What is a protein



 

Proteins, colloquially too Egg whites called, are macromolecules made up of amino acids. The amino acids mainly consist of the elements carbon, hydrogen, oxygen, nitrogen and - more rarely - sulfur. Proteins are one of the basic building blocks of all cells. They not only give the cell structure, but are the molecular “machines” that transport substances, pump ions, catalyze chemical reactions and recognize signal substances.

Building blocks of proteins are certain as proteinogenic, i.e. protein-building, designated amino acids that are linked by peptide bonds to form chains. In humans, there are 21 different amino acids: the 20 long-known and selenocysteine. The amino acid chains have a length of up to several 1000 amino acids, whereby one actually calls amino acid chains with a length of less than 100 amino acids as peptides and only speaks of proteins from a larger chain length. The molecular size of a protein is usually given in kilo-Daltons (kDa). Titin, the largest known human protein with approx. 3600 kDa, consists of over 30,000 amino acids and contains 320 protein domains.

The number of possible different amino acid chains is gigantic. With a chain length of 100, 21 different amino acids result in the unimaginable number of 21100 or 10132 Link options. This number exceeds the number of all atoms in the universe (“only” 6 · 1079 Particles) around the 10th52times.

The composition of a protein, and thus its structure, is encoded in the respective gene.

The word protein was founded in 1838 by Jöns Jakob Berzelius from the Greek word πρωτευω (proteuo, "I take first place", from πρωτος, protos, "First", "most important") derived. This is to underline the importance of proteins for life.

Significance for the organism

The tasks of proteins in the organism are varied. Examples are:

  • As structural proteins, they determine the structure of the cell and thus ultimately the nature of tissues, for example the hair structure, and the entire body structure.
  • As enzymes, they take on biocatalysis functions. They enable or prevent chemical reactions in living beings by accelerating or slowing them down.
  • As ion channels, they regulate the ion concentration in the cell, and thus its osmotic homeostasis as well as the excitability of nerves and muscles.
  • In the muscles, certain proteins change their shape and thus ensure the contraction of the muscles and thus movement.
  • As transport proteins, they take on the transport of substances that are important to the body such as B. Hemoglobin, which is responsible for the transport of oxygen in the blood, or transferrin, which transports iron in our blood.
  • Some (mostly smaller proteins) act as hormones to control processes in the body.
  • As antibodies, they serve to ward off infection.
  • As blood coagulation factors, the proteins prevent excessive blood loss when a blood vessel is injured and an excessive coagulation reaction with blockage of the vessel on the other hand.
  • As a reserve substance, the proteins serve the body as energy suppliers when hungry. The proteins stored in the liver, spleen and muscles can be used for gluconeogenesis and thus for energy production in a hunger state in order to maintain the vital processes of the body.

Mutations in a gene cause changes in the structure of the protein that the gene encodes. This can result in errors in protein activity. Such errors, sometimes with the complete loss of protein activity, are the basis of many hereditary diseases.

Spatial structure

 

For the functioning of the proteins, their spatial structure (their folding) is particularly important. The protein structure can be described on four levels:

  • As Primary structure of a protein is the sequence of the individual amino acids within the polypeptide chain. Put simply, one could imagine a chain in which each pearl represents an amino acid (notation: AS1–AS2–AS3–AS5 etc.). The primary structure only represents the amino acid sequence, but not the spatial structure.
  • The Tertiary structure is the spatial arrangement of the polypeptide chain that is superordinate to the secondary structure. It is determined by the forces and bonds between the residues (i.e. the side chains) of the amino acids. The binding forces that stabilize this three-dimensional structure are, for example, disulfide bridges (covalent bonds between the sulfur atoms of two cysteine ​​residues) or, above all, non-covalent interactions such as the aforementioned hydrogen bonds. In addition, hydrophobic, ionic and van der Waals forces play an important role. It is through these forces and bonds that the protein continues to fold.
  • In order to be able to function, many proteins have to assemble into a protein complex, the so-called Quaternary structure. This can either be an aggregation of different proteins or an association of two or more polypeptide chains that have arisen from one and the same polypeptide chain, the so-called precursor (see: insulin). The individual proteins are often linked to one another by hydrogen bonds and salt bridges, but also by covalent bonds. The individual subunits of such a complex are called Protomers designated. Some protomers can also function as independent proteins, but many only achieve their functionality in complexes. The immunoglobulins (antibodies), in which two identical heavy and two identical light proteins are linked via a total of four disulfide bridges to form a functional antibody, can serve as an example of complexes made up of several proteins.

Many complex proteins cannot fold spontaneously, i.e. adopt their physiological structure, but instead need folding aids, so-called chaperones. The chaperones bind to newly formed (or damaged, denatured) amino acid chains and help them to get their structure while consuming chemical energy.

There are two main groups of proteins:

  • the globular proteinswhose tertiary or quaternary structure looks approximately spherical or pear-shaped and which are usually readily soluble in water or saline solutions (for example the protein of the albumen, Ov albumin called),
  • the fibrillar proteinswhich have a thread-like or fibrous structure, are mostly insoluble and belong to the supporting and structural substances (for example the keratins in the hair and fingernails, collagen, actin and myosin for muscle contraction).

Protein surface

For the sake of simplicity, often only the backbone (Backbone) of the protein (e.g. images above right). In order to understand the function, however, the surface of the protein is of great importance. Since the side chains of the amino acids protrude into space from the backbone, they also make a decisive contribution to the structure: The course of the backbone determines the general three-dimensional structure, but the contours of the surface and their chemical properties are determined by the side chains.

Denaturation

The secondary and tertiary structure and thus also the quaternary structure of proteins can change through chemical influences such as acids, salts or organic solvents, as well as physical influences such as high or low temperatures or pressure, without the Sequence of amino acids (primary structure) changes. This process is called denaturation and is usually irreversible, i.e. the original three-dimensional spatial structure cannot be restored without help. The best-known example of this is the protein in hen's eggs, which solidifies when cooked because the spatial structure of the protein molecules has changed. The original liquid state can no longer be restored. Restoring the denatured protein to its original state is called renaturing.

People denature, i.e. cook, their food in order to make it easier to digest. Denaturation changes the physical and physiological properties of the proteins, such as B for the fried egg, which is denatured by the heat in the pan. A high fever can therefore be life-threatening for humans. Because the body's own proteins are denatured during fever due to the excessively high body temperature and can no longer fulfill their vital tasks in the organism. For example, some red blood cell proteins already denature at 42 ° C. The fever actually has a protective function, not a destructive one. Because the high temperature of a fever is supposed to destroy intruders and foreign bodies, so-called antigens, and render them harmless. These antigens usually denature at lower temperatures than the body's own proteins.

The pieces that result from chemical cleavage of the protein chains (proteolysis) are called peptones.

Protein deficiency

Protein performs a large number of roles in our body. It is necessary to build up and maintain the body's cells and helps heal wounds and diseases. An adult person should consume around 1 gram of protein per kilogram of body weight with their food every day. The need is still increasing in pregnant and breastfeeding women.

A deficiency can have dire consequences:

  • Hair loss (hair consists of 97-100% proteins - keratin)
  • Lack of drive
  • In the worst case scenario, Kwashiorkor, a protein deficiency disease, occurs. People (mostly children) who suffer from Kwashiorkor can be recognized by their large bellies. The organism tries to cover the protein deficiency with water, so that after a while the water is deposited in the body (edema). Other symptoms are:
    • Muscle weakness
    • Stunted growth
    • Fatty liver
    • Edema
  • Persistent protein deficiency leads to marasmus and death.

However, protein deficiencies are extremely rare in industrialized countries and only occur in extremely low-protein diets. The average German mixed diet, on the other hand, contains 100 grams of protein per day, more than enough protein. Although protein powder is often advertised as essential for amateur athletes, "Our usual diet ... also covers the protein needs of athletes", according to a report by the Baden-Württemberg Ministry of Nutrition and Rural Areas. However, according to studies, muscle building through strength training is hardly possible if you gain less than 1 gram of protein per kilogram of body weight per day. A daily protein intake of 1.5 grams per kilogram of body weight is considered sufficient for strength athletes.

Protein synthesis

Main article: Protein synthesis

We ingest proteins with our food. During our digestion, these proteins are broken down into their components - the amino acids. The human organism is particularly dependent on nine amino acids (out of 21 that are required in total) because they are essential, which means that the body cannot produce them itself. The amino acids are transported to every cell in the blood. The amino acid sequence is encoded in deoxyribonucleic acid (DNA). In the ribosomes, the “protein production machinery” of the cell, this information is used to assemble a protein molecule from individual amino acids, whereby the amino acids are linked in a very specific sequence specified by the DNA.

Protein suppliers

High protein foods are:

  • flesh
  • fish
  • Eggs
  • Dairy products (cheese)
  • nuts
  • Grain
  • Legumes (soy: 41.6%)
  • Potatoes (only 2%, but a lot of essential amino acids)

Studies by Thomas Osborne and Lafayette Mendel from 1914 showed that rats given animal protein gained weight faster than rats given only vegetable protein. From this it was concluded prematurely that animal protein was “higher quality” than vegetable protein. However, later studies by McCay at Berkeley University showed that rats given vegetable protein are healthier and live roughly twice as long.

Detection of proteins

3d illustration

For a better understanding of structure and function, it is essential to display the spatial shape of proteins using suitable graphics programs. In principle, a simple molecular observation program is sufficient for this (Viewer), of which there are now a large number of freely available and commercially available examples. When representing biomolecules, however, a simplification to the protein backbone with the secondary structural elements helix, sheet, etc. is necessary (see section Protein surface). At the same time, there is a large range of corresponding software packages, but most of them also have additional functionalities. Examples:

The most common file format for the structures of proteins is the pdb format of the freely accessible Protein Data Bank. A pdb file contains line-by-line entries for each atom in the protein, sorted by amino acid sequence; in the simplest case these are the atomic type and Cartesian coordinates. It is therefore a system-independent plain text format.

literature

  • Jeremy M. Berg, John L. Tymoczko, Lubert Stryer: biochemistry. Spectrum Academic Publishing House, Heidelberg 2007 (6th edition). ISBN 3-82-741800-3
  • Hubert Rehm: The Experimenter - Protein Biochemistry / Proteomics. Spektrum Akademischer Verlag, Heidelberg 2002 (4th edition). ISBN 3-82-741195-5

See also

  • Peptide, polypeptide, peptide bond, protein domain,
  • Enzyme, metalloenzyme, metalloenzyme, metalloprotein, glycoproteins
  • Protein synthesis, proteolysis
  • Chaperone, heat shock proteins
  • Proteome, proteomics, yeast two-hybrid system, protein overexpression in yeast
  • Intein

Categories: Fabric Group | protein