Proteins are found in every cell and usually make up more than half of the dry weight. They serve as molecular “tools” and, depending on the particular structure, perform different tasks, for example by enabling cell movements , transporting metabolites , pumping ions , catalyzing chemical reactions or recognizing signal substances . Muscles, heart, brain, skin and hair also consist mainly of proteins.
Word origin and history
The word protein was first used in 1839 in a publication by Gerardus Johannes Mulder . This designation was proposed to him in 1838 by Jöns Jakob Berzelius , who derived it from the Greek word πρωτεῖος proteios for 'fundamental' and 'primary', based on πρῶτος protos for 'first' or 'primary'. Behind this was the mistaken idea that all proteins are based on a common basic substance. This resulted in a violent argument with Justus von Liebig .
The fact that proteins are made up of amino acid chains via peptide bonds was first suspected in 1902 at the 14th meeting of German naturalists and doctors, independently of Emil Fischer and Franz Hofmeister , who both gave lectures. Fischer introduced the term peptide.
The building blocks of proteins are certain amino acids known as proteinogenic, i.e. protein-building, which are linked to form chains by peptide bonds . In humans, there are 21 different amino acids - the 20 known for a long time, as well as selenocysteine . The human organism is particularly dependent on eight amino acids because they are essential , which means that the body cannot produce them itself but has to take them in with food. The amino acid chains can have a length of up to several thousand amino acids, whereby amino acid chains with a length of less than approx. 100 amino acids are referred to as peptides and are only referred to as 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 amino acid sequence of a protein - and thus its structure - is encoded in deoxyribonucleic acid (DNA). The genetic code used for this has hardly changed during the evolution of living things. In the ribosomes , the cell's “protein production machinery”, this information is used to assemble a polypeptide chain from individual amino acids , whereby the amino acids determined by a codon are linked in the sequence specified by DNA. Only with the folding of this chain in aqueous cell environment, the three-dimensional shape then produced a specific protein molecule.
The haploid human genome contains around 20,350 protein-coding genes - much fewer than assumed before the genome was sequenced . In fact, only about 1.5% of the total genomic DNA code for proteins, while the remainder consists of genes for non-coding RNA , as well as introns , regulatory DNA and non-coding deoxyribonucleic acids . Since many of the protein-coding genes - for example through alternative splicing of the primary transcript ( precursor mRNA ) of a gene - produce more than one protein, the human body contains far more than 20,350 different proteins. In addition, we now know proteins whose formation is based on exons of genes or gene segments in spatially distant chromosome regions, sometimes even on different chromosomes . The traditional one-gene-one-enzyme hypothesis (also: one-gene-one mRNA -one protein hypothesis) is no longer tenable for higher organisms today.
Number of amino acids involved
Small peptides are referred to as oligopeptides , whereby dipeptides are composed of only two amino acids , tripeptides of three, tetrapeptides of four amino acids, etc. Larger peptides with more than ten amino acids are called polypeptides . Most proteins are chains of 100 to 300 amino acids, rarely more than a thousand (see bar graph). The largest known protein consists of a chain of over 30,000 peptidically linked amino acids and is found in muscle cells : titin .
Proteins need a certain size to function. Oligopeptides can be used as signal substances - for example as hormones or neurotransmitters - but more than 50 amino acids are usually required for an enzyme function. A protein cannot contain an unlimited number of amino acids, if only because only a limited amount of amino acids is available. In addition, the time it takes to assemble an amino acid chain depends on the number of amino acids (see protein biosynthesis ).
The spatial structure determines how the proteins work. The protein structure can be described on four levels:
- The sequence of the individual amino acids in a polypeptide chain is called the primary structure of a protein. Put simply, one could imagine a chain in which each chain link represents an amino acid (notation from the amino / N- to the carboxy / C-terminus: AS 1 –AS 2 –AS 3 –AS 4 -…). The primary structure only describes the amino acid sequence , but not the spatial structure of the protein. This also includes the signal sequence .
- As secondary structure , the composition of the protein from most frequently occurring motifs is referred to for the spatial arrangement of amino acids. A distinction is made between the following structure types: α-helix , β-sheet , β-loop , β-helix and disordered, so-called random-coil structures . These structures result from hydrogen bonds between the peptide bonds of the polypeptide backbone . Every amino acid in a protein has characteristic angles between the individual atoms of the backbone ( dihedral angle ). The angle (N-terminal) in front of the carbon atom with the side chain of an amino acid is called the φ angle, and the angle after that as the ψ angle. These can be numbered and plotted against each other in a Ramachandran plot in order to display secondary structures. Alternatively, a Janin plot can be used.
- The tertiary structure is the spatial arrangement of the polypeptide chain above 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 be either an assembly of different proteins or an association of two or more polypeptide chains from the same polypeptide chain, the precursor protein (Engl. Precursor emerged) (cf .: insulin ). The pre- (with signal or activation sequences to be proteolysed) and the preproproteins (with signal and activation sequences to be proteolysed) are called precursor proteins. 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 . 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 .
- Some proteins are arranged in a “superstructure” or “superstructure” that goes beyond the quaternary structure, but is also molecularly predetermined, such as collagen in the collagen fibril or actin , myosin and titin in the sarcomere .
The division into primary to quaternary structure makes it easier to understand and describe the folding of proteins. Under physiological conditions, a defined primary structure unfolds into a specific tertiary structure. In other words: the information content, which is already contained in the primary structure as a linear amino acid sequence , is expressed in the form of a specific three-dimensional protein structure.
For this folding of the polypeptide chain into the characteristic three-dimensional shape of the native protein, however, special environmental conditions are required - such as an aqueous medium, a pH value in a certain narrow range, a temperature within certain limits. They are fulfilled in the cell's environment within its membrane . Nevertheless, many complex proteins would not spontaneously fold into the structure that is functional in the cell, but instead need folding aids, so-called chaperones . The chaperones bind to newly formed ( nascent ) polypeptides - or denatured or damaged amino acid chains - and, while consuming chemical energy, help them achieve a physiologically functional structure.
Classification of proteins
Proteins can be divided into two main groups according to their external shape:
- the globular proteins whose tertiary or quaternary structure or spherical approximately looks pear-shaped and which are usually well soluble in water or salt solutions (for example, the protein of albumen , OV albumin called),
- the fibrillar proteins , which 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 ).
|Molecular shape||Non-protein portion|
|Globular Proteins||Fibrillar proteins|
Simplistically is representative of the complex protein structure often the backbone ( backbone ) mapped the protein (eg. As 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 the biochemical properties of the protein are determined by the side chains.
For a better understanding of structure and function, it is essential to display the spatial shape of proteins using suitable graphics programs.
The most common file format for the atomic position data 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 atomic type and Cartesian coordinates. It is therefore a system-independent plain text format. On the basis of this file, z. B. in Jmol the 3D structure can be displayed. If the natural 3D structure has not yet been determined, only the protein structure prediction can help .
The purification and enrichment of proteins from biological material is an important step in the biochemical identification and characterization of newly discovered proteins.
In biotechnology, and especially with recombinant proteins , reproducible, careful protein purification - usually on a large scale - is an important prerequisite for using these proteins in diagnostics or therapy .
The following verifications, which do not represent absolute measurements and all have their limitations (e.g. incorrect measurements due to interfering substances; reference to a certain standard protein, etc.), are used to quantify proteins:
- UV absorption
- Xanthoprotein reaction
- Millon's reaction
- Ninhydrin reaction
- Biuret reaction
- Bradford test
- Lowry test
- BCA test
- Folin's reagent
- Amino acid analysis
A variety of methods can be used to prove the identity of a protein. Indirect evidence can also be provided via properties other than the primary structure which, however, follow from it, e.g. B. on the presence of its function ( enzyme kinetics ) in the sample vessel or on immunological properties that are used, among other things, in a Western blot .
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 effects 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, which means that the original three-dimensional spatial structure cannot be restored without help. The best-known example of this is the egg white in hen's egg, 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.
With control over the fire, people were also able to cook, which not least means that food can be prepared more easily and easily. Denaturation when heated changes the physical and physiological properties of proteins, for example fried eggs, which are changed by the heat in the pan. Even very high fever can denature the body's own proteins above a certain temperature. These proteins can then no longer fulfill their tasks in the organism, which can be life-threatening for humans.
For example, some red blood cell proteins already denature at 42 ° C. The regulated rise in fever therefore remains below such temperatures. The increase in body temperature during a fever means an acceleration of the metabolic processes (see RGT rule ) and thus allows the immune system to react more quickly. This fever is generated by the body itself (see pyrogen ) in order to be able to better defend itself against invading pathogens or foreign bodies (see also antigen ). Many of the foreign proteins already denature at lower temperatures than the body's own.
Hydrolysis and oxidation
The fragments resulting from hydrolytic cleavage of the protein chains ( proteolysis ) are a mixture of peptides and amino acids ; If these are formed under the catalytic effect of pepsin , they are called peptone , in the case of trypsin, tryptone .
Proteins can be oxidized by reactive oxygen species . This process is called protein oxidation and plays an important role in aging and a number of pathological processes. The oxidation can mean an extensive loss of function and lead to the accumulation of degenerated proteins in the cell.
Proteins can have the following very special functions in the organism :
- Protection, defense against microorganisms
- Body structure, movement
- Collagens that are up to 1 / 3 may account for the total body protein, structural proteins are the skin, connective tissue and bone. As structural proteins , they determine the structure of the cell and thus ultimately the nature of the tissue and the entire body structure.
- In the muscles, myosins and actins change their shape and thus ensure muscle contraction and thus movement.
- Keratin structures such as hair / wool, horns, nails / claws, beaks, scales and feathers
- Silk threads in spiders and insects
- Substance turnover (metabolism), transport, signal function
- Enzymes take on biocatalysis functions , i. In other words, they enable and control very specific (bio) chemical reactions in living beings.
- As ion channels , proteins regulate the ion concentration in the cell , and thus its osmotic homeostasis and the excitability of nerves and muscles .
- 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 the blood.
- There are membrane receptors in cell membranes ; mostly complexes of several proteins (also called multiprotein complexes ) that recognize and bind substances outside the cell. This results in a change in conformation which is then recognized as a transmembrane signal inside the cell.
- Some (mostly smaller proteins) act as hormones to control processes in the body.
- As blood coagulation factors , the proteins prevent excessive blood loss in the event of an injury to a blood vessel on the one hand and an excessive coagulation reaction with blockage of the vessel on the other.
- Auto-fluorescent proteins in jellyfish.
- Reserve material
Mutations in a certain gene can potentially cause changes in the structure of the corresponding protein, which can have the following effects on function:
- The mutation causes a loss in protein function; Many hereditary diseases are based on such errors, sometimes with complete loss of protein activity .
- The mutation causes an enzyme to increase its enzyme activity. This can have beneficial effects or also lead to a hereditary disease.
- Despite the mutation, the function of the protein is retained. This is known as a silent mutation.
- The mutation causes a functional change that is beneficial for the cell, organ or organism. An example would be a transmembrane protein that is in front of the mutation only in a position to stoffwechselbaren metabolites A record, while also the metabolite after the mutation B can be taken up regulated and z thereby. B. increases food diversity.
Protein in food
Proteins fulfill numerous tasks in the human body and are important for all organ functions, especially in the healing of wounds and diseases. In order to build up, maintain and renew body cells , people need food that contains protein. In relation to body weight (KG), the need is highest in the growth phases at the beginning of life.
In the first month of life, the baby should take in a daily amount of protein of around 2.5 grams per kilogram of body weight (g / kg body weight); towards the end of the first year of life, around 1.3 g / kg body weight is sufficient. From the age of two, the German Nutrition Society recommends age-dependent reference values between 1.0 and 0.8 g / kg for the daily intake of protein with food. In small children (1.0) the requirement is higher than in older children and adolescents (0.9), in younger adults (0.8) lower than in those over 65 years of age (estimated 1.0) - in each case based on normal weight , not actual body weight. Overweight people do not need more than people of normal weight. In contrast, the need for pregnant women is around 20% higher (1.0), and for breastfeeding women it is even higher (1.2). There is, however, a review from 2010, which was also taken into account in a report by an expert commission of the Food and Agriculture Organization of the United Nations (2013), which estimated the daily protein requirement for average adults to be significantly higher at (0.91–1.2). According to the DGE, the need for protein does not increase with physical activity. Kidney damage due to increased protein consumption has been refuted several times in long-term studies.
The protein ingested with food is digested in the stomach and intestines , broken down into smaller components and broken down into building blocks. Cells of the intestinal mucosa reabsorb these and release the individual amino acids into the ( portal ) bloodstream that leads to the liver. The human organism cannot produce some of the amino acids itself, but needs them as a building block for its own proteins. Protein in the diet must therefore be sufficient to cover the requirements for each of these indispensable ( essential ) amino acids.
- The protein deficiency disease Kwashiorkor occurs mainly in so-called developing countries and mostly affects malnourished children, whose advanced disease can be recognized by a hunger stomach caused by the storage of water. Due to the lack of blood proteins such as albumin , edema (hunger edema) also occurs in other parts of the body . Other symptoms include a.
However, a protein deficiency occurs very rarely in industrialized countries and only with extremely low-protein diets. The average German mixed diet contains 100 grams of protein per day, more than enough protein. Although protein powder is recommended in advertising as recommended for amateur athletes, “our normal diet [...] also covers the protein needs of athletes”, as stated in a report by the Ministry of Nutrition and Rural Areas of Baden-Württemberg.
High protein foods (in alphabetical order) are:
- Legumes ( soy and lupins approx. 40%, beans and peas approx. 20%) and other plant seeds such as rapeseed (approx. 40%)
- Dairy products ( cheese and quark )
Another source of protein is the quinoa plant, which contains all 9 essential amino acids in addition to its high protein content (around 14 g per 100 g). Aware of its importance as a source of food, the former UN Secretary General Ban Ki-moon declared 2013 to be the year of quinoa, as it should serve as an important source of food and fight hunger in developing countries, especially in times of climate change.
Valence of proteins
The Protein Digestibility Corrected Amino Acid Score (PDCAAS) is accepted by the Food and Agriculture Organization of the United Nations (FAO / WHO) and the US Food and Drug Administration as "the best way" to determine protein quality. The key figure takes into account both the amino acid composition and its digestibility. The content of essential amino acids is of particular importance . In addition, there is the older and now outdated concept of biological value . In 2013, the FAO also introduced the Digestible Indispensable Amino Acid Score (DIAAS) as an assessment method for determining protein quality. The main difference is that the DIAAS measures real ileal digestibility. The DIAAS is generally not shortened and can also assume values over 100%. The DIAAS only needs to be reduced to 100% when considering the total protein intake, regardless of whether it is a mixed diet or a single food such as baby food.
The total amount of proteins used as renewable raw materials in material use is given for Germany as a rule at around 55,000 t per year. There is no precise information about the origin of these proteins, but it can be assumed that they are largely of animal origin.
The majority of plant proteins for the feed industry expended, so as by-products in vegetable oil pressure and extraction arising Preßrückstände (z. B. rapeseed and soybean cakes , extraction meal ) and by-products of the extraction of starch from cereals . Plants that are grown for the main use as protein crops, such as lupins , protein peas and field beans , are only of minor importance - the total area for cultivating these plants as renewable raw materials in Germany is around 30 hectares per year. Around 1,000 t of wheat proteins are used in the chemical industry every year.
Animal proteins, on the other hand, are of central importance for the chemical-technical and biotechnological industry. Gelatine , in particular, plays a key role in this, and in Europe it is obtained primarily from cracks in beef , pork rind and bones of cattle and pigs. In Germany around 32,000 t of edible quality gelatine are produced annually, the total European production is 120,000 t (70% pork rind, 18% bones, 10% beef split, 2% other). Around 90,000 tonnes are used in Germany, two thirds of which are used in the food sector and about half of the remainder is used for animal feed. Around 15,000 t are used in the chemical and pharmaceutical industries. The main areas of application are in the pharmaceutical industry , with coatings for tablets and vitamin preparations (hard and soft capsules) and gelatin suppositories. In addition, gelatine is used for hemostatic sponges and as a blood plasma replacement. In analog photography , gelatine is the basis for the photosensitive layers on film and photo paper . Modern printer papers for printing color images are also coated with gelatine.
In addition to gelatine, casein is an important source of protein for the chemical industry. The protein obtained from milk protein is mainly used as a coating material for glossy papers and as an additive for coating colors (approx. 1–2% depending on the manufacturer). It is also used as a label adhesive on glass bottles. Around 8,000 to 10,000 tons of casein are used in Germany every year.
The use of proteins from blood meal for the production of bio-based plastics (e.g. plant pots) is still in development, as is a process for the biotechnological production of fibers from silk proteins for processing in foams, nonwovens or foils.
About 6,000 to 7,000 t of proteins are autolysis products made from yeast ( yeast extracts ). These are used primarily in the pharmaceutical and food industries as well as in the biotechnological industry as a nutrient solution for microorganisms.
Extraction of vegetable proteins
Vegetable proteins can be obtained from soy, peas, lupins or rapeseed. To do this, the protein has to be isolated from the plant: the flakes or flour are mixed with water and mashed . In the next step, the low-protein fibers and solids are separated from the protein-rich solution with the help of industrial centrifuges. Then comes the so-called precipitation . Here the pH of the protein-rich solution is adjusted to the isoelectric point . This causes the protein particles to settle. These are then in turn separated from the liquor using centrifuges. In order to remove all components of the mother liquor from the precipitated and separated protein, the protein is again mixed with water and separated again with the help of centrifugal force.
Industrial use of new protein sources
A company is breeding house flies ( Musca domestica ) millions of times on the agricultural science site of the University of Stellenbosch in Elsenburg (South Africa) as a full replacement for protein-rich fish meal with protein from fly larvae meal . One kilogram of fly eggs can produce around 380 to 420 kilograms of protein in just 72 hours.
With corresponding large-scale production, a large part of the global fish meal production could be saved and the world's oceans relieved of industrial fishing. In 2012 the company “Agriprotein” plans to go into mass production. Every day 65 t of blood from conventional slaughterhouses are needed to pull 100 t of fly larvae over a period of approx. 3 days over a length of around 12 mm each. By drying, grinding into maggot meal and subsequent pelleting, 20 t of the protein product can be obtained every day. Another pilot plant in Germany is funded by the German government with 50%. The company received the People and Environment Achievement Award in 2012 .
Production and optimization of recombinant proteins
The production of recombinant proteins with a precisely defined amino acid sequence and possibly other changes (e.g. glycosylation ) takes place both in the laboratory and on an industrial scale, either through peptide synthesis or biotechnologically through overexpression in various organisms and subsequent protein purification .
In the course of protein engineering , properties of the desired protein can be changed in a targeted manner (via the protein design ) or randomly (via a directed evolution ). In principle, the same processes can be used industrially as in the laboratory, but the use of useful plants by pharming is best suited for large-scale use, in which bioreactors are used in clean rooms . Genetic engineering methods are used to obtain the appropriate organisms .
The technical production of native proteins takes place worldwide mainly in pharmacy ( biopharmaceuticals ) and for the industrial use of enzymes as detergent additives ( proteases , lipases , amylases and cellulases ) or in milk processing ( lactases ). Proteins for the food industry do not necessarily have to be produced in native form as biological activity is not always required, e.g. B. with cheese or tofu.
"Seidenglanz proteins", which as an additive for shampoo are applied for human and grooming for animals (to ostensibly gloss to produce) are obtained from the remains of pupae of the silkworm , after detachment of these enveloping silk threads produced.
- Protein family
- Membrane protein , metalloenzyme , metalloprotein
- Heat shock proteins
- Yeast two hybrid system
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