In the following article we want to tell you a little more about proteins. Of our three macronutrients (carbohydrates, proteins, fat), proteins represent the most diverse group of molecules. They play a role in virtually all biological structures and processes in our bodies.
When it comes to nutrition, we often just talk about how proteins are important for muscle maintenance and should be included in our diet in a certain ratio. In this context, proteins are readily generalized to a uniform substance. Of course, there is no one protein, but thousands of different proteins in different sizes, shapes and most importantly: with different tasks.
Proteins are not only important for muscle maintenance!
We will go into more detail about the many functions later. Here are just a few examples: There are structural proteins, transport proteins, enzymes, storage proteins and much more. But why can proteins actually perform so many different tasks? To understand this, let’s take a closer look at the structure.
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1.1. The modular protein system
The smallest structural unit of proteins is called amino acids. There are 20 proteinogenic – that is, protein-forming amino acids. All proteins are composed of these 20 amino acids. Like the Lego bricks in a Lego box, amino acids can also be put together in countless combinations. Each combination provides a different protein. Some proteins consist of only a few hundred amino acids, others of several thousand.
20 amino acids form the building unit of our proteins.
Most amino acids can be produced by the body itself. The rest are essential and must be supplied through food. Essential amino acids are lysine, leucine, isoleucine, threonine, valine, methionine, phenylalanine and tryptophan.
1.2. What distinguishes the individual amino acids?
Amino acids are very small molecules that differ in their structure and properties. However, it is crucial for the classification as an amino acid that all amino acids have both a so-called “amino group” and a “carboxyl group”. These are specific groups of atoms in the molecule that have a significant influence on the properties of the molecule as a whole. Such special groups of atoms are also called “functional groups”. In proteins, it is precisely these two groups of molecules that form the links between the individual amino acids. This bond is called a peptide bond. In this way, the individual amino acids link up to form long chains. Imagine a train with many wagons. Each amino acid is a wagon and has a coupling at the front and back. On one side the wagon has a hook, on the other – an eyelet. The hook from one wagon always fits into the eyelet of a second wagon. Hook and eye are the amino and carboxyl groups. You can only ever connect hook (amino group) and eye (carboxyl group) together, never hook & hook or eye and eye. Thus, the amino acid wagons always look in the same direction, but theoretically, infinitely long connections are possible. By the way, there is an infinite number of each amino acid – for example, a protein can also contain the same amino acid 20 times. Thus, very long combinations are possible.
Short chains are called peptides, long chains proteins.
If up to 100 amino acids are strung together in a chain, they are called peptides (short proteins), and if the chains are even longer, they are called proteins. You can also think of it a bit like a string of pearls.
Different properties of amino acids lead to different properties of proteins.
We have just mentioned that the different amino acids have different properties. Some amino acids have basic or acidic properties, are water-soluble (hydrophilic) or water-insoluble (hydrophobic), negatively or positively charged, and so on. The properties of the individual amino acids are later decisive for the properties and functions of the protein. If many hydrophilic amino acids are present, the protein will probably be very soluble in water. With increased hydrophobic amino acids, the protein will not dissolve as well in water.
1.3. Diversity of forms in proteins
Even though you cannot see such a single protein molecule, it is important to realize that proteins have a spatial structure. They take up space in the room and they have a very specific shape that is unique to each protein.
The long chain of amino acids strung together is the so-called primary structure of proteins. Each protein has a unique sequence of amino acids that is characteristic of that protein. In addition to this primary structure, however, a protein continues to fold into a very specific shape.
Proteins fold compactly.
Depending on which amino acids are present in a protein and in which order they are lined up, subsections of the protein fold into specific structures. These are mainly two-dimensional structures (so-called folded sheets) or helical structures. These structures are called secondary structure.
In simplified terms, this can be explained as amino acids preferring to interact with amino acids that are similar to them. However, if these amino acids are not directly next to each other, the bead chain must fold so that these amino acids come closer together. A protein can be composed of helices only or of leaflets, or it can have both helices and leaflet structures. The overall 3-dimensional appearance of the protein is called the tertiary structure. For example, there are more spherical proteins, such as hemoglobin, or long fibrous proteins, such as collagen.
For a protein to dissolve well in water, it must fold so that water-soluble amino acids are on the outside and water-insoluble amino acids are on the inside.
Not only the amino acid sequence but also the folding are important factors influencing the different properties of proteins. Let’s take a look at two examples. Whey proteins are spherical proteins. They fold in such a way that the water-soluble amino acids are mainly located on the surface of the sphere and the water-insoluble amino acids are located more inside the sphere. This makes the protein soluble in water. A transport protein in our blood, such as hemoglobin, which can transport oxygen, must of course also be readily soluble in water, since our blood is an aqueous solution and hemoglobin could not transport anything otherwise. Such molecules thus have increased water-soluble amino acids on the surface. But of course there are also proteins that are not water-soluble. Structural proteins or muscle proteins, for example, have a very compact structure and are therefore not water-soluble. It would be stupid if we suddenly dissolved while swimming.
1.4. Denaturation - change in protein structure
Let’s summarize again: Each protein has a specific sequence of amino acids and takes on a characteristic shape. Both are quite crucial for their biological function. But what happens now if I change this structure?
The structural change of proteins is called coagulation or denaturation.
During denaturation, the tertiary and secondary structure of proteins are altered or destroyed. The structure of proteins is sensitive to various external influences, such as heat, acid, enzymes or even salts.
Heat and acid can cause proteins to coagulate.
Heat denaturation is probably something everyone can imagine, because each of you has cooked an egg before. The heat from boiling or frying causes the proteins in the egg to unfold. Amino acids that were previously hidden inside the molecule are suddenly on the surface and can react with parts of other proteins. Instead of the previously ordered structure, you now suddenly have a lot of interlocking chains, and instead of the liquid, you now have a hard-boiled egg.
It works very similarly for yogurt. Only here, acid is used and not heat. The lactic acid bacteria eat the lactose contained in the milk and produce lactic acid. The acid causes the caseins (certain milk proteins) in the milk to lose their solubility and precipitate. They then accumulate together, forming a 3-dimensional protein network that ensures that the yogurt becomes firm.
In the case of cheese, by the way, the whole process takes place enzymatically. The rennet (an enzyme necessary for cheese production) cuts off the water-soluble amino acid groups from the casein surface, so to speak. This also causes the casein to become insoluble and solid cheese to form.
Cheese, yogurt, hard-boiled eggs – examples of coagulated protein.
However, protein denaturation is not only important in the production of cheese and yogurt. Nutritionally, it is also important that dietary proteins are denatured first. Unfolded proteins can be much better broken down into their individual parts by our enzymes. By the way, our stomach acid is also there to denature the proteins contained in food.
2. where do proteins actually occur?
I have already mentioned one or the other protein in the previous text. Many proteins today are also used with the English name, such as the Whey. Therefore, here is a brief overview of the origin and designation of the most important dietary proteins:
2.1. Milk proteins
Caseins and whey proteins are both milk proteins.
Milk contains two protein fractions. The whey proteins – there is not one whey protein, but different proteins, which are assigned to the whey proteins based on their properties – and the caseins. Again, a group of different proteins. Whey proteins are sensitive to heat and form the milk skin in pudding or cocoa, for example….
Whey is the English name for whey protein.
In English, whey proteins are referred to as Whey. Caseins are heat stable but not acid stable and are responsible for the solidification of yogurt or cheese.
2.2. Egg proteins
Different proteins are found in both the egg yolk and the egg white. Egg yolk additionally contains fat. The most abundant protein in terms of quantity is ovalbumin, which is found in egg white.
2.3. Meat as a source of protein
In addition to muscle proteins, such as myosin and actin, to name two important representatives, there are also connective tissue proteins, such as collagen and elastin. Collagen plays a more important role in our diet. It is found mainly in hides, tendons, bones, and connective tissue-containing meats.
2.4. Plants as a source of protein
Plants are also becoming increasingly important as a source of protein. Plant sources of protein include hemp seeds, rice, peas and lupine.
In the other articles of our protein series, we will highlight, among other things, the importance of proteins in food production and the different value of proteins from animal and plant sources.
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Wondering how much protein is allowed on a low-carb keto diet? You can find the answer in our in-depth article on ketogenic diets.