What Does it Mean to Denature a Protein?

Posted on 04 Jun 2013 15:12

What's the Big Deal About Protein Denaturing?

Raw foodies, and those selling so-called raw whey make a lot of noise about proteins being denatured by cooking and so losing their natural goodness. So what does it mean for a protein to be denatured? Well, proteins are big molecules with a complex 3-dimensional shape. For a protein within your body, this shape is integral to its function. So, they have it right when they say that denaturing of a protein renders it "nonfunctional."

What you need to understand is that, for the most part, digestion breaks down proteins into smaller sub-units: dipeptides, tripeptides, and single amino acids. The first step in protein digestion? Well, the only thing that happens to proteins in the mouth is it's crushed and broken up by chewing, and moistened by saliva. The true digestion begins in the stomach, where hydrochloric acid is released. What does this acid do? It denatures the proteins.

What is Protein Denaturing?

The denaturing of a protein means that it begins to unfold out of its 3-dimensional shape. This is of the utmost importance as it allows digestive enzymes to gain better access to the protein bonds. During digestion in the stomach, muscular action is churning the food contents so that they can be better mixed with the acid, resulting in more thorough denaturation! The resulting semi-fluid mixture is called chyme, by the way.

Pepsin is also triggered to be released by the hydrochloric acid and this enzyme begins to break some of the protein bonds, resulting in polypeptides, which are long chains of amino acids, but shorter than they were to begin with. This accounts for about 10-20% of the protein digestion.

From there, protein digestion is continued in the small intestine. The pancreas makes an alkaline juice which is released into the small intestine to neutralize the HCL, so that other enzymes can do their work. Protein digesting enzymes called proteases, released by both the pancreas and the small intestine, break down the chains of amino acids into even smaller chains. Special cells in the lining of the small intestine also release other enzymes called peptidases, and these break the chains into little chains of two or three amino acids, called di- and tripeptides, as well as some single amino acids. These dipeptides, tripeptides, and single aminos are then absorbed by facilitated diffusion or active transport, which mostly occurs in the cells that line the intestinal duodenum and jejunum. In these cells the final step occurs, and the protein chains are broken down by other peptidases into individual amino acids.

The intestinal cells themselves use some of these amino acids, but most of them are transported, by facilitated diffusion, into the portal blood vessels and go right to the liver which uses them or releases them to the general blood circulation. Although it is not unheard of for di- or tripeptides, or even whole proteins, to be absorbed, this is extremely rare (except in fetal and neonatal mammals). Protein digestion and absorption by the body is very efficient and pretty much all of it is used. The little protein that is not digested goes into the large intestine where it is excreted.

Cooking Denatures Proteins?

So what of this cooked and so "denatured" food? 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. In fact, professional chefs know that you can basically "cook" a protein by applying an acid to it. Most proteins, when they are heat treated and denatured in this way, become more available to the body for digestion. For instance, when you cook egg whites, they turn white and solid. This means they are denatured. The protein from cooked egg whites is actually more available to the body and will be more efficiently absorbed than uncooked egg white.

The image below is a great illustration of this, using the simple analogy of paper clips as proteins. Imagine that the proteins in the liquid egg whites are orderly and distinct like the paper clips, and when they are denatured by heat these paper clips unfold and sort of get wound up with each other. Now, imagine if you are an enzyme trying to digest these proteins. When the paper clips are in their normal and functional configuration, much of their parts are folded away and close together in such a way that it is difficult to access them. Once they are denatured and opened up, as you can see from the picture, the stuff that makes up the paper clip is easier to get to. This is a perfect analogy for protein denaturation and digestion.

egg white protein denaturation by cooking analogy with paper clips

The paper clips represent the proteins in the egg white and what
happens to them once they are denatured by heat, or by acid.
image by RMADLA via wikimedia

egg white protein denaturation by cooking analogy with paper clips

The paper clips represent the proteins in the egg white and what
happens to them once they are denatured by heat, or by acid.
image by RMADLA via wikimedia

Most enzymes, which are also proteins, that are present in the food will also be denatured by the stomach's HCL and then broken down like any other protein. These are enzymes that would normally require a more neutral PH to function. Now, some have made a big deal about some plant enzymes being acid stable. However, they skip right over proving (in any way) that such enzymes have a role in human physiology.

Lactase Enzyme Products such as Lactaid

However, is there a such thing as as an enzyme, which is a protein, that you can take and it will not be made nonfunctional in the acidic environment of the stomach? Yes. Many lactose intolerant people successfully take commercial preparations of lactase enzyme, allowing them to ingest varying quantities of milk and dairy products. Now, the humans are not the only organisms to produce lactase enzymes. Other organisms do as well. For instance, bacteria such as E. coli and L. bulgaricus produce such an enzyme. And so do some yeasts, such as Saccharomyces fragilis, Torula cremoris and Torula utilis. However, the optimal pH of the lactase enzymes these organisms produce is usually more alkaline than the low pH of the stomach. This means they like a pH range of anywhere from 5 to 7, with 7 being more or less neutral. The pH in the active stomach can be from 1 to 3. A preparation made from the lactase produced by these organisms would not survive in the gut, unless an effective enteric coating was used. Which is how some lactase products work. On the other hand, a very common solution is to use the lactase produced by another organism, which is acid-active and acid-stable. This organism is a fungus called Asperguillus niger and from it, a lactase enzyme can be isolated which is active in a very wide range of pH. Actually, A. niger produces a number of useful enzymes. The product Beano contains another enzyme called Alpha-galactosidase, which helps to break down complex carbohydrates, which can help decrease flatulence and other GI distress from eating beans and other problematic starches.

So yes, it is possible to consume an enzyme and have it not only not be rendered useless by the stomach, but to have it work in the stomach. But this does not mean that all the various enzymes that plants contain will function in your body this way! The body has specific enzymes that are used to digest the foods we eat. Some analogues, as seen, can be found elsewhere in nature and used to produce products to help with maldigestion. These products are purposefully and specifically prepared. They do not happen by accident and they are not ubiquitous.

I go more into digestive enzymes in my article Do You Need Living Enzymes from Your Diet to Digest Food?. You can read the full article version or listen to the video version of that post here:

This page created 04 Jun 2013 15:12
Last updated 29 Mar 2018 21:38

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