces of this process are not yet fully understood. In particular, it is unclear whether either hydrophobic … Continue reading


What is Protein Hydrolysis?

Protein hydrolysis is the conversion of proteins into amino acids and peptides. It can be done enzymatically as well as chemically. During hydrolysis, bonds between amino acids are disrupted in order to form free amino acids. Protein hydrolysis takes place naturally within organisms due to enzymes such as pancreatic proteases, etc. When we consume protein-rich food, they are hydrolyzed into small peptides during the digestion process by enzymes.

Figure 02: Protein Hydrolysis

Protein hydrolysis is important since it allows isolation of individual amino acids. As an example, histidine can be isolated from red blood cells. Similarly, cystine can be isolated from hydrolysis of hair. In some instances, when it is needed to quantify the total amino acid content in a sample, multiple hydrolysis procedures should be carried out. Acid hydrolysis is the most common technique of protein analysis. Moreover, alkaline hydrolysis can also be employed to measure tryptophan. Therefore, the method of choice for the hydrolysis of proteins depends on their sources.


Let's Talk Chemistry

To fully understand what protein denaturation is and how it might impact your nutrition, we need to talk about the chemical structure of proteins. As you might remember from high school biology, proteins are made up of small substances, called amino acids.

These amino acids are then arranged in a very specific order and shape to make up individual proteins. In fact, it's this amino acid content that makes proteins the valuable macronutrients that they are.

Once you ingest protein through your food, your body takes them apart and repurposes those amino acids to build anything it needs, from hormones to cells and tissues.

This process of breaking down and reorganizing a protein's structure is what we call protein denaturation and can occur through a variety of processes. As mentioned, the heat, acid, and mechanical action of your digestive system all denature proteins.


From the Stomach to the Small Intestine

The stomach empties the chyme containing the broken down egg pieces into the small intestine, where the majority of protein digestion occurs. The pancreas secretes digestive juice that contains more enzymes that further break down the protein fragments. The two major pancreatic enzymes that digest proteins are chymotrypsin and trypsin. The cells that line the small intestine release additional enzymes that finally break apart the smaller protein fragments into the individual amino acids. The muscle contractions of the small intestine mix and propel the digested proteins to the absorption sites. In the lower parts of the small intestine, the amino acids are transported from the intestinal lumen through the intestinal cells to the blood. This movement of individual amino acids requires special transport proteins and the cellular energy molecule, adenosine triphosphate (ATP). Once the amino acids are in the blood, they are transported to the liver. As with other macronutrients, the liver is the checkpoint for amino acid distribution and any further breakdown of amino acids, which is very minimal. Recall that amino acids contain nitrogen, so further catabolism of amino acids releases nitrogen-containing ammonia. Because ammonia is toxic, the liver transforms it into urea, which is then transported to the kidney and excreted in the urine. Urea is a molecule that contains two nitrogens and is highly soluble in water. This makes it a good choice for transporting excess nitrogen out of the body. Because amino acids are building blocks that the body reserves in order to synthesize other proteins, more than 90 percent of the protein ingested does not get broken down further than the amino acid monomers.


What happens to absorbed amino acids?

Once the amino acids are in the blood, they are transported to the liver. As with other macronutrients, the liver is the checkpoint for amino acid distribution and any further breakdown of amino acids, which is very minimal. Dietary amino acids then become part of the body’s amino acid pool.

Assuming the body has enough glucose and other sources of energy, those amino acids will be used in one of the following ways:

  • Protein synthesis in cells around the body
  • Making nonessential amino acids needed for protein synthesis
  • Making other nitrogen-containing compounds
  • Rearranged and stored as fat (there is no storage form of protein)

If there is not enough glucose or energy available, amino acids can also be used in one of these ways:

  • Rearranged into glucose for fuel for the brain and red blood cells
  • Metabolized as fuel, for an immediate source of ATP

In order to use amino acids to make ATP, glucose, or fat, the nitrogen first has to be removed in a process called deamination, which occurs in the liver and kidneys. The nitrogen is initially released as ammonia, and because ammonia is toxic, the liver transforms it into urea. Urea is then transported to the kidneys and excreted in the urine. Urea is a molecule that contains two nitrogens and is highly soluble in water. This makes it ideal for transporting excess nitrogen out of the body.

Because amino acids are building blocks that the body reserves in order to synthesize other proteins, more than 90 percent of the protein ingested does not get broken down further than the amino acid monomers.

Attributions:

  • Lindshield, B. L. Kansas State University Human Nutrition (FNDH 400) Flexbook. goo.gl/vOAnR, CC BY-NC-SA 4.0
  • “Protein Digestion and Absorption”, section 6.3 from the book An Introduction to Nutrition (v. 1.0), CC BY-NC-SA 3.0

Image Credits:

  • Fig 6.17. “Protein digestion in the human GI tract” by Alice Callahan is licensed under CC BY 4.0 edited from “Digestive system diagram edit” by Mariana Ruiz, edited by Joaquim Alves Gaspar, Jmarchn is in the Public Domain
  • Fig 6.18. “Protein digestion in the stomach” from “Protein Digestion and Absorption,” section 6.3 from An Introduction to Nutrition (v. 1.0), CC BY-NC-SA 3.0
  • Fig 6.19. “Denaturation of proteins” by Alice Callahan is licensed under CC BY 4.0 edited from “Process of denaturation” by Scurran is licensed under CC BY-SA 4.0
  • Fig 6.20. “Enzymatic digestion of proteins” by Alice Callahan is licensed under CC BY 4.0 edited from “Process of denaturation” by Scurran is licensed under CC BY-SA 4.0
  • Fig 6.21. “Summary of protein digestion” by Alice Callahan is licensed under CC BY 4.0 edited from “Process of denaturation” by Scurran is licensed under CC BY-SA 4.0


Watch the video: Effect of heat on PROTEINS. EGG Protein Albumin. Denaturation u0026 Coagulation. Food Chemistry (September 2022).


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